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How can paleontologists identify morphologically different fossils as members of the same species?

How can paleontologists identify morphologically different fossils as members of the same species?


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I recently saw a documentary about Tyrannosaurus rex, which detailed the growth stages of the dinosaur. Apparently, it underwent a huge growth spurt at around 14 years of age, growing into the massive predator so well known in the popular imagination. From what else I've read, this much seems uncontroversial.

However, it made another claim which is that pre-teen Tyrannosaurs had a radically different morphology from adults. The skull was smaller, the legs much longer and the two-fingered forearms far larger in proportion to the rest of the body.

Given that there is some limited evidence for the animal living and hunting in social groups, the program speculated animals of different ages may have filled different hunting roles. The faster, more agile teens driving prey toward the massive, slower adults which then tackled the actual killing. Lions in Africa hunt in packs in this way, but whether Tyrannosaurs did remains speculative.

Anyway, on seeing the body plan of the young Tyrannosaur, I immediately wondered at how any palaeontologist could identify it as the same species as the adult. And lo, the documentary went on to say that, for some years, it was presumed to be a different species.

So the question is: given the radically different body shapes of these two forms and the limited evidence one can glean from the fossil record, how were scientists able to conclusively identify that young and adult tyrannosaurs were, in fact, members of the same species?


Radically different body shapes and sizes represent overall phenotype. Overall phenotype was used briefly in the 1960s and 1970s for species identification under a statistical distance method called "phenetics". However, phenetics has been rejected as a valid method of species identification. Similarly, although ecologists widely use the Biological Species Concept (Mayr) based on reproduction, taxonomists generally prefer some form of the Phylogenetic Species Concept. In addition, paleontologists find species concepts that require assumptions about reproduction in fossils to be difficult. The phylogenetic species concept uses the presence of fixed morphological characters to identify individuals as members of a species. Fixed characters need not be major, obvious differences. They need only be present in all members of a hypothesized species, and absent outside the species. That they are fixed in all members of a species implies a lack of gene flow, but the definition does not require that. So in the example you gave, juvenile T-rex must have some unique characteristic morphological feature that define them as members of the species (a synapomorphy). In addition, when there are a series of intermediate juvenile fossils, it becomes possible to link them together.


How do we know two different fossil sets are the same species?

Let's say a paleontologist finds two complete sets of fossils and they look similar, but with some pretty obvious differences. They are later determined to be the same species. How is that done? Can we know for sure that they weren't always just similar looking animals?

Of course, the implication here is that they would have evolved from a common ancestor and certain traits were selected to improve their "efficiency". It makes sense to me that the similar skeletal structure would suggest that's what happened. I'm wondering if there are any tests that can confirm that.

There's no good way to be sure, and there can be a lot of debate over it. Given that usually we only have the skeleton to work with--and almost never a full one--it's likely that our species boundaries for extinct organisms are broader than they would be if the organisms were still alive.

The short answer is you don't a lot of the time. For larger organisms like dinosaurs you will have more certainty than for microorganisms like forams. You have to understand the the concept of "species" is very loose and if you ask 50 different taxonomists you'll probably get 50 different answers. Some even argue that everything important happens on a population level and therefore the labeling of species isn't particularly important. There are toooooons of clades every year that are broken up into multiple groups after further study or genetic analysis and that's present day species that we have unlimited access to. So you can imagine how difficult it would be to classify extinct specimens that you only have a few examples of and no viable DNA for. Cryptic speciation is prolific in many clades and there's a ton a present day groups of organisms that you absolutely need living specimens or DNA to identify (example needing a spore print for a mushroom or micro organisms that can only be differentiated by DNA). Basically present day taxonomy is one of the biggest cluster fucks you will ever come across so there is no way that we can expect it to be any better for extinct organisms. (Remember we've only identified an estimated

15% of the species on earth currently and the present day species only constitute about 1% of those that have ever lived!)


Contents

Living fossils have two main characteristics, although some have a third:

  1. Living organisms that are members of a taxon that has remained recognisable in the fossil record over an unusually long time span.
  2. They show little morphological divergence, whether from early members of the lineage, or among extant species.
  3. They tend to have little taxonomic diversity. [5]

The first two are required for recognition as a living fossil status some authors also require the third, others merely note it as a frequent trait.

Such criteria are neither well-defined nor clearly quantifiable, but modern methods for analyzing evolutionary dynamics can document the distinctive tempo of stasis. [6] [7] [8] Lineages that exhibit stasis over very short time scales are not considered living fossils what is poorly-defined is the time scale over which the morphology must persist for that lineage to be recognized as a living fossil.

The term "living fossil" is much misunderstood in popular media in particular, in which it often is used meaninglessly. In professional literature the expression seldom appears and must be used with far more caution, although it has been used inconsistently. [9] [10]

One example of a concept that could be confused with "living fossil" is that of a "Lazarus taxon", but the two are not equivalent a Lazarus taxon (whether a single species or a group of related species) is one that suddenly reappears, either in the fossil record or in nature, as if the fossil had "come to life again". [11] In contrast to "Lazarus taxa", a living fossil in most senses is a species or lineage that has undergone exceptionally little change throughout a long fossil record, giving the impression that the extant taxon had remained identical through the entire fossil and modern period. Because of the mathematical inevitability of genetic drift, though, the DNA of the modern species is necessarily different than that of its distant, similar-looking ancestor. They almost certainly would not be able to cross-reproduce, and are not the same species. [12]

The average species turnover time, meaning the time between when a species first is established and when it finally disappears, varies widely among phyla, but averages about 2–3 million years. [ citation needed ] A living taxon that had long been thought to be extinct could be called a Lazarus taxon once it was discovered to be still extant. A dramatic example was the order Coelacanthiformes, of which the genus Latimeria was found to be extant in 1938. About that there is little debate — however, whether Latimeria resembles early members of its lineage sufficiently closely to be considered a living fossil as well as a Lazarus taxon has been denied by some authors in recent years. [1]

Coelacanths disappeared from the fossil record some 80 million years ago (upper Cretaceous) and, to the extent that they exhibit low rates of morphological evolution, extant species qualify as living fossils. It must be emphasised that this criterion reflects fossil evidence, and is totally independent of whether the taxa had been subject to selection at all, which all living populations continuously are, whether they remain genetically unchanged or not. [13]

This apparent stasis, in turn, gives rise to a great deal of confusion — for one thing, the fossil record seldom preserves much more than the general morphology of a specimen. To determine much about its physiology is seldom possible not even the most dramatic examples of living fossils can be expected to be without changes, no matter how persistently constant their fossils and the extant specimens might seem. To determine much about noncoding DNA is hardly ever possible, but even if a species were hypothetically unchanged in its physiology, it is to be expected from the very nature of the reproductive processes, that its non-functional genomic changes would continue at more-or-less standard rates. Hence, a fossil lineage with apparently constant morphology need not imply equally constant physiology, and certainly neither implies any cessation of the basic evolutionary processes such as natural selection, nor reduction in the usual rate of change of the noncoding DNA. [13]

Some living fossils are taxa that were known from palaeontological fossils before living representatives were discovered. The most famous examples of this are:

    fishes (2 species)
  • Metasequoia, the dawn redwood discovered in a remote Chinese valley (1 species) (2 species) (10 species) (1 species) (1 species) (59 species)

All the above include taxa that originally were described as fossils but now are known to include still-extant species.

Other examples of living fossils are single living species that have no close living relatives, but are survivors of large and widespread groups in the fossil record. For example:

  • Ginkgo biloba
  • Syntexis libocedrii, the cedar wood wasp (typified on coccoid dinocysts: occasionally calcareous cell remnants)

All of these were described from fossils before later found alive. [14] [15] [16]

The fact that a living fossil is a surviving representative of an archaic lineage does not imply that it must retain all the "primitive" features (plesiomorphies) of its ancestral lineage. Although it is common to say that living fossils exhibit "morphological stasis", stasis, in the scientific literature, does not mean that any species is strictly identical to its ancestor, much less remote ancestors.

Some living fossils are relicts of formerly diverse and morphologically varied lineages, but not all survivors of ancient lineages necessarily are regarded as living fossils. See for example the uniquely and highly autapomorphic oxpeckers, which appear to be the only survivors of an ancient lineage related to starlings and mockingbirds. [17]

The term living fossil is usually reserved for species or larger clades that are exceptional for their lack of morphological diversity and their exceptional conservatism, and several hypotheses could explain morphological stasis on a geologically long time-scale. Early analyses of evolutionary rates emphasized the persistence of a taxon rather than rates of evolutionary change. [18] Contemporary studies instead analyze rates and modes of phenotypic evolution, but most have focused on clades that are thought to be adaptive radiations rather than on those thought to be living fossils. Thus, very little is presently known about the evolutionary mechanisms that produce living fossils or how common they might be. Some recent studies have documented exceptionally low rates of ecological and phenotypic evolution despite rapid speciation. [19] This has been termed a "non-adaptive radiation" referring to diversification not accompanied by adaptation into various significantly different niches. [20] Such radiations are explanation for groups that are morphologically conservative. Persistent adaptation within an adaptive zone is a common explanation for morphological stasis. [21] The subject of very low evolutionary rates, however, has received much less attention in the recent literature than that of high rates

Living fossils are not expected to exhibit exceptionally low rates of molecular evolution, and some studies have shown that they do not. [22] For example, on tadpole shrimp (Triops), one article notes, "Our work shows that organisms with conservative body plans are constantly radiating, and presumably, adapting to novel conditions. I would favor retiring the term ‘living fossil’ altogether, as it is generally misleading." [23]

The question posed by several recent studies pointed out that the morphological conservatism of coelacanths is not supported by paleontological data. [24] [25] In addition, it was shown recently that studies concluding that a slow rate of molecular evolution is linked to morphological conservatism in coelacanths are biased by the a priori hypothesis that these species are ‘living fossils’. [26] Accordingly, the genome stasis hypothesis is challenged by the recent finding that the genome of the two extant coelacanth species L. chalumnae and L. menadoensis contain multiple species-specific insertions, indicating transposable element recent activity and contribution to post-speciation genome divergence. [27] Such studies, however, challenge only a genome stasis hypothesis, not the hypothesis of exceptionally low rates of phenotypic evolution.

The term was coined by Charles Darwin in his On the Origin of Species from 1859, when discussing Ornithorhynchus (the platypus) and Lepidosiren (the South American lungfish):

. All fresh-water basins, taken together, make a small area compared with that of the sea or of the land and, consequently, the competition between fresh-water productions will have been less severe than elsewhere new forms will have been more slowly formed, and old forms more slowly exterminated. And it is in fresh water that we find seven genera of Ganoid fishes, remnants of a once preponderant order: and in fresh water we find some of the most anomalous forms now known in the world, as the Ornithorhynchus and Lepidosiren, which, like fossils, connect to a certain extent orders now widely separated in the natural scale. These anomalous forms may almost be called living fossils they have endured to the present day, from having inhabited a confined area, and from having thus been exposed to less severe competition. [28]

Other definitions Edit

Long-enduring Edit

A living taxon that lived through a large portion of geologic time. [ citation needed ]

Queensland lungfish (Neoceratodus fosteri) is an example of an organism that meets this criterion. Fossils identical to modern Queensland lungfish have been dated at over 100 million. Modern Queensland lungfish have existed as a species for almost 30 million years. The contemporary Nurse shark, a threatened species has existed for more than 112 million years making this species one of the oldest if not actually the oldest extant vertebrate species.

Resembles ancient species Edit

A living taxon morphologically and/or physiologically resembling a fossil taxon through a large portion of geologic time (morphological stasis). [29]

Retains many ancient traits Edit

A living taxon with many characteristics believed to be primitive. [ citation needed ]

This is a more neutral definition. However, it does not make it clear whether the taxon is truly old, or it simply has many plesiomorphies. Note that, as mentioned above, the converse may hold for true living fossil taxa that is, they may possess a great many derived features (autapomorphies), and not be particularly "primitive" in appearance.

Relict population Edit

Any one of the above three definitions, but also with a relict distribution in refuges. [ citation needed ]

Some paleontologists believe that living fossils with large distributions (such as Triops cancriformis) are not real living fossils. In the case of Triops cancriformis (living from the Triassic until now), the Triassic specimens lost most of their appendages (mostly only carapaces remain), and they have not been thoroughly examined since 1938.

Low diversity Edit

Any of the first three definitions, but the clade also has a low taxonomic diversity (low diversity lineages). [ citation needed ]

Oxpeckers are morphologically somewhat similar to starlings due to shared plesiomorphies, but are uniquely adapted to feed on parasites and blood of large land mammals, which has always obscured their relationships. This lineage forms part of a radiation that includes Sturnidae and Mimidae, but appears to be the most ancient of these groups. Biogeography strongly suggests that oxpeckers originated in eastern Asia and only later arrived in Africa, where they now have a relict distribution. [17]

The two living species thus seem to represent an entirely extinct and (as Passerida go) rather ancient lineage, as certainly as this can be said in the absence of actual fossils. The latter is probably due to the fact that the oxpecker lineage never occurred in areas where conditions were good for fossilization of small bird bones, but of course, fossils of ancestral oxpeckers may one day turn up enabling this theory to be tested.


How can paleontologists identify morphologically different fossils as members of the same species? - Biology

But, WHAT ARE SPECIES. We (sometimes) know them when we see them, but how do we recognize them? What is our species concept (more accurately species criterion )? This is known in biology as the "species problem" .

Difficulties with species delineation:

"Species" are our attempt scientifically to codify traditional "kinds," populations of interbreeding critters that are more or less morphologically uniform. Seems easy, but when you scrutinize living diversity in detail, a number of problems come up:

jmerck/nature/animals/images/hybrid23627s.jpg" />
Hybrid Galápagos iguana

To see how weird things can get, consider the case of hybridogens like exist in the molly genus Poeciliopsis . In this morphologically distinct "species," each individual is female and gets her paternal genome through her mother's mating with males of at least two closely related normal species.

Thus, hybridization spans the range from extremely maladaptive to essential to the perpetuation of hybridogenic "species."


Yellow-shafted flicker (right), red-shafted flicker (left) from The Cornell Lab of Ornithology

Obviously, if they were to be extirpated from the interior of the continent, a speciation-event might result. Indeed, Pleistocene glaciations seem to have produced this effect in many avian sister-taxa, like blue jays (East) and Steller's jays (West.)


Resident (left), transient (center), and offshore (right) orcas, the last from Homer News

  • "Residents" are sedentary fish-eaters.
  • "Transients" are nomadic marine-mammal hunters.
  • "Offshores" are nomadic fish-eaters.

Species criteria:

The paradox of species criteria: We feel compelled to define species precisely when, in nature, their boundaries are fuzzy, indistinct, and best described probabilistically.

  • Merck: Two populations represent distinct species when they are sufficiently different that hybridization between their individuals reduces the fitness of their offspring.
  • Holtz: Any species concept that requires us to view lions and tigers as belonging to the same species is a bad one.

But we're not the experts. Let's refer to the view of Charles Darwin, who felt that species were merely well delineated varieties whose distinctiveness from other organisms arose from the fact that intermediate forms were now extinct.

None of this has prevented biologists from attempting to develop hard and fast species concepts. Major attempts at species definitions that have gained significant traction include:

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Kekaimalu the wholphin and her baby from MSNBC
  • Reproductive isolation is meaningless in asexual organisms
  • has not been tested for the vast majority of living organisms
  • fails to account for the truly vast numbers of morphologically distinct populations that are prevented from interbreeding simply by geography or ecology, and may do so under altered circumstances. (E.G. Wholphin (right), improbable offspring of captive bottlenose dolphin and false killer whale.)
  • is impossible to test for fossils (including fossils which we would otherwise include within modern species)
  • ignores that fact that reproductive isolating mechanisms (distinctive calls and displays, mechanical or karyological barriers to successful mating) are evolved responses to the challenge of reduced fitness in hybrids, and may not be present in distantly related species for whom hybridization is unlikely to occur anyway.
  • By the BSC, Panthera is a species with highly variable morphotypes.
  • By the PSC, Panthera is a higher-order group of closely related species.

Paleontological applications: There have been many minor variations on these themes. For paleontologists, however, the Phylogenetic Species Concept is most frequently employed, simply because we just can't test reproductive isolation in fossil taxa. We can, however, test hypotheses of phylogeny - the branching pattern of evolution, in such a way as to determine whether individuals might have belonged to single lineages.


Ceratopsid display structures from Wikipedia
  • Some degree of similarity is necessary in fossils before the thought enters our heads that they might belong to the same species. E.G.: Dinornis robustus a particularly large extinct Moa, and Dinornis struthioides a small and slender one were considered distinct species (albeit sympatric) until genetic analysis revealed that all D. robustus were female and all D. struthioides were male. Oops.
  • One case where paleontologists might approximate a test of reproductive isolation focuses on the display structures that the critters, themselves, used to know with whom they should be mating. The Specific Mate Recognition Concept bases species identification on these features. Not applicable in many cases, but foolish to ignore when it's there. Examples: Coke's hartebeast and sympatric topi. Cranial display structures in ceratopsid dinosaurs. (Right - Color is hypothetical.)

Speciation: The Origin of Species

  • Just as species boundaries are fuzzy in space, they are fuzzy in time. When does a species begin and end? In some cases, we can definitely say:
    • Speciation events in which one evolving lineage splits into two mark the beginning of the two daughter species.
    • Extinction events mark the end of a species.

    • Suggests sympatric speciation
    • "Extinctions" of any member successional "species" would be pseudoextinction : an arbitrary division of the anagenetic series "originations" would be similarly arbitrary

    • Consistent with allopatric speciation as the result of the appearance of a geographic barrier to gene flow or the peripheral isolation of a population. (E.G.: The Hawaiian goose is a peripheral isolate of the Canada goose.)
    • Gives discrete origination point (the cladogenetic event)

    So now it's your turn: Speculate on how biostratigraphic patterns would differ in worlds in which anagenesis or cladogenesis predominated.


    2. Species and species concepts

    Above image: A very small sampling of some of the millions of different species from Earth's past and present.

    Life's hierarchies

    One of the most fundamental features of the natural world is that there are hierarchies of relationship that connect all living things.

    To the best of our knowledge, all life on Earth today shares a single common ancestor that lived billions of years ago. Every organism that we know of--whether alive today or that died hundreds of millions of years ago--descended from this ancestor. Further, there is a line of relationship--of parents and their offspring--that connect you to this ancient shared common ancestor. There are also, however, many side branches that are not part of your direct ancestry.

    Think about your own family tree: you descended directly from your grandparents via your mother, but your grandparents also may have had other children besides your mother (that is, your aunts or uncles), who in turn had children of their own (your cousins).

    Hierarchy of familial relationships. Numbers in red boxes indicate percentages of genetic relationship relative to yourself. Image source: "Gringer" (Wikimedia Commons public domain).

    The same is true when considering evolutionary patterns at larger scales. Evolution has produced a bush of branches, none leading to one type of organism in particular (just as your own family tree does not lead to any particular "most important" person (contrary to what your cousins might try to tell you over Thanksgiving dinner!). Because of this, evolution must be thought of as hierarchical, just as a family tree is hierarchical. We call evolution's family tree the " tree of life ."

    Source: "Tree of Life video HD, narrated by David Attenborough (Youtube).

    Another fundamental aspect of evolution and the history of life is that biological diversity exists in discontinuous packages. Evolutionary history has produced numerous branches, each composed of groups of organisms that are more similar to each other than they are to groups on other branches. Ants look like ants, hawks look like hawks, and frogs look like frogs. If you carefully study their bodies, however, you will find that frogs and hawks share much more in common with each other than they do with ants (most obviously, they both have bones, which ants lack). Within any of these broad groups, it is possible to define subgroups more-and-more narrowly until one is presented with very similar clusters of individual organisms. We call these clusters species , and more than 1.2 million of them have been described by scientists thus far. Systematists (scientists who study biological diversity, or biodiversity , and the relationships among organisms) estimate, however, that there may be between 10 and 30 million species alive on Earth today. This estimate does not include the number of species known only from the fossil record.

    A very small sampling of some of the millions of different species from Earth's past and present. Image by Jonathan R. Hendricks.

    The Systematics chapter of the Digital Encyclopedia of Ancient Life covers the practice of assigning scientific names to species and higher level groupings. It also introduces phylogenetic trees , which are branching diagrams that depict the degrees of relationship among species or higher taxonomic groups (that is, they represent certain portions of the tree of life).

    An example of a phylogeny, in this case showing the relationships among most major groups of life. The small "You are here" indicator in the upper left corner of the phylogeny indicates the position of Homo sapiens (you and me). Source: David Hillis lab.

    For now, though, let's simply concern ourselves with the nature of species as biological entities. Their existence is clearly a phenomenon that evolution needs to explain. Their characterization and definition, however, has long proven vexing, even if each of us has our own "gut feeling" for what they represent.

    What are species?

    For a long time the notion of what species are and what they represent has often been treated as one of the great philosophical mysteries, along with whether gravitational waves exist.

    Source: "Gravitational waves hit the Late Show" on The Late Show with Stephen Colbert (Youtube).

    whether life exists elsewhere in the universe.

    Source: "Drake equation visualized by Carl Sagan" on the program Cosmos (Youtube).

    and how might history have turned out differently if Rose had made a little room for Jack on that floating door at the end of Titanic?

    Source: "A "Titanic" Myth: Would jack Have Survived if Rose Had Shared the Door?" by Mythbusters on Science Channel (Youtube).

    Any discussion of species presents two importation questions:

    1. What "are" species as evolutionary entities? (Or, what are species in nature?) and
    2. How should we recognize and define species? (Or, what are species in practice?).

    Nearly all of the over one million described living species of plants, animals, and other lifeforms have been named on the basis of their morphological appearances (or, how they look). But, species are groups of organisms that not only resemble each other closely in terms of their anatomy. They also seem to comprise individuals of both sexes that will at times reproduce with one another. We can very easily go out into the natural world and identify these minimal clusters of individuals consisting of at least one male and one female, defining them by one or more distinct morphological features that they--and only they--possess.

    Consider the American robin, or Turdus migratorius (a rather crude-sounding name, if you think about it). This common bird is a harbinger of spring for many Americans who live in cold, northern climates.

    A short video about the American robin. Source: "The American Robin" by LesleytheBirdNerd.

    The American robin occurs (at least during certain times of the year) from Mexico all the way to the Northwest Territories of Canada.

    Range map of the American robin. Yellow indicates the robin's breeding range green, its year-round range and blue, its wintering range. Source: Image by Ken Thomas (public domain).

    Throughout their range, individual American robins always posses the same basic appearances and types of behaviors. We suppose, based on the scientific name of their genus (i.e., Turdus) that one of those behavioral proclivities includes pooping on freshly washed cars. (Just kidding! Turdus is actually the Latin name for a thrush, the broader grouping of birds to which robins belong). Another notable behavior of robins is their odd, though distinctive song.

    Video demonstrating the call of the American Robin. Source: "Robin Call" by Corey Schmaltz, TheBackyardBirder.

    Within this very large cluster of individual birds we’d have a hard time using morphological features to recognize narrower groupings. American robins all more-or-less look alike.

    We call such similar groupings of individual organisms a species. The existence of things like species of robins, or trilobites, or mushrooms, or flowers is one of the manifest patterns in nature and it also is therefore probably telling us something significant about the way the evolutionary process works.

    A trilobite (Paradoxides), mushroom (fly agaric, Amanita muscaria), and flower (California poppy, Eschscholzia californica). Mushroom image source: Michael Maggs (Wikimedia Commons Creative Commons Attribution-Share Alike 2.5 Generic license). Other images: Jonathan R. Hendricks.

    How are species defined?

    Biologists have developed numerous technical definitions of species. Most students learn only the " biological species concept " in their introductory life science courses. This concept, championed by ornithologist Ernst Mayr, argued that species "are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." Under this definition, a mule (the infertile offspring of a male donkey and a female horse) is not a species because it cannot reproduce with other mules.

    A pack mule, seemingly oblivious to the fact that it is not a species. Source: image by Dario Urruty (public domain via Wikimedia Commons).

    The biological species concept, however, presents some practical challenges. For example, to "prove" that two individuals (a male and female) belong to the same species, one would need to confirm that they mate in nature, then go on to produce offspring that are also fertile. This would be next to impossible to study in many habitats (e.g., for species that live on the bottom of the ocean).

    The " morphological species concept " is somewhat more practical. It defines species as groups of individuals that are morphologically similar to one another and are morphologically distinct from other such groups. Another way of saying this is that members of the same species look alike members of different species look different. A related variant of the morphological species concept is the " phylogenetic species concept ," which defines species as "the smallest aggregation of (sexual) populations or (asexual) lineages diagnosable by a unique combination of character states" (Wheeler and Platnick, 2000, p. 58).

    It turns out that the morphological and phylogenetic species concepts are not ideal, however, because they don't really define what species represent in an evolutionary context. Indeed, species are not just a collection of carefully defined morphological characters their features are just the information that we use to identify them. This is similar to how bar codes identify foods at the grocery store checkout line, but do not represent any meaningful qualities about the foods themselves (e.g., appearance, taste, odor, etc.).

    There are many, many other species concepts (as suggested by the image below). We could go into great detail about these, but we won’t do that because many of them aren’t particularly relevant, or even necessarily accurate.

    A handful of the species concepts (and reflections on species) that adorn the ceiling of the paleontology graduate student office in Snee Hall at Cornell University. Images provided by Caren Shin.

    Instead, we are going to focus on the one species concept that we think really matters: the " evolutionary species concept ." While originally conceived by paleontologist George Gaylord Simpson, our modern understanding of this species concept derives mostly from retired University of Kansas biology professor Ed Wiley.

    University of Kansas Curator Emeritus of Ichthyology, Edward Wiley.

    Before getting into the particulars of the Evolutionary Species Concept, let's consider some generalities. First, you have already learned about the Biological Species Concept, which stresses the ability of organisms to reproduce (or not) as being central to their reality in nature. We do think that this is true (at least for many sexually reproducing organisms). The ability of some similar organisms to reproduce with each other, but not members of other groups, gives them cohesion and distinctiveness. Consider that if there are two different “looking” groups that are continually interbreeding with each other, then eventually they will become a single, blended group that combines features of both groups: they won’t look different any more. We would only expect the two groups to stay distinct and persist over geological time scales if they are not reproducing with each other. This element of geological time is a key component of the Evolutionary Species Concept.

    Wiley and colleague Richard Mayden characterized the Evolutionary Species Concept thusly in 2000: "An evolutionary species is an entity composed of organisms that maintains its identity from other such entities through time and over space and that has its own independent evolutionary fate and historical tendencies" (p. 73).

    While this is a very technical definition, note each important attribute:

    • "an entity composed of organisms that maintains its identity from other such entities"—members of the same species interbreed with one another, thus maintaining their genetic distinctiveness relative to other species however, organisms belonging to separate species do not interbreed with each other (this is similar to the Biological Species Concept).
    • "through time and over space"—species have their own unique temporal (in some cases spanning millions of years) and geographic histories (note that both time and geography are missing from the Biological Species Concept).
    • "independent evolutionary fate and historical tendencies"—the histories of closely related species are independent and the fate (e.g., extinction) of one does not have anything to do with the fate of the other(s).

    Species as hypotheses

    The Evolutionary Species Concept captures what we think species "are" in nature. Like the Biological Species Concept, the Evolutionary Species Concept is not practical for actually going outside and identifying species (whether extant or from the fossil record). Specialists on different types of organisms use very different tools, approaches, and criteria for recognizing species. For example, genetic data are increasingly being used to differentiate some modern species and are required for recognizing different species of morphologically simplistic organisms such as bacteria. Nearly all named species of plants and animals (both extant and long extinct), however, were named on the basis of their morphology alone. For instance, most species of clams and snails were described only from their shells (essentially just one organ system).

    The fundamental problem for recognizing species is the issue of differentiating intraspecific variation (=within species variation) from interspecific variation (=variation between species). (Note: remember "intramural sports" when you think of the prefix "intra" - they are competitions between groups at a single college or university remember "intercollegiate sports" when you think of the prefex "inter" - they are competitions between different colleges or universities.) The intra- and interspecific variation that one is most often faced with is morphological&mdashand this is always the case, of course, when considering species in the fossil record&mdashbut may also be genetic, or even behavioral.

    Beetle specimens on display at the Melbourne Museum. How many species do you think are represented? Do any of the specimens belong to the same species? Photograph by Jonathan R. Hendricks.

    How much variation is enough to differentiate two similar species? If two groups overlap in a particular range of measurements, should they be considered different species? What about if they vary genetically by a tenth of a percent? How much variation is enough variation to recognize two (or more) species? Unfortunately, there are not universal answers to these questions. In fact, these issues sometimes divide taxonomists into two separate camps, lumpers and splitters . Lumpers are taxonomists who assume that species show lots of natural variation. Splitters define species narrowly and do not accept much variability when they circumscribe species. Which view is correct?

    At the end of the day, species—as a practical matter—should be thought of as hypotheses to be tested in the face of evidence. Regardless of what species "are", their recognition in nature is a business subject to the biases, preconceptions, and data available to the biologists and paleontologists who study them. Special care must be taken to describe new species (the taxonomic rules for how species are named are described in the Systematics chapter). Retired University of Washington biologist Alan Kohn perhaps put it best: "absolutely no honor is associated with describing a new species. There is only responsibility — a heavy responsibility, for the author of a new species stakes his reputation on his defense of the new name as denoting a real, previously undescribed species, representing breeding populations of many individuals, outside the range of variation of all previously described species" (Kohn, 1980, p. 7 bold emphasis in original).

    The extinct cone snail Conus adversarius. The species is unique among cone snails in having a shell that opens to the left (sinistral coiling) all others have shells that open to the right (dextral coiling). Note the subtle morphological differences between the individual specimens. Source: Jonathan R. Hendricks and the Neogene Atlas of Ancient Life (Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License).

    Even once a new species is formally named and described, it remains a hypothesis. New specimens might be discovered, for instance, that show features that were not preserved in the originally described specimens. Such additional information could potentially reveal the presence of two species, rather than just one (thereby rejecting the hypothesis of a single species). Alternatively, the discovery of additional specimens might instead reinforce the original description, adding strength to the hypothesis of there being just one species.

    Just as all scientific hypotheses are best supported by multiple lines of evidence, hypotheses of species boundaries are similarly strengthened by the presence of multiple lines of evidence. This is why it is generally poor practice to describe new species on the basis of single specimens, especially when they are fragmentary. Such specimens cannot demonstrate natural variation and may often represent extreme morphologies of previously described species.

    One early and paleontologically sigificant example of using multiple lines of evidence to define species comes from research on a group of marine invertebrate animals called bryozoans (or, "moss animals"). Bryozoans are colonial animals that live in modern marine and freshwater environments. They also have a diverse fossil record.

    Interactive 3D model of a fossil bryozoan (Polypora elliptica) from the Pennsylvanian Thrifty Formation of Texas (PRI 76722). This s pecimen is from the collections of the Paleontological Research Institution , Ithaca, New York.

    Paleontologists Jeremy Jackson and Alan Cheetham were interested in learning whether ancient bryozoan species that were defined only on the basis of their morphology ("morphospecies") were equivalent to modern bryozoan species. To resolve this problem, they studied the morphology and genetics of modern species and found that these "morphospecies are genetically distinct" and that their morphological boundaries are heritable. This was good news, as it meant that species known only from the fossil record were directly comparable as units of analysis to modern species that are genetically unique.

    The study of evolution and the description of life's biodiversity both benefit from such multi-pronged approaches, which have been given the name " integrative taxonomy ." Much research is now underway to describe (or re-describe) biodiversity using as many lines of evidence as possible. This is a great example of scientific hypothesis testing at work in systematic biology.

    References and further reading

    Allmon, W. D., and M. M. Yacobucci (eds.). 2016. Species and speciation in the fossil record. University of Chicago Press, Chicago, 384 pp.

    Brooks, D. R., and D. A. McLennan. 2002. The nature of diversity: an evolutionary voyage of discovery. University of Chicago Press, Chicago, 676 pp.

    Eldredge, N. 1985. Unfinished Synthesis. Oxford University Press, New York, 248 pp.

    Futuyma, D. J., and M. Kirkpatrick. 2013. Evolution, 4 th edition. Oxford University Press, New York, 594 pp.

    Hendricks, J. R., E. E. Saupe, C. E. Myers, E. J. Hermsen, and W. D. Allmon. 2014. The generification of the fossil record. Paleobiology 40: 511-529.

    Jackson, J.B.C., and A. H. Cheetham. 1990. Evolutionary significance of morphospecies: a test with cheilostome Bryozoa. Science 248: 579-583.

    Mayr, E. 1982. The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Harvard University Press, Cambridge, MA, 996 p.

    Padial, J. M., A. Miralles, I. de la Riva, and M. Vences. 2010. The integrative future of taxonomy. Frontiers in Zoology 7:16.

    Wheeler, Q. D., and R. Meier (eds.). 2000. Species Concepts and Phylogenetic Theory: A Debate. Columbia University Press, NY, 230 p.

    Wheeler, Q. D., and N. I. Platnick. 2000. The phylogenetic species concept (sensu Wheeler and Platnick). Pp. 55-69 in Wheeler, Q. D., and R. Meier (eds.), Species Concepts and Phylogenetic Theory: A Debate. Columbia University Press, NY, 230 p.

    Wiley, E. O., and B. S. Lieberman . 2011. Phylogenetics, 2 nd edition. J. Wiley & Sons, New York, 432 p.

    Wiley, E. O., and R. L. Mayden. 2000. The evolutionary species concept. Pp. 70-89 in Wheeler, Q. D., and R. Meier (eds.), Species Concepts and Phylogenetic Theory: A Debate. Columbia University Press, NY, 230 p.

    Content usage

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    How can paleontologists identify morphologically different fossils as members of the same species? - Biology

    Biological species concept

    The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.

    Appearance isn't everything
    Organisms may appear to be alike and be different species. For example, Western meadowlarks (Sturnella neglecta) and Eastern meadowlarks (Sturnella magna) look almost identical to one another, yet do not interbreed with each other — thus, they are separate species according to this definition.

    The Western meadowlark (left) and the Eastern meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding.

    Adding to the problem
    We already pointed out two of the difficulties with the biological species concept: what do you do with asexual organisms, and what do you do with organisms that occasionally form hybrids with one another? Other difficulties include:

      What is meant by "potentially interbreeding?" If a population of frogs were divided by a freeway, as shown below, that prevented the two groups of frogs from interbreeding, should we designate them as separate species? Probably not — but how distantly separated do they have to be before we draw the line?

    are species with a geographic distribution that forms a ring and overlaps at the ends. The many subspecies of Ensatina salamanders in California exhibit subtle morphological and genetic differences all along their range. They all interbreed with their immediate neighbors with one exception: where the extreme ends of the range overlap in Southern California, E. klauberi and E. eschscholtzii do not interbreed. So where do we mark the point of speciation?

    This trilobite lineage below evolved gradually over time:

    Should we consider trilobite A as a separate species from trilobite D, and if so, where should we divide the lineage into separate species?


    How can paleontologists identify morphologically different fossils as members of the same species? - Biology

    "Of what use are the great number of petrifactions, of different species, shape and form which are dug up by naturalists? Perhaps the collection of such specimens is sheer vanity and inquisitiveness. I do not presume to say but we find in our mountains the rarest animals, shells, mussels, and corals embalmed in stone, as it were, living specimens of which are now being sought in vain throughout Europe. These stones alone whisper in the midst of general silence." -- Aphorism 132, Philosophia Botanica (1751), Carolus Linnaeus

    "I am fully convinced that species are not immutable but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species." -- Introduction, On the Origin of Species by Means of Natural Selection (1859), Charles Darwin

    "The usual concept of species can be stated as follows (Mayr 1970): "Species are groups of interbreeding natural populations that are reproductively isolated from other such groups." This concept is grandly called "the biological species concept." But that is an arbitrary appropriation of a term with a more general and earlier meaning. I will instead use the term "reproductive species concept."" -- "Ecological species, multispecies, and oaks" (1991), Leigh Van Valen

    BIG QUESTION: How do we identify fossils? What are species?

    Parts is Parts: Homology, Analogy & Comparative Anatomy
    In order to recognize how organisms are similar, or different, we need to compare its body parts. The important thing is to recognize the equivalent body parts: no sense in comparing a leg with a tail, or a jaw with a stomach. It had been noted by early anatomists that related organisms were built on the same "body design" (in German, Bauplan or "building plan"). In each of these, the underlying structure was repeated from organism to organism: these parts are considered to be homologous.

    Homologous structures are the same body part, but might be shaped or modified differently. The wing of a bat, the front leg of a horse, the flipper of a whale, and the arm of a human are all homologous, and have the same basic parts: a single upper arm bone, a pair of bones below some rounded wrist bones some long bones in the palm of the hand and series of long bones down each finger. But even though they are homologous, they have different functions.

    In contrast, structures that have the same function but are derived from different body parts are analogous. The wings of bats are modified arms, but the wings of insects are modified gill flaps.

    The science of comparative anatomy was developed to describe, compare, and contrast the homologous structures of different kinds of organisms. Given a language of comparative anatomy, we can show how two specimens are similar and how they are different. And we can characterize different types of organisms based on their distinctive combination of features.

    Taxonomy: the Naming of Names
    Taxon (pl. taxa ): a named group of organisms.

    Naturalists have long noted that there exist units of natural diversity, species , in which the members share certain distinctive features with each other. Traditionally, each culture had its own name for the animals, plants, and other organisms in their region. But EACH culture had its own set of names, so the same type of animal might have many different names. During the 1600s and 1700s, methods were proposed for a formal scientific set of names. Following the work of Carl von Linne' (Linnaeus) in the 1700s (most specifically, the rules he set down in the Systema Naturae ("System of Nature") in 1758 later workers added and modified the system (primarily with the addition of new "ranks")), species were recognized as one unit within a nested hierarchy of larger clusters of organisms: taxa (singular, taxon literally, "named thing").

    • All names are in Latin or Greek , or are modified into Latin form
    • Each name must be unique
    • All names are fit into a nested hierarchy (species into genera, genera into families, and so forth)
    • In traditional Linnaean taxonomy, there is a set of official ranks (from smallest to largest, species, genus, family, order, class, phylum) (later workers added additional intermediate ranks, such as tribes, subfamilies, superfamilies, subphyla, etc.)
      • The primary unit is the species (pl. species )
        • Refers to a "specific" kind of organism
        • Refers to a more "generic" category than species
        • Definition of a "genus" is even more problematic than that for species, since it is composed of one or more "species"
        • Each genus has a type species : all other species are assigned to the genus based on their similarity to the type species
        • Genera have one word names (e.g., Panthera , Homo , Ginkgo , Tyrannosaurus )
          • The genus name is always Capitalized and italicized (or underlined if you don't have access to italics)
          • The genus name ("generic nomen") is ALWAYS capitalized, the second part ("trivial nomen") is ALWAYS in lower case, and the name is ALWAYS italicized or underlined
          • Species names can be abbreviated by using only the first letter of the genus name, followed by a period ( NEVER by a hyphen): H. sapiens and T. rex are correct H. Sapiens or T-Rex are WRONG!! (Subtle hint: do not use the incorrect form on your homework or tests)
          • Traditionally, higher order taxa all had specific ranks: Kingdom, Phylum, Class, Order, Family. And as more diversity to life was found, these got subdivided and subdivided or grouped and grouped again, so we got Subclasses and Infraorders and Superfamilies and Tribes (and Subtribes!). But in the later 20th Century it was recognized that these formal ranks were actually meaningless!! An "order" doesn't mean anything in particular, and two "classes" aren't actually equivalent in any special way. So (as we'll see next week) we've abandoned these higher order ranks, but we STILL use the names of the taxa. Those DO represent real things: groups of living things.

          Type Specimens and Type Species: Another aspect of Linnaean taxonomy is that each species must have a particular type specimen. This is a particular individual preserved specimen (extant animal) or fossil (extinct animal) that is the "name holder" for that species. A type specimen is specifically referred to in the original description and diagnosis of the species. It need not be the most complete specimen known at the time (although that helps, as the more complete it is, the better the chance a less-complete individual can be compared to it!). The type specimen plus all the additional (referred specimens) are collectively called the hypodigm. Ultimately, if a species is regarded as being "valid" (that is, representing a real species in Nature), the type specimen is the only individual that is absolutely certain to belong that that species.

          Similarly, each genus has a particular type species. This is the particular species to which the genus name is linked. If a genus is valid, the type species is the only species that is absolutely guaranteed to be within that genus.

          As an example, CM 9380 (in the collections of the Carnegie Museum of Natural History) is the type specimen of Tyrannosaurus rex, and Tyrannosaurus rex is the type species of the genus Tyrannosaurus.

          Parataxonomies: There is a formal set of names for some types of fossils that is parallel to, but independent of, the biological nomenclature of actual species and genera and the like. These are parataxonomies. For instance, there is an "ootaxonomy" of "oospecies" and "oogenera" and "oofamilies" of fossil eggs, and a whole complex of ichnospecies for trace fossils. These are even given italicized Latinate names and use rules of priority and the like. But these are names of the eggs, burrows, footprints, etc., and NOT of the organisms that produce them.

          • Taxonomists who consider a particular set of specimens to represent many taxa are called splitters those who consider a particular set to represent few taxa are called lumpers
          • If a taxonomist feels that some specimens of a genus belong to an as-yet unnamed species, they can split these specimens off as a new species (which a new type specimen)
          • On the other hand, if a taxonomist considers that two previously named species are not distinct enough from each other to truly be distinct species (that is, the taxonomist regards the two names as synonyms ), they may lump them together:
            • In these cases, the Rule of Priority is used: whichever of the names was published first, even if only by days, is the name that must be used the older of the two names is the senior synonym and the younger one(s) the junior synonym(s):
            • Splitting: the American Devonian species of trilobite Phacops rana (named by Green in 1832) was eventually regarded as being sufficiently different from the type species of Phacops (P. latifrons, named by Bronn in 1825) to belong to a new genus. No other genus name was already available for rana, so Struve proposed "Eldredgeops" in 1990. We now regard that American species as Eldredgeops rana (Green 1832) (the coiner's name is in parentheses to mark that this was not the original genus the species was in.)
            • Lumping: the Javan "apeman" was initially given the name Pithecanthropus erectus Dubois 1892. Subsequent work showed that it was not sufficiently different from our own species and related ones (like Neanderthals) to warrant its own genus name. Since we regard both the extinct Java species and our own as belonging to the same genus, and since Homo Linnaeus 1758 has priority over Pithecanthropus Dubois 1892, we now call the Javan form Homo erectus (Dubois 1892)

            Sometimes, by accident, two taxa wind up with the same name. These are said to be homonyms. In this case, the senior (earlier proposed) of the two names occupies the name (i.e., it gets to keep it!). The junior homonym needs a new name: maybe there is another name already proposed that could be used, but if not it needs a new name. For instance, a dinosaur was given the name Syntarsus in 1969 unfortunately, a modern beetle was given that name back in 1869! So the beetle occupies Syntarsus, and the dinosaur wound up being renamed Megapnosaurus in 2001.

            For those interested in a website concerning some unusual Linnaean species names, click here.

            But, What ARE Species?
            What is a species? Above we see the rules for these names, but it doesn't tell us about what it is being named.

            Darwin did not regard species as a distinct "kind" of biological entity. Instead, he considered them as essentially the same thing as geographic or stratigraphic variations (see these below), but ones in which extinction has removed the intermediate forms that otherwise would blend into the closest living relative group.

            20th Century biologist Ernst Mayr (and most contemporary biologists) formalized their definition of a species as a "an array of populations which are actually or potentially interbreeding, and which are reproductively isolated from other such arrays under natural conditions". (Almost certainly you learned some version of this in high school and BSCI classes.) It works pretty well for the first pass: it emphasizes isolation, and thus species would represent pools of shared genes which do not get mixed with their closest relatives. Mayr and his followers refer to this as the "biological species concept" (or "BSC"), but as paleontologists Leigh Van Valen (see quote above) pointed out, this is an over-reach on the part of its proponents, and it is better termed a "reproductive species concept".

            But there are some problems with this. For one: hybrids (crosses between two separate species) do occur naturally, and many of these are actually fertile! And for paleontologists: we can't test interfertility between populations because they are dead!

            • Impossible to define for asexual species, unless each clone lineage is a separate species, despite being otherwise no different from each other than are two individuals are within sexually reproducing species
            • Extremely difficult to test in most wild populations, and (worst for paleontology) IMPOSSIBLE to test for fossils!
            • Existence of rare but real natural hybrids show that such isolation is not complete by any means
            • Existence of ring species shows that you can go through a continuum with no boundaries and yet reach conditions of genetic incompatibility between end members.
            • Species are recognized by some shared aspect or attribute held in common by members of the species (similarity criterion)
            • Species are distinguished from other species by some attribute that makes them different from other (difference criterion)

            Although related, they really aren't talking about the same things necessarily.

            There are other species definitions and concepts that people have tried to apply, but none have been able to universally encapsulate the diversity out there.

            • Species originating within another species: This is a particular problem for the phylogenetic species concept. We recognize that some species (indeed, likely many!) originate from subpopulations of previously existing species the REMAINING subpopulations are free to persist under the original morphology, range, habitat, gene exchange, etc. A notable example includes a number of extinct Hawaiian geese and the surviving Branta sandvicensis (nene), which represent a cluster of new species arising from isolated populations of Canada geese (Branta canadensis), the rest of which's populations remained in the original morphology and habitat. Similarly, polar bears (Ursus maritimus) arose as isolates within the cluster of populations of brown bear (Ursus arctos).
            • Introgression: This is a problem for the BSC. The rise of genomic analysis has revealed that not only is hybridization in the wild not a rare phenomenon, it does not (as some BSC advocates hoped and claimed) only produce rare individuals of reduced fitness, and thus do not contribute to the adaptive success of variants within the population. Instead, it has been discovered that introgression (the repeated backcross of hybrids with their parent species, resulting in significant gene transfer from one species to another) is actually disturbingly common. Indeed, it is appearing time and time again in many, many groups of animals (it had long been recognized to be common in plants.) In just the last several years it has been documented among many types of mammals, including various species of bears, of big cats, of mysticetes (baleen whales), of proboscideans (including the survival of genes from extinct elephantids in the surviving species), and in the genus Homo. Note that in all of these but whales the studies have included recent (Pleistocene) fossil DNA, showing this phenomenon is on-going. And in at least the human case, the Neanderthal genes introduced into some modern human populations code for traits with positive adaptive significance: that is to say, individuals that are descendants of these hybrids have the potential for increased selective success compared to other individuals.
            • Species originating by hybridization: This introgression pattern achieves greater expression in entire species which arise by hybridization between closely related taxa. Note: this is a distinct phenomenon from the hybridogens mentioned last time: those are species where every individual has to be produced by a hybrid breeding event. These cases were initially established by hybridization, but later contained populations of interbreeding individuals with distinct morphologies and genomes from either ancestral lineage. These cases remain controversial, but two examples that seem to be reasonably well-supported are: the red wolf (Canis rufus) of Southeastern North America, with ancestry in the grey wolf (C. lupus) and either (or BOTH) the eastern wolf (C. lycaeon) and the coyote (C. latrans) and the Jefferson mammoth (Mammuthus jeffersonii), a morphologically-distinct form genetically nested among the Columbian mammoth (M. columbi) but with a substantial amount of woolly mammoth (M. primigenius) genes.
            • There really are clusters of biodiversity with shared history, interbreeding, ecology, etc.
            • In general (barring ring species, etc.), these variation within each cluster is less than the the distance to the next such cluster
            • At lower levels of analysis, reticulation (interbreeding) dominates inheritance over divergence (branching)
            • As you move to higher level branching tends to dominate, but (as we now know) reticulations do not cut off at some arbitrary point. (Even though they are essentially absent [so far as we know] above the old-style family level, reticulations are present by rare at very high levels in the form of various sorts of endosymbiosis, as in the origins of lichens, eukaryotes, plants, etc.)
            • So the word of advice is that at present no species concept/criterion is perfect, so hold off on accepting them as Holy Writ!
              • Keep in mind, the goal of Science is to describe Nature. Modify rules to fit Nature, not Nature to fit rules.

              And specifically with regards to fossils: in the end--with the rare exception of fossil genomes--all fossil species are morphospecies, since we really can't see other aspects of them. And since there is the time factor that neontologists don't have to deal with, we see stratigraphic variation which segue into chronospecies. (We'll talk more about this issue and rates of change, later.)

              In real life, species do seem to have "fuzzy boundaries", and the distinction between different closely related species on the one hand and clusters of variation within a species are nearly impossible to tell. In fact, biologists go through shifts of fashion towards increasing splitting (the former idea) and lumping (the latter) over time. Currently the fashion is towards splitting: consequently, whereas in much of the 20th Century we recognized only one species each of African elephant, gorilla, orangutan, Nile crocodile, and orca, early 21st Century taxonomists recognize two or more. (On the flip side, dinosaur paleontologists seem to be following the opposite trend, lumping once-separate species and genera into each other).

              As with many things, we run into problem with typological thinking: the idea that there are ideal types of things, and that we judge a specimens membership in a group by how well it conforms from that type. Instead, we find that variation is the reality. So we need to use population-based thinking. (Next lecture we will add tree-based thinking.)

              Ultimately, for paleontologists we are stuck looking only at shapes (and in fact, only the shapes of those hard parts that survive fossilization).

              • Sexual dimorphism : different sexes are different sizes and shapes and have different structures
              • Ontogenetic (growth): babies look different from juveniles look different from subadults look different from adults (can be even more extreme in animals that undergo metamorphosis, like amphibians and many insects)
              • Geographic : populations in different regions might have slightly different sizes, color patterns, proportions, behaviors, etc. For example, some biologists consider the populations of orangutans, tigers, African elephants, etc. as distinct species others simply regard them as regional variants
              • Stratigraphic : lineages (ancestor and descendant populations) may shift in some traits or characteristics over time
              • Individual : one of the great "discoveries" of Darwin and Wallace (we'll meet them next lecture), the recognition that no two individuals in a population are identical! (Before them, many people thought that there existed the perfect "type" of each kind of organism, and all variation is degeneration from that perfection. Darwin and Wallace showed that the variation is the reality)
              • Taphonomic: Not an issue that we have to deal with so much with modern species, but a great issue with fossil ones. We never have 100% of the organism preserved as fossils, and are lucky to get 100% of the hard tissues in complex organisms. So the difference that we see between two specimens may be due to differential quality or quantity of preservation rather than actual biological differences.
              • Sexual (aka courtship) displays: attract potential mates
              • Territorial displays: defend territory (which might also include mates)
              • Defensive display: more generally, to warn off potential threats
              • Specific Recognition : features distinctive to each species. Especially common where multiple closely-related forms live in the same environment

              Some of these forms of display result in preservable "showy" features of the anatomy that might help us identify species more easily, or ironically confound us into thinking that two different sexes represent two different species!

              Sexual strategies: male and female animals have different priorities in terms of reproduction. Males can in principle fertilize many many individuals, while females typically have fewer sex cells (eggs) available at any given time. With less cells to use, females often are "choosier" in terms of mates. So many species evolve displays in which males somehow "show off" (in terms of physical features, ritual motions, combat between rivals, etc.) and females evaluate the display.

              For example:

              • Are you looking at fossils of two sexes in the same species, or two different species?
              • Many sexual display features (and sex organs!) are soft tissue, so they would only rarely fossilize
              • If you have only a few individuals, are you looking at true dimorphism, or just end members in a continuous spectrum?
              • One morph (often the male) is rarer
              • One morph (often the male) is showier
              • Distinction between alleged males and females is less pronounced prior to sexual maturity
              • Changes to dimorphic forms is rapid once sexual maturity is reached
              • Frequency of the two morphs remains relatively constant in strata where the species is found.

              In very rare cases the eggs or embryos have been found inside a fossil, which rather unambiguously shows them to be female. Otherwise, there can be circumstantial evidence. For instance, if the species has crests, horns, etc., and these are some rarer showier crests, these might more likely be male.

              An alternative to sexual displays for showy structures, however, is specific recognition systems (SRS). In this cases, different species have unique characteristics within their ecosystem to recognize other members of the species from all other species they encounter. For cases of olfactory and aural SRS we are lost with regards to fossils. But we have potential with visual SRS.

              • Likely to be most pronounced when related species are sympatric (since they live together, there is greater need to distinguish A from B)
              • Differences should be in obvious traits, not subtle internal ones
              • Need NOT be sexually dimorphic!

              But it need not be so profound a change to be an issue. For some fossil species there remain debates over whether smaller individuals of a particular group from a formation are the juveniles of the larger species, or are smaller species that lived sympatrically (in the same time and place). Without a large enough sample size and sufficient numbers of intermediate stages, this might be very difficult to resolve. To Lecture Schedule


              How can paleontologists identify morphologically different fossils as members of the same species? - Biology

              Lumping or splitting in the fossil record
              November 2013


              The 1.8-million-year-old skull was found in 2005 in the Republic of Georgia

              Rarely does pure science take top billing in the news, but this past month saw a notable exception. The front page of the New York Times was occupied by the image of an ancient hominid skull caked in dirt. This 1.8 million-year-old fossil, excavated in the Republic of Georgia, represents the oldest complete adult cranium of a hominid yet discovered. That alone would be significant news, but the context in which the fossil was preserved adds even more weight to the discovery.

              The new cranium was found in the same location as four other skulls that were deposited around the same time — each in the burrow of an ancient carnivore. Clearly, our ancestors were not at the top of the food chain! But more significant than their grisly deaths is the shapes of their skulls. They are not particularly similar to one another. Each clearly belongs to the Homo lineage, but if they had been found in distinct locations or were from different time periods, they likely would have been classified as different species. Yet, we know that these human relatives were all living in the same place at the same time.

              The international team of researchers behind the work contends that all five skulls (and by extension many similar fossils discovered previously in Africa) belong to the same species. They suggest that the physical differences between these fossils simply reflects the normal variation among individuals of the same species — just as you look different from every other human on Earth and individual chimpanzees all look different from one another. In support of their hypothesis, the researchers measured the physical differences between the fossil Homo skulls and found them to be no greater than the range of variation within modern human or chimpanzee populations. Perhaps then, many of our ancient human relatives were erroneously assumed to be of different species based on unimportant differences in shape and because they were found in different localities. The researchers argue that the distinction between fossils assigned to Homo erectus, H. rudolfensis, and H. habilis should be re-examined. They may represent not distinct branches on the tree of life, but a single limb.

              Some publications have described this discovery as upsetting everything we know about human evolution, but that is an exaggeration. Our main understandings of human evolution (e.g., that our ancestors parted evolutionary ways with chimpanzees' ancestors about six million years ago and evolved into modern humans in Africa) remain unchanged. Furthermore, argument and reinterpretation are normal, healthy parts of the process of science. In fact, this particular debate — whether the Georgian skull and Homo fossils of similar ages represent a single species or multiple species — is a common one throughout biology. It is often described as "lumping" versus "splitting."

              The argument about whether groups of organisms should be "split" into distinct species is particularly common in paleontology. Generally, closely related groups of organisms constitute distinct species when they are on separate evolutionary tracks, which usually occurs because of lack of interbreeding between the two groups. However, when it comes to fossils, most lines of evidence that would inform us about evolutionary trajectories and reproduction have degraded away over time. Only in rare cases can genetic information be recovered from fossils, and few fossils are preserved with any clues about the behavior of the organism. Species are not defined by their appearances, but when it comes to long-extinct organisms, physical traits (i.e., morphology) may be the only information we have to go on!

              There are many reasons that two members of the same species might look different — sex (e.g., in some species, males are larger), age (e.g., many organisms develop distinct features, like horns, as they grow older), disease (e.g., poor nutrition, genetic diseases, and infectious diseases all have the potential to change the shapes and sizes of bones), and natural genetic variation from one individual to the next (e.g., some individuals may just naturally have longer legs than others). Such differences could be resolved if the organisms were alive, but with only fossils to go off of, paleontologists have to do the best they can with the available evidence.

              As biologists discover new evidence, they reconsider their old ways of classifying fossil forms. This can result in lumping previously distinct species into one. For example, recently three distinct-looking pachycephalosaur dinosaurs (Dracorex hogwartsia, Stygimoloch spinifer, and Pachycephalosaurus wyomingensis) were recognized as the juvenile, adolescent, and adult forms of the same species. A similar lumping has occurred within the triceratops lineage. And paleoanthropologists have long argued about whether 13,000-year-old, three-foot tall hominid bones from Indonesia are those of diseased modern humans or a unique lineage with short stature.


              Three distinct-looking Pachycephalosaur dinosaurs, recognized as the juvenile, adolescent, and adult forms of the same species.

              In this context, the discovery of the Georgian hominid fossils and the debate they inspired should come as no surprise. Exactly how many different hominid lineages make up our branch of the tree of life? With the currently available evidence, we can't yet be sure, but one thing is certain: we haven't seen the last of this debate! As new Homo and other fossils are discovered and analyzed, lumpers and splitters will continue to refine our understanding of the history of life on Earth.

                Horner, J. R., and Goodwin, M. B. (2009). Extreme cranial ontogeny in the Upper Cretaceous dinosaur Pachycephalosaurus. PLoS ONE. 4: e7626.


              How can paleontologists identify morphologically different fossils as members of the same species? - Biology

              DEFINING SPECIES AND SPECIATION

              While Darwin entitled his book "On the Origin of Species", the book dealt primarily with a mechanism of evolution (natural selection) in which variation was critical. But what will selection do with this variation? Change the frequency of dark morphs of moths, or morphs of snow geese, or change the mean and the variance of the distribution of heights in human populations? What is the result of disruptive selection? How do we decide that natural selection has actually lead to the origin of new species? The answer to these questions depends on one's species concept . The concept of species is an important but difficult one in biology, and is sometimes referred to the "species problem". Some major species concepts are:

              Typological (or Essentialist, Morphological, Phenetic) species concept. Typology is based on morphology/phenotype. Stems from the Platonic "forms". Still applied in museum research ( type method ) where a single specimen ( type specimen ) is the basis for defining the species. In paleontology all you have is morphology: typology is practiced and species are defined as morphospecies (e.g., snail shells in fossil beds). Problems: what about sexual dimorphism: males and females might be assigned to different species. Geographic variants: different forms viewed as different species? What about life stages: caterpillars and butterflies? If typology is let run it can lead to oversplitting taxa: each variant is called a new species ( Thomomys ) pocket gophers with > 200 subspecies .

              Evolutionary species concept . "A species is a series of ancestor descendent populations passing through time and space independent of other populations, each of which possesses its own evolutionary tendencies and historical fate" (George Gaylord Simpson). Simpson was a paleontologist and emphasis on stability over time is best appreciated in the fossil record. Inherently morphological, but his claim is that morphologies have genetic bases, so it is indirectly a genetical definition. Problem: gaps in the fossil record impose arbitrary boundaries between species, especially those undergoing gradual size/shape evolution. Compare with Cladistic species concept (pg. 418). How speciation affects existing taxa can alter one's view of species.

              Biological species concept . from population-level thinking of the modern synthesis.

              "Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups" (Ernst Mayr Museum of Comparative Zoology, Harvard).

              "Species are systems of populations the gene exchange between these systems is limited or prevented in nature by a reproductive isolating mechanism or several such mechanisms." (Theodosius Dobzhansky Rockefeller and Columbia Universities).

              Not the first to claim the importance of reproductive continuity : "a set of individuals who give rise through reproduction to new individuals similar to themselves" (John Ray, 1682). "A species is a constant succession of similar individuals that can reproduce together." (George Louis Buffon, 1707-1788). Note the characteristic Mayr: "biological" species concept implies that all other species concepts are non-biological.

              Recognition concept . species are groups of individuals that share a common fertilization system (a "specific mate recognition system", SMRS of Hugh Paterson, South Africa). Emphasis is on those characteristics of species that tend to hold them together something that members of a species share . Biological species concept stresses that which makes a species different from other species cant define species without reference to other species. Contrast isolation vs. recognition . See figure 15.2, pg. 409.

              There are other species concepts (now you know why it this has been called the 'species problem'): Ecological, Pluralistic, etc. One philosophical approach is to ask whether species are "individuals" or "classes".

              There are some conceptual and practical problems with the Biological Species Concept: Are species real or are they arbitrary categories imposed by biologists? Populations: where do they begin and end often arbitrary and grade into other populations Genus, Family, Order, etc. are these human constructs? Is a genus of bees = a genus of birds in terms of levels of organization? What are the typological grounds for the boundaries. What about "species" that can freely mate such as species of orchids that can mate sometimes between genera ( wide cross ). What about asexual species ? They don't reproduce with other species so every individual is a species?? Mayr would hold that species are real units. Views species boundaries as being defined by limits of gene exchange: each species is a group of populations held together by exchange of genes in a genetic system that allows free recombination among the chromosomes of this system. Holds that species are real objective units with definable limits - basic units of evolution . No mistake that the Biological Species concept was advanced by two zoologists who worked with organisms that did not present some of the more obvious problem of plants and bacteria (Nevertheless, there is clear discontinuity in the phenotypes of bacteria).

              Isolating "Mechanisms" (misleading term: is it a mechanism in that it evolved for the purpose of isolating or did isolating "mechanisms" evolve in one context and serve to prevent mating on another?). Premating mechanisms prevent interspecific crosses. Temporal or Ecological isolation (don't meet due to different time of emergence or occur in different habitats). Ethological (behavioral) isolation (meet but don't mate) e.g. fireflies. Mechanical isolation (can't transfer sperm, morphological incompatibilities). See table 15.1, pg. 405.

              Postmating isolating mechanisms inhibit or prevent interspecific crosses

              gametic mortality (sperm transferred but does not fertilize eggs). zygote mortality (egg is fertilized but zygote dies). hybrid inviability (F1 hybrid has reduced viability: incomplete development). hybrid sterility (F1 hybrid viable but sterile) e.g., mule

              Premating isolation prevents wasting of gametes : highly susceptible to improvement by natural selection. Damselflies: character displacement of wing spot density. Rapid speciation events often involve behavioral isolation: Hawaiian Drosophila: hundreds of species in the past several million years. Postmating isolation does not prevent the wasting of gametes and its improvement by natural selection is indirect. Isolating mechanisms may work in concert if one breaks down, another will prevent gene exchange (e.g., bird songs and plumage patterns). This issue of the opportunity for selection to act on pre- vs. postmating isolating mechanisms is important in the discussion of Reinforcement in the next lecture.

              Breakdown of isolating mechanisms will lead to hybridization (crickets in eastern North America hybridize in a hybrid zone along the Appalachian ridge). Are hybridizing "species" really species? If the hybrids backcross to either type, introgression can occur ("the incorporation of genes from one species into the gene pool of another species"). Many examples of hybridization in both plants and animals. Often referred to as semispecies , i.e., not complete species.

              Population structure: are populations the unit of evolution? (Ehrlich and Raven 1969, Science 165:1288-1232) Species are just "phenetic clusters". But why do populations cluster into "species". Checkerspot butterfly studied on Jasper Ridge near Stanford CA by Paul Ehrlich and colleagues (1975 Science 188:221-228). Different populations fluctuate independently: suggests little gene exchange between populations (But Slatkin's analysis of allele frequency data suggest otherwise.

              Geographic variation in reproductive isolation. If a series of populations can mate sequentially, but the end populations cannot, is one species two?? Mayr would say that since they do not meet the issue is not biologically relevant. Do you agree?

              Polymorphism. Mimicry complexes of African swallowtail. Papilio dardanus exists as one morph where no noxious species are found (Madagascar). Where noxious models are present the same species ( Papilio dardanus ) takes on different forms depending on the local model the mimetic forms look like completely different species but are one.

              Sibling species . Morphologically indistinguishable, but are reproductively isolated. Not always easy to test for reproductive isolation and no morphological grounds on which to separate populations.

              Descriptions of the geography of population location/overlap helps focus on how geography might influence gene flow . If gene exchange between two populations is completely stopped, what will happen? Allopatric populations/species exist in different areas (do not overlap or abut) sympatric populations/species occupy the same geographic locality parapatric populations/species have abutting but not overlapping ranges a peripatric distribution refers to peripheral isolates.


              How can paleontologists identify morphologically different fossils as members of the same species? - Biology

              Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their distribution. Abiotic factors, such as temperature and rainfall, vary based on latitude and elevation, primarily. As these abiotic factors change, the composition of plant and animal communities also changes.

              Patterns of Species Distribution

              Ecologists who study biogeography examine patterns of species distribution. No species exists everywhere for example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size. Other species are generalists: species which live in a wide variety of geographic areas the raccoon, for example, is native to most of North and Central America.

              Since species distribution patterns are based on biotic and abiotic factors and their influences during the very long periods of time required for species evolution, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century. Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers. Biologists estimate that Australia, for example, has between 600,000 and 700,000 species of plants and animals. Approximately 3/4 of living plant and mammal species are endemic species found solely in Australia.

              Australia: Australia is home to many endemic species. The (a) wallaby (Wallabia bicolor), a medium-sized member of the kangaroo family, is a pouched mammal, or marsupial. The (b) echidna (Tachyglossus aculeatus) is an egg-laying mammal.

              The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time. Broad groups that evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana. The presence of Proteaceae in Australia, southern Africa, and South America is best explained by the plant family’s presence there prior to the southern supercontinent Gondwana breaking up.

              Biogeography: The Proteacea family of plants evolved before the supercontinent Gondwana broke up. Today, members of this plant family are found throughout the southern hemisphere (shown in red).



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