Fossil Species: Species Concepts and Criteria

Species (pl. "species"; Latin for "kind"): generally considered the fundamental unit of biological diversity. Certainly is the primary entry in databases of diversity, abundance, occurence, etc. from modern and fossil assemblages, ecosystems, etc.

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:

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

The upshot: Even among living creatures, the delineation of "species" can be daunting. Consider the killer-whale Orcinus orca: Three morphological types are known from the Pacific Northwest (above) that are behaviorally distinct. and possibly not interbreeding.

Four analogous morphs are known from Antarctica. Are these end-members of a global morphocline, hybridizing populations, or reproductively isolated? In intelligent animals like these, we also don't know if their behavioral differences are inherited or cultural. We are far from knowing how many "species" of orca exist.

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.

And yet, living things do seem to group into morphologically distinct populations rather than grading across wide ranges of morphospace without breaks, even those that reproduce asexually. Traditionally, species are morphospecies: "a diagnosible cluster of individuals within which there is a pattern of ancestry and descent, and beyond which there is not." However, individual variation is a basic attribute of ALL organisms; and geographic variations are very common as well. At what point are two different geographic populations different at "the species level"? Your instructors' personal rules of thumb:

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:

Liger from Ligerworld
Warning! Don't think that these criteria are simply different ways to reach the same underlying truth. In many cases, their practical application might be the same, but they are different concepts. Consider pantherine cats: In the wild, lions, leopards, jaguars, etc. are morphologically and behaviorally distinct, but in captivity they produce fertile hybrids frequently.

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

Speciation: The Origin of Species

So far, we have only considered the present time slice. When we look at the past, other issues rise up. Remember that despite the foregoing, "paleospecies" are necessarily morphospecies. Whatever species concept we favor, we can't absolutely assess their mating habits.

Two major models of species origins in geologic time:

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

Additional reading:

To Syllabus.

Last modified: 20 September 2010