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".

"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:

Asexual "species:" Many creatures reproduce primarily or entirely asexually. How do species concepts apply to them?

Bdelloid rotifer

Hybrids: Creatures belonging to morphologically and behaviorally distinct groups cane become "confused" and mate with the wrong partner, especially when normal ecological or geographic barriers break down. (E.G. the hybrid of the Galápagos Land and Marine iguana shown at right.) In nature, this is far from unusual, and they span a broad spectrum from clearly maladaptive sterile hybrids to fertile ones. To see how weird things can get, consider the case of hybridogens such as the molly Poeciliopsis from Sonora. In this "species," each individual is female and gets is paternal genome through its 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."

Hybrid Galápagos iguana

Sibling species: Sister taxa (i.e. descendents form a recent common ancestor) that are morphologically indistinguishable, but distinguished by molecular, karyological, or behavioral traits. E.G. stonebashers of the genus Pollimyrus, who tell one another apart by difference in their calls.

Pollimyrus from Aquarium Systems Ltd.

Ring species: Species spanning a broad morphological continuum across a wide geographic range in which members of a local population can interbreed with members of adjacent populations, but end members of the continuum cannot interbreed.

Ensatina salamanders

And yet, living things do seem to group into morphologically distinct populations, 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"; or, more succinctly, "a bunch of critters that look and act and grow basically the same." 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"? Merck's personal rule of thumb: Populations represent distinct species when they are sufficiently different that hybridization between there individuals reduces their evolutionary fitness.

But I'm not the expert. Major attempts at species definitions that have gained significant traction include:

• Ernst Mayr's Biological Species Concept: "a species is an array of populations which are actually or potentially interbreeding, and which are reproductively isolated from other such arrays under natural conditions." Problems, however, are that such reproductive isolation is not testable in asexual organisms, has not been tested by 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 is impossible to test for fossils (including fossils which we would otherwise include within modern species).

• Evolutionary Species Concept: "an evolutionary species is a lineage evolving separately from others and with its own unitary evolutionary role and tendencies."

• Reformulated after the cladistics revolution into the phylogenetic species concept: the smallest taxon with origin in a lineage splitting event.

• Does make one important observation: the ability to interbreed with relatives is a symplesiomorphy (shared primitive trait), while the ability NOT to interbreed with relatives is the synapomorphic (derived) condition. This latter idea became the basis for the Isolation Species Concept

• Specific Mate Recognition Concept: species are recognized by those traits used by the organisms themselves to prefer or reject mating. (Obvious problems for asexual organisms and for fossils)

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.

• "Paleospecies" are necessarily morphospecies. Whatever species concept we favor, we can't absolutely assess their mating habits.
• 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.
  But we must account for more than that.

Anagenesis: fossil populations changing from one form to another gradually Suggests sympatric speciation "Extinctions" of any member successional "species" would be pseudoextinction: an arbitrary division of the anagenetic series; "originations" would be similarly arbitrary And yet, evolving lineages may undergo pulses of evolution that make the identification of segments of the lineage possible. These are called chronospecies and are, by necessity, somewhat arbitrary.

Cladogenesis: Origination of species through speciation - i.e. lineage splitting event. 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.

To Syllabus.

Last modified: 22 August 2008