Thursday, July 14, 2005

Species concepts   posted by Razib @ 7/14/2005 02:51:00 AM

Since we mysteriously tend to show up rather high on some science related google queries, I have decided to copy many of the "species concepts" found in Coyne and Orr's Speciation below (adapted from table 1.1 on page 27). Google does have a list of sites with a lot of information, but none of them strike the balance of brevity and thoroughness I would like. I think a familiarity with the general outlines of the "Species Controversy" can also help people grapple with within-species population substructure. Obviously species exist, right? But drawing hard and fast lines based on deterministic axioms aren't as easy as you might think, good enough for government work, but definately nothing that could ever quench physics-envy.

First, the Biological Species Concept - Species are groups of interbreeding natural populations that are reproductively isolated from other such groups (Mayr 1995).

Genotypic Cluster Species Concept - A species is a [morphologically or genetically] distinguishable group of individuals that has few or no intermediates when in contact with other such clusters (Mallet 1995).

Recognition Species Concept - A species is the most inclusive population of individual biparental organisms which shares common fertilization system (Patterson 1985).

Cohesion Species Concept - A species is the most inclusive population of individuals having the potential for phenotypic cohesion through instrinsic cohesion mechanisms (Templeton 1989).

Ecological Species Concept - A species is a lineage (or a closely related set of lineages) which occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range (Van Valen 1976).

Evolutionary Species Concept - A species is a single lineage of ancestral descendent populations or organisms which maintains its identity from other such lineages which has its own evolutionary tendencies and historical fate (Wiley 1978, modified from Simpson, 1961)

Phylogenetic Species Concept 1 - A phylogenetic species is an irreducible (basal) cluster of organisms that is diagnosably distinct from other such clusters, and within which there is a paternal pattern of ancestry and descent (Cracraft 1989).

Phylogenetic Species Concept 2 - A species is the smallest [exclusive] monophyletic group of common ancestry (de Queiroz and Donoghue 1988).

Phylogenetic Species Concept 3 - A species is a basal, exclusive group of organisms, all of whose genes coalesce more recently with each other than with those of any organisms outside the group, and that contains no exclusive group with it (Baum and Donoghue 1995; Shaw 1998).

Personally, the "Species Controversy" is not that interesting to me, my attitude toward species is strictly ends-based, that is, what does the "concept" species convey to you in terms of information, framing a basis for further research and contributing to model building? Coyne and Orr take the same general approach from what I can gather. It seems that some of the species concepts, especially the phylogenetic ones, are favored by biologists whose sole focus is Sytematics. But for all its problems the Biological Species Concept seems the more useful for day to day research in most areas, and helps guide us along the path to the Most Perfect System of the World. I am really more interested in the dynamics of alleles in gene space, and "species" are simply one verbal description of that space. On the adaptive landscape that I have spoken of recently the boundaries between species can be thought of as very deep trenches, or even untraversable "holes" which separate the fitness peaks which correspond to the species.