A recent PLoS One paper by Birky and colleagues attempts to define species for asexual animals using the theoretical framework that has been developed by population geneticists. Many species concepts focus on reproductive isolation and are not applicable for defining what is necessary and sufficient for delimiting what species are in non-sexually reproducing organisms slike some fungi, protists or even multicellular animals like crayfish, certain mites and rotifers.
For this week’s ResearchBlogCast I chose the Birky paper after reading about it on the Marmorkrebs blog by Zen Faulkes. You can listen to the ResearchBlogCast at ResearchBlogging.org, where each week Razib Khan, Dave Munger and I discuss a peer-reviewed article from the RB aggregator.
Birky, C., Adams, J., Gemmel, M., & Perry, J. (2010). Using Population Genetic Theory and DNA Sequences for Species Detection and Identification in Asexual Organisms PLoS ONE, 5 (5) DOI: 10.1371/journal.pone.0010609
In many asexual taxa, DNA sequences alone are used to detect and assign species, but this is only meaningful in the context of a well-defined species concept that you are blatant about operating under. I say “operating under” because I personally feel that scientists need to recognize that different species concepts are useful under different scenarios or for different taxa. This is called pluralism in the species concept literature. Many biologist from the ecology and evolutionary biology are not forthcoming about what species concept(s) they are operating under when they write up their results.
The new framework being proposed draws on previous work by the authors characterizing the mode of speciation in bdelloid rotifers. They updated their model to make it more general to other asexually reproducing taxa. Birky and colleagues refer to it as the 4x rule and argue that clades are sufficiently diverged by a number of generations equal to 4 times the effective population size. At this genetic distance, speciation is deep enough to be discerned from random genetic drift and other stochastic processes. It does not have to be exactly 4 times, but is actually the ratio of the average sequence diversity between groups to the nucleotide diversity among individuals within a groups. The point being that this is when two clades are reciprocally monophyletic at a statistical probability of 95% confidence.
What these authors try to do is introduce a rigorous way to define species that is based in existing theory. They differentiate this from DNA barcoding because the barcoding approach identifies species already defined by traditional taxonomy and uses empirically determined limits, not justified by any theoretical foundations. This also differs from traditional taxonomy because the authors feel that discernible phenotypes need not be present to distinguish between species, only reciprocal monophyly.
What I don’t really understand yet is how this really differs from the phylogenetic species concepts of the 1980s and ’90s. It may be that the population genetics species concept present a more rigorous and less subjective way to define species within a phylogenetic species concept. For instance, 2 closely-related taxa that are on the verge on being an obvious phylogenetic species may be discerned by using Birky and colleagues methodology as a decision rule.