Asexual species identifications

Marmorkrebs are difficult beasts. As I’ve mentioned before, there is no species name for them yet, partly because of how people define species. At the practical level, most crayfish are identified by the sex organs of the males (which Marmorkrebs don’t have). At the conceptual level, many people define species by interbreeding populations (which parthenogenetic organisms don’t do).

Birky and colleagues recently proposed a way to define species for parthenogenetic organisms. As near as I can understand it, their argument runs like this.

First, they’re going to define species using DNA. They just think morphology is too subtle and too prone to mislead.

Second, the criteria that they’re going to use to separate species is going to revolve around two key concepts: genetic drift and adaptation to a niche.

For any organism, even parthenogenetic clones like Marmorkrebs, there is a certain probability that mutations will occur each generation. Even when there is no selection pressure for that mutation, the frequency of the mutation in the population will change just due to chance over time, even becoming fixed or eliminated. That’s genetic drift.

Birky and company define a species as a group of organisms that show genetic changes that are too large to be accounted for by drift alone. They argue that this is indicative of a population that has undergone adaptation to a specific ecological niche.

The details of their proposal involve a fair amount of math, which, for the purposes of writing a blog post, I didn’t feel the need to become intimately acquainted with. At first glance, however, this approach seems generally fruitful. They apply their methods to six different asexual groups, and seem to make some headway on defining them. I think they could also apply their approach to sorting defining an asexual species that is derived from sexual ancestors, although they don’t discuss this.

Some potential glitches in their approach are that they effectively rule out the possibility that two species could be created by drift alone, which I think many evolutionary biologist would be uncomfortable with. They also mention briefly the idea of “higher taxa,” but how to define those higher taxa was not laid out nearly as clearly as for species.

But they do provide hope that Marmorkrebs, and many other asexuals, can get recognized as species, as they should be, in my opinion.

Reference

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): e10609. DOI: 10.1371/journal.pone.0010609