Sunday, September 28, 2008

Is Local Adaptative Variation Tied to Mate Choice?

With the growing popularity of ecological speciation, instances of local adaptation have taken on added significance. In addition to being powerful examples of natural selection in action, local adaptation is now recognized as a potential starting point for the process of speciation. In order to invoke speciation or incipient speciation, however, it's necessary to show that some degree of reproductive isolation exists among local variants. Although this hypothesis is often tested indirectly with molecular markers, relatively few authors have succeeded in conducting direct behavioral assays of reproductive isolation in nature.

In the latest issue of Journal of Herpetology, Erica Bree Rosenblum uses a series of relatively simple behavioral experiments to examine reproductive isolation between the light and dark color morphs of the Lesser Earless Lizard (Holbrookia maculata). These forms evolved over the last 6,000 years to match substrate variation in the southwestern United States. While she found that light individuals are more likely to court other light individuals, this decision does not appear to be the result of body coloration: similar results were obtained even when light individuals were painted to resemble dark individuals. She suggests that behavioral signals or other aspects of the animal's coloration that are not obviously associated with crypsis (e.g., throat coloration) may be the mechanisms driving mate choice in Holbrookia.

The conclusion that the trait involved in adaptive divergence for improved crypsis is decoupled from the trait(s) involved in mate choice is surprising because it requires a more complicated model of speciation than when the same trait is filling both roles. Specifically, successful speciation under this scenario requires linkage between two traits that likely have distinct genetic foundations. Because the evolution of such linkage has always been viewed as one of the main impediments to ecological speciation it's going to be interesting to see how this story plays out, and how general this pattern is among other putative examples of ecological speciation.


Luke J. Harmon said...

To me, this might be better characterized as "partial," or even "failed" ecological speciation. That is, there is strong selection against intermediate forms, and some reduction in gene flow, but not full speciation. This seems compatible with theory. If there were a "magic" gene linking ecology and mate choice, then speciation could proceed to completion.

Glor said...

I think the jury is still out. It's possible that the traits are linked and this is an example of incipient ecological speciation. It even seems possible that one trait or the other involved in this study is a "magic" trait. If the signaling behavior of the organism is determining mate choice this trait too may be influenced by variation in the substrate environment.

Luke J. Harmon said...

In any case, we're on it - Bree and I were down there just three weeks ago, scouting out the situation. Since there are three species (of lizards) on the dunes, we have replicates to look at.