Mechanisms of divergence and character displacement

In chapter six Schluter addresses competition and how it may lead to divergence through character displacement. Character displacement is defined as “the process of phenotypic divergence caused or maintained by interspecific resource competition.”

Schluter states that there are three main patterns found in nature that indicate character displacement has occurred. The first is exaggerated divergence in sympatry, where species found together are more different phenotypically than in areas where they do not coexist. The second is trait over-dispersion. This occurs when the mean phenotypes found within a species are more evenly spaced along a trait axis than would be expected from a random sample. The final pattern, species-for-species matching, is defined as replicated guild structure between independently evolving sets of species. There are six criteria for assessing whether character displacement has occurred between different species:

1. Phenotypic differences between species should have a genetic basis.
2. Chance should be ruled out as an explanation of the pattern.
3. Differences must represent evolutionary shifts rather than species sorting.
4. Shifts in resource use should match changes in morphology.
5. Environmental differences between sympatric and allopatric sites must be controlled for.
6. Independent evidence should be gained that similar phenotypes compete for resources.

So what’s the relevance of character displacement to the lizards of white sands? Is character displacement likely to occur between these three species sometime in the future? For lizards in the white sands area there are fewer competitors, so the interspecific competition for resources may not be strong enough to lead to the evolution of character displacement. However, there are also fewer resources in white sands than in the rest of the desert, which should increase interspecific competiton. Looks like it’s time for some field work examining the overlap in the niches of the different species of white sands lizards!

Schluter also listed some other mechanisms besides competition that may promote divergence. The one that seemed most relevant to white sands is the process of the evolution of food webs. Through this mechanism divergence occurs in newly colonized areas where there were originally few inhabitants, and a new food web is essentially generated from scratch. When this happens many species and populations undergo trophic level transitions, where they begin to take advantage of resources from other trophic levels than they did previously. White sands has few inhabitants, and so colonists may diverge through the process of exploiting new resources and through the generation of a new food web.

Thanks for reading…take it away Simone!