Behavioral Isolation

This week we read chapter 6 or Speciation, titled “Behavioral and Nonecological Isolation.” Coyne and Orr describe 4 main categories of “evolutionary forces” that lead to behavioral isolation. These include 1) selection on mate preference, 2) selection on certain traits, 3) genetic drift, and 4) nongenetic mechanisms.

1) In the first category, initial selection pressures act on mate preferences of individuals either directly or indirectly. Direct selection on mate preferences increases the immediate fitness of the chooser by improving the chooser’s ability to acquire resources, or allowing the chooser to avoid deleterious features associated with mating. Indirect selection on preference includes selection that does not increase the chooser’s immediate fitness. For example, in the runaway model of selection preference for a certain trait is genetically correlated with the gene conferring that trait.

2) In the second category there is selection on traits that improve the attractiveness of the bearer to the opposite sex, improve the ability of the bearer to overcome competition from members of the same sex, or that facilitate species recognition. This last category includes trait changes that evolve through natural selection with preferences coevolving. It also includes the evolution of reinforcement, which improves the ability of individuals to discriminate conspecifics from heterospecifics in order to reduce maladaptive hybridization.

3) Genetic drift may affect behavioral isolation if nonselective changes in allele frequencies affect a signal or a preference.

4) Nongenetic mechanisms of isolation include cultural drift, and host parasitism.

It seems to me that direct selection on traits would be most important for behavioral isolation of populations of white sands lizards and dark soils lizards, especially through species recognition. The two subcategories for the species recognition section seem quite similar to me. In the first scenario, natural selection causes a trait to change, and preferences for that trait coevolve because mating with individuals with that trait will improve the fitness of offspring. In the second scenario, evolution favors discrimination between individuals with different traits to reduce the chance of producing less fit offspring. It almost seems like two sides of the same coin; recognize a trait and mate with that individual to create more fit offspring, or recognize a trait and don’t mate with that individual to avoid making less fit offspring.

Coyne and Orr distinguish the two by emphasizing that in the first scenario there need not be a closely related sympatric species present. Divergent selection in different habitats could cause changes in traits and preferences for those traits, which could lead to behavioral isolation between locally adapted populations. The book cites the example of divergent selection on body size for resource acquisition in benthic and limnetic morphs of sticklebacks .

In the case of reinforcement, there must be a related sympatric species present, and the fact that hybrids are less fit leads to selection favoring the avoidance of hybridization through increased ability to recognize conspecifics. So I guess if the difference between these two ideas is just whether there is divergence of locally adapted populations or of sympatric sister taxa, then the lizards in white sands would fall under the first scenario. This distinction is still a little fuzzy to me so if anyone has any comments please feel free to let me know where I’m going wrong.