We study sexual selection mostly to understand the evolution of costly, extravagant traits in males, for example flashy colors. Another reason is that sexual selection may play a role in the formation of new species (speciation). I mostly study sexual selection by female choice, meaning that certain females may prefer these flashy males; one way this could happen is that females might prefer particular males simply because they are conspicuous, or they look like food. This is called sensory bias. The simplest example is the guppy Poecilia reticulata where the females display a preference for orange objects because that’s what they eat. Males mimic this orange color and attract females. This is interesting because natural selection directly sets up the female preference: even if the preference has a cost, natural selection still favors its evolution, because it helps females survive. This could make new species form because different perceptual biases could be favored in different places.
Recent Servedio Lab graduate Alicia Frame asked whether sexual selection can then pick up these preferences and exaggerate them beyond what natural selection calls for. Sexual selection here would be indirect, meaning that the preference itself is no longer acted upon, but sexual selection on the attractive male trait could make the preference stronger. I’ll explain the mathematical details in another post, but basically whenever there’s direct selection on a trait (selection favors a trait), there is indirect selection on other traits that correlate with it. In this case the traits (the signal and the preference) occur in different individuals, but it’s simpler to think of two traits on the same individual organism. A simple example would be size of the brain and size of the body in humans. If there was selection to have a bigger brain in early hominids, then individuals with bigger bodies would also be favored by selection.
Alicia and our advisor Maria used a newer version of a biologically realistic model: there are “yellow fish” and “blue fish” that live in either yellow or blue environments. Males that contrast with their background environment are more conspicuous and thus more easily spotted by predators and preferred by females. Indirect selection is caused by correlations between traits. To have a correlation, you need lots variance in both correlated traits. What Alicia and Maria found was that when all females are choosy (the preference is there because of natural selection), there is not enough variation in the male trait for indirect selection to strengthen the female preference. This was true even when the male trait mutated back and forth between conspicuous and inconspicuous color morphs, creating more genetic variance. They did find exceptions, but I find this general result really interesting: natural selection alone is most important in determining the strength of the trait. If there is any strengthening, making the male trait even more ridiculous and conspicuous, then it will have to be the result of direct benefit to the female. Indirect selection is just too weak.
Alicia published this paper in the Open Access journal Ecology and Evolution, so anyone can read it free of charge. Here’s the abstract:
Evidence suggests that female preferences may sometimes arise through sensory bias, and that males may subsequently evolve traits that increase their conspicuousness to females. Here, we ask whether indirect selection, arising through genetic associations (linkage disequilibrium) during the sexual selection that sensory bias imposes, can itself influence the evolution of preference strength. Specifically, we use population genetic models to consider whether or not modifiers of preference strength can spread under different ecological conditions when female mate choice is driven by sensory bias. We focus on male traits that make a male more conspicuous in certain habitats—and thus both more visible to predators and more attractive to females—and examine modifiers of the strength of preference for conspicuous males. We first solve for the rate of spread of a modifier that strengthens preference within an environmentally uniform population; we illustrate that this spread will be extremely slow. Second, we used a series of simulations to consider the role of habitat structure and movement on the evolution of a modifier of preference strength, using male color polymorphisms as a case study. We find that in most cases, indirect selection does not allow the evolution of stronger or weaker preferences for sensory bias. Only in a “two-island” model, where there is restricted migration between different patches that favor different male phenotypes, did we find that preference strength could evolve. The role of indirect selection in the evolution of sensory bias is of particular interest because of ongoing speculation regarding the role of sensory bias in the evolution of reproductive isolation.
Alicia is now working on a postdoc in toxicology at the EPA in Research Triangle Park.
Alicia M. Frame, Maria R. Servedio (2012). The evolution of preference strength under sensory bias: a role for indirect selection? Ecology and Evolution DOI: 10.1002/ece3.273