Evolution by Sexual Selection

Charles Darwin published his second book “Sexual selection and the descent of man” in 1871 150 years ago, to try to explain, amongst other things, the evolution of the peacock’s train, something that he famously thought was problematic for his theory of evolution by natural selection. He proposed that the peacock’s train had evolved because females preferred to mate with males with more elaborate trains. This idea was very controversial at the time and it wasn’t until 1991 that a manuscript testing Darwin’s hypothesis was published. The idea that a character could arise as a result of a female preference is still controversial. Some argue that there is no need to distinguish sexual from natural selection and that natural selection can adequately explain the evolution of extravagant characteristics that are characteristic of sexually selected species. Here, I outline the reasons why I think that this is not the case and that Darwin was right to distinguish sexual selection as a distinct process. I present a simple verbal and mathematical model to expound the view that sexual selection is profoundly different from natural selection because, uniquely, it can simultaneously promote and maintain the genetic variation which fuels evolutionary change. Viewed in this way, sexual selection can help resolve other evolutionary conundrums, such as the evolution of sexual reproduction, that are characterised by having impossibly large costs and no obvious immediate benefits and which have baffled evolutionary biologists for a very long time. If sexual selection does indeed facilitate rapid adaptation to a changing environment as I have outlined, then it is very important that we understand the fundamentals of adaptive mate choice and guard against any disruption to this natural process.

Strong spatial population structure shapes the temporal coevolutionary dynamics of costly female preference and male display

Female mating preferences for exaggerated male display traits are commonplace. Yet, comprehensive understanding of the evolution and persistence of costly female preference through indirect (Fisherian) selection in finite populations requires some explanation for the persistence of additive genetic variance (Va) underlying sexual traits, given that directional preference is expected to deplete Va in display and hence halt preference evolution. However, the degree to which Va, and hence preference-display coevolution, may be prolonged by spatially variable sexual selection arising solely from limited gene flow and genetic drift within spatially structured populations has not been examined. Our genetically and spatially explicit model shows that spatial population structure arising in an ecologically homogeneous environment can facilitate evolution and long-term persistence of costly preference given small subpopulations and low dispersal probabilities. Here, genetic drift initially creates spatial variation in female preference, leading to persistence of Va in display through migration-bias of genotypes maladapted to emerging local sexual selection, thus fuelling coevolution of costly preference and display. However, costs of sexual selection increased the probability of subpopulation extinction, limiting persistence of high preference-display genotypes. Understanding long-term dynamics of sexual selection systems therefore requires joint consideration of coevolution of sexual traits and metapopulation dynamics.

Evolutionary origins of sexual dimorphism : Lessons from female-limited mimicry in butterflies.

The surprising female-limited mimicry observed in some species is a text-book example of sexually-dimorphic trait submitted to intense natural selection. Two main hypotheses have been proposed to explain female-limited mimicry in butterflies. Predation pressure favouring mimicry could be higher in females because of their slower flight, and overcome developmental constraints favouring the ancestral trait that limits the evolution of mimicry in males but not in females. Alternatively, the evolution of mimicry in males could be limited by sexual selection, generated by females preference for non-mimetic males. However, the evolutionary origin of female preference for non-mimetic males remains unclear. Here, we hypothesise that costly sexual interactions between individuals from distinct sympatric species might intensify because of mimicry, therefore promoting female preference for non-mimetic trait. Using a mathematical model, we compare the evolution of female-limited mimicry when assuming either alternative hypotheses. We show that the patterns of divergence of male and female trait from the ancestral traits can differ between these selection regimes but we specifically highlight that divergence in females trait is not a signature of the effect of natural selection. Altogether, our model reveals the complex interplay between sexual and natural selection shaping the evolution of sexually-dimorphic traits.