Nicholas Alexandre led this discussion on Mar. 18th 2020 about sexual imprinting setting stage for speciation. This paper provides potential solutions for two speciation dilemma: (1) LD between preference and trait within an ancestry tends to be broken down by hybridization; (2) assortative mating reduces genetic variation underlying signal and preference, which hampers evolution of assortative mating towards complete behavioural isolation. These polymorphic frog populations are convenient for investigating mate signals and preference. It turns out that sexual imprinting maintains trait and preference association without requirements for strong genetic basis of trait or preference. This non-genetic preference-trait association could bypass the reduction of genetic variation maintaining genetic polymorphism to further fuel the evolution of assortative mating. The frequency-dependent nature of sexual imprinting favours the locally common mating signal and could lead to fixation of the common signal. This sexual imprinting scenario is essentially a ‘one-locus model of speciation’ (Felsenstein 1981).
It is unclear whether such behavioral isolation would be sufficient to initiate speciation, the evolution of reproductive isolation. Irwin (2020) simulation shows that strong but incomplete (10X own-type preference over the alternative type) assortative mating would not be strong enough to prevent gene flow and fusion between diverged lineages. This is because when the hybrids formed over the rare hybridization events would prefer each other than either of the parental population, forming a genetic “stairwell” linking the gene flow between parental lineages. Alternatively, postmating hybrid fitness reduction even with a little amount (10% decrease of hybrid fitness than the parentals), is efficient at attenuating gene flow. Therefore, there genetic incompatibility is absent, it is hard to rely on behavioral isolation alone to speciate, although it could facilitate the evolution of reproductive isolation as well.
In Yang et al. (2019) system, it will be interesting to examine postmating isolation mechanism such as genetic incompatibility. Maybe these intrinsic genetic incompatibility had imposed selection for any type of assortative mating (genetic or non-genetic).
Reference
Yang, Y. Servedio M. R. & Richards-Zawacki C. L. 2019. Imprinting sets the stage for speciation. Nature. 574: 99-109.
Felsenstein, J. 1981. Skepticism towards Santa rosalia, or why are there so few kinds of animals? Evolution. 35:124-138.
Irwin, D. E. 2020. Assortative mating in hybrid zones is remarkably ineffective in promoting speciation. Am. Nat.