Variation in male sailfin molly, preference for female size: does sympatry with sexual parasites, drive preference for smaller conspecifics?

Reinforcement as an initiator of population divergence and speciation

When hybridization results in reduced fitness, natural selection is expected to favor the evolution of traits that minimize the likelihood of hybridizing in the first place. This process, termed reinforcement (or, more generally, reproductive character displacement), thereby contributes to the evolution of enhanced reproductive isolation between hybridizing groups. By enhancing reproductive isolation in this way, reinforcement plays an important role in the final stages of speciation. However, reinforcement can also contribute to the early stages of speciation. Specifically, because selection to avoid hybridization occurs only in sympatric populations, the unfolding of reinforcement can lead to the evolution of traits in sympatric populations that reduce reproduction between conspecifics in sympatry versus those in allopatry. Thus, reinforcement between species can lead to reproductive isolation—and possibly speciation—between populations in sympatry versus those in allopatry or among different sympatric populations. Here, I describe how this process can occur, the conditions under which it is most likely to occur, and the empirical data needed to evaluate the hypothesis that reinforcement can initiate speciation.

The impact of social context on male mate preference in a unisexual--bisexual mating complex

Male sailfin mollies Poecilia latipinna were tested in five different treatments that varied in the relative frequency of heterospecific gynogens (Amazon molly Poecilia formosa) to conspecific females to determine whether social interactions among males within a population causes some males to mate with heterospecific females. Male P. latipinna inseminated a significantly higher proportion of conspecific females and fertilized a significantly higher number of conspecific eggs regardless of the treatment. Nonetheless, preference for conspecific females was not exclusive as a range of 20 to 50% of heterospecific females were fertilized. Social interactions among males may best explain the results and may therefore play an important role in the maintenance of unisexual--bisexual mating complexes.