Convergent evolution of the mating behaviour of founder-flush populations of the housefly
Courtship bouts of six founder-flush populations (two-pair founder-flush) and two nonbottlenecked controls of the housefly were videotaped over the course of 26 generations in order to evaluate the stability of mating behaviour. Limited-choice mate preference tests were conducted periodically to assess levels of homogamic preference. Both founder-flush and control treatments showed significant evolutionary potential in courtship, along with homogamic and heterogamic preferences. The founder-flush populations were significantly differentiated from the controls in courtship repertoire, but all of the populations pursued convergent evolutionary trajectories in adapting to the laboratory, resulting in dissolution of homogamic preferences. The phenotypic shifts in courtship and mate preferences were unrelated to evolutionary trends in overall mating vigour; therefore, the convergence in courtship could not be attributed to either a fitness meltdown due to inbreeding or the purge of deleterious alleles. Only one founder-flush population showed some independence from the selectional pressures for convergence; therefore, the single two-pair founder-flush event was generally inadequate to stimulate stable incipient speciation. This study thus demonstrates how convergent evolution can dissolve founder-flush effects.