Female preference for novel males constrains the contemporary evolution of assortative mating in guppies

Progress toward local adaptation is expected to be enhanced when divergent selection is multidimensional, because many simultaneous sources of selection can increase the total strength of selection and enhance the number of independent traits under selection. Yet, whether local adaptation ensues from multidimensional selection also depends on its potential to cause the build-up of reproductive barriers such as sexual signals and preference for these signals. We used replicate experimental introductions of guppies (Poecilia reticulata) in nature to test whether an abrupt and dramatic shift in multiple important ecological dimensions (at a minimum: parasitism, predation, and diet/resources) promoted the contemporary evolution of assortative mating. After 8–12 postintroduction guppy generations in the wild, we bred descendants of each population in a common-garden laboratory environment for 2 generations, after which we recorded the preferences of females from each population for males from all populations. We found contemporary evolution of male traits (size, body condition, color) that should influence mate choice, but no evidence for the occurrence of positive assortative preferences. That is, females in a given evolving population did not prefer males from that population over males from other populations. Instead, females tended to prefer novel males (i.e., disassortative mating), which likely acts as a mechanism preventing the evolution of reproductive isolation. Preferences for novelty may explain why many cases of local adaptation do not lead to the evolution of reproductive barriers and ecological speciation.