Coevolution and the origins of reproductive isolation

点击翻译按钮获取中文摘要

Since Darwin, the predominant view of how new species arise is through adaptation occurring in isolated populations experiencing different ecological selection pressures. However there are “non-ecological” mechanisms by which reproductive isolation (RI) can evolve when populations are divided. Among these, it has been suggested that intersexual coevolution (IC) may be a particularly potent “engine of speciation”. IC occurs through reciprocal adaptation of one sex to features of the other sex in reproduction, cooperation or conflict between the sexes. Because IC is driven by open-ended cycles of adaptation and counter-adaptation within populations, isolated populations may diverge simply because, by chance, new favourable mutations arising in one population are different from those arising in another. RI arising through such a process has been called mutation order divergence (MOD). There are few convincing tests of the MOD hypothesis because separated natural populations inevitably have ecological differences to one degree or another, often have unknown histories, and may be difficult to experiment with. I will take advantage of a powerful long-term evolution experiment with the fruit fly, Drosophila, to quantify mechanisms contributing to the early stages of speciation. Numerous studies with this species have shown that IC is measureable over time scales of 10's of generations. My 15-population experiment has been running for over 25y and has populations isolated from common ancestry for as many as 2,600 generations, evolved under 3 different treatments. Populations from each treatment have markedly different characteristics, especially related to body size, development and age of reproduction, constituting a source of ecological divergence. Populations in the same treatment have identical handling histories but are genetically isolated, as if occupying separate islands with identical ecology. Pilot studies show considerable pre-mating isolation has evolved between populations selected for different characteristics, like preferring like in mate choice, and some physical mating incompatibilities between them. Future research will expand this work on mate choice, look at post-mating pre-fertilization competition between sperm, fertilization success, and the success of first- and second-generation hybrids between populations. With perhaps the longest running evolution experiment undertaken in an animal species, control over the environment, knowledge of the genome of the species and the specific populations being used, and near perfect knowledge of the history of the lines since domestication, we have an opportunity to characterize the nature of evolving reproductive isolation in unprecedented detail.