Experimental adaptation and speciation in rotifers

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Evolutionary biology seeks to explain two features of the natural world: the fit of organisms to their environment (adaptation) and the diversity of different species. Diversity depends on a balance between two key processes, division of existing species into two or more distinct daughter species (speciation) and extinction. In sexually-reproducing organisms, species are defined by successful interbreeding within species combined with barriers to interbreeding between species. Thus speciation consists of the evolution of new barriers to interbreeding between populations. The evolution of these barriers can be initiated by divergent natural selection that generates adaptation of populations within a species to different environments. To complete the speciation process, this adaptation needs to be combined with other barriers to successful interbreeding such as assortative mating. Theory suggests that two major forces oppose divergent adaptation and the progression towards speciation. These are gene flow, due to the movement of individuals from one population to another, and recombination, the genetic process that breaks up combinations of genes generated by natural selection or other factors. However, it is difficult to test the effects of these forces in natural systems because of the complex and unknown history of extant species or divergent populations. In this project, we will take a different approach, using experimental control of migration and recombination in pairs of populations under divergent selection to test their effects, independently and jointly, on both adaptation and the evolution of barriers to interbreeding. We will use rotifers in the Brachionus plicatilis species group. These are small aquatic animals that are easily cultured in the laboratory with a generation time of only a few days. They have both sexual and asexual modes of reproduction and this will allow us to control recombination by varying the frequency of the sexual mode. We will maintain replicated pairs of lines without sexual reproduction, with rare sex and with frequent sex. Within each pair, each line will be subjected to a different novel, stressful environment, resulting in divergent selection. We will also control gene flow between paired lines by having replicates without connection, replicates where one individual is moved between lines each time we renew cultures, replicates with a higher rate of movement and replicates that are completely mixed. We will follow the progress of divergent adaptation in these pairs of lines over more than 100 generations and also test whether they evolve other barriers to interbreeding by measuring assortative mating and the fitness of hybrids. We will sequence the genomes of our starting populations and of the experimentally-evolved lines. This will allow us to determine whether gene flow and recombination influence the number and distribution of genetic changes underlying the evolutionary changes we observe. Experimental evolution has rarely been used in the context of speciation research but there is now an increasing recognition that experimental work is needed to complement theory and analysis of natural systems. With this project, we will apply this approach to central issues in speciation research using a powerful model system that has not previously been used in this context. The further understanding of adaptation and speciation to which this project will contribute is critical for the management of biological diversity in a changing environment.