Abstract
Getting a comprehensive understanding of microevolution in natural populations requires proper quantification of the important selective forces exerted on these populations. Over the last decade, long-term studies and resurrection-ecology (revival of resting stages) have been the main approaches to study life history trait evolution over many generations in the wild. My work aims at understanding how simple ecological factors (e.g. temperature) and complex interactions between and within species (host-parasite and male-females antagonistic interactions) shape evolutionary processes in natural populations. To this end, I used the brine shrimp Artemia as a model system and combined laboratory and field studies. First, I investigated thermal niche evolution with a resurrection ecology approach, using dormant-egg time series from an Artemia population introduced from temperate to tropical salterns in the mid-80's. This experiment shows that survival at the high temperatures typical of the new environment increased linearly through time after the introduction, suggesting a sustained rate of adaptation over more than 100 generations. Second, I used the same approach to study adaptation between sexes in another Artemia population. I found that sexual conflicts result in fluctuating male-female coevolutionary dynamics in natura, over a time scale of ~100 generations. Third, I studied the relative role of one cestode and two microsporidian parasites in mediating the competition between a native asexual host and an invasive bisexual host. I found that all three parasites were either host- or genotype-specific and that the castrating cestode parasite specifically infected the native species, suggesting that this parasite actually played a major role in the competition between native and invasive hosts. Interestingly, all three parasites manipulated the swarming behavior of their host, most likely to increase their transmission. Fourth, I performed population genetic studies of diploid and polyploid Artemia parthenogenetica and their Asian bisexual close relatives. Diploid asexual Artemia produce rare males and I found indication that these males allow some rare sex in this otherwise parthenogenetic species. In addition, hybridization between divergent Artemia species has led to the origin of at least three independent polyploid lineages.