The role of competition and frequency-dependent selection in population differentiation and the maintenance of genetic polymorphisms

Erik Svensson, Lund University Graduate students:
Jessica Abbott	Thomas Gosden

In our research, we combine concepts and methods from ecological population genetics, quantitative genetics and evolutionary ecology. We are especially interested in how ecological processes, such as competition, translates into various forms of selection, such as disruptive natural or sexual selection and frequency-dependent selection. These forms of selection are powerful evolutionary forces since they can both maintain genetic polymorphisms and affect the genetic architecture of a species. Field work is carried out in natural populations of marked individuals of damselflies (Sweden) and and lizards (California). By following marked individuals in the field, measuring various fitness components such as survival and reproductive success, and mapping fitness variation on to variation in phenotypic traits, we can investigate the interplay between frequency-dependent selection and other evolutionary forces such as gene flow and genetic drift. We use study systems of conspicuous heritable morphs to investigate how correlational selection can favour the buildup and maintenance of co-adapted trait complexes within populations. A major goal is to understand how divergence in phenotypic traits between morphs and populations can lead to speciation.

Frequency-dependent selection and genetic polymorphisms in evolution Frequency-dependent selection plays a major role in many evolutionary models, such as the maintenance and evolution of sex, host-parasite co-evolution, male reproductive strategies, polymorphisms, interspecific competition and speciation. In spite of its huge theoretical importance, our empirical knowledge about the role of frequency-dependent processes in natural systems is still very limited. We study frequency-dependent selection in natural populations of damselflies (Ischnura elegans) and side-blotched lizards (Uta stansburiana). We focus on species with discrete and conspicous genetic polymorphisms (e. g. colour polymorphisms) and quantify fitness components in natural and semi-natural settings. We are especially interested in how frequency-dependent selection may cause correlational selection between different characters, and reversals in the direction of selection between life-stages (conflicting selection). We have recently started to develop explicit population genetic models to understand morph frequency-dynamics across several generations in collaboration with Roger Härdling.

Ecological vs. non-ecological speciation mechanisms The relative role of ecological and non-ecological factors in speciation have increased markedly during the last years, but the relative importance of natural selection, sexual selection and sexual conflict remain largely unknown. Recent theoretical models have shown that each of these factors could promote speciation alone or in combination, but rigorous empirical field studies to disentangle between the various models are still few. Tests of ecological speciation theories require studies in the wild, where the impact of predation, resource competiton and mate competition can be assessed. Experimental and observational studies are performed in natural and free-living populations of damselflies (Calopteryx spp., Ischnura elegans). Special emphasis is directed towards the roles of predation and male-female mating conflicts in causing selection on phenotypic traits that are involved in species recognition.

Evolutionary quantitative genetics of physiology, life-history and signalling traits

In collaboration with Barry Sinervo we are investigating the evolutionary quantitative genetics of fitness-related traits in a free-living population of side-blotched lizards (Uta stansburiana) in California. This population is polymorphic with respect to throat colour, and strong correlational selection for different optimal character combinations leads to striking differences between morphs in morphology, physiology, life-history traits and behaviour. We are quantifying the consequences of correlational selection by measuring genetic correlations between throat colour and other traits, e. g. immunocompetence and steroid hormones. We are also interested in maternal effects mediated via e. g. egg size. In addition to the quantitative genetic studies, we will also further investigate the genomic architecture in this species by using Quantitative Trait Loci (QTL) analysis to detect genes of major effect and dissect the anatomy of the genetic correlations we have found.

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