The role of population structuring in adaptive evolution
All species are structured into subgroups or subpopulations by more or less obvious boundaries. Physical barriers may directly prevent gene flow through habitat fragmentation, whereas temporal or behavioural differences concerning dispersal and/or reproduction may indirectly have the same effect. To better understand how gene flow prevents or facilitates local adaptations, and how local adaptations may prevent gene flow, detailed knowledge about the interaction between organisms and their environment is needed; what are the traits under selection and what are the selection pressures?
We will study the importance of gene flow in adaptive evolution in two different fish systems combining neutral genetic and functional data. In spite of the natal homing behaviour of the European grayling, some gene flow among populations has been demonstrated as an isolation-by-distance pattern in neutral genetic markers (microsatellites). Preliminary experiments using a common-garden design have revealed rapid evolution of life history traits (e.g., spawning time, egg size and early development), this will be explored further. We will also undertake reciprocal transplant experiments in the wild to estimate the fitness of dispersing and non-dispersing individuals, and to assess to what extent the phenotypic variation is heritable, or due to plastic responses. Candidate genes will be analyzed for differential expression patterns (cf. Theme 2). Coastal populations of the Atlantic cod receive large, but temporally variable, numbers of larvae from exogenous offshore populations. Rapidly changing environmental (ocean currents) and ecological parameters (offshore larval production) thus have a direct effect on gene flow and are expected to counteract selection for local adaptations. We will study certain ecological traits of local coastal populations (spawning behaviour, egg buoyancy, etc.) as potential evolutionary responses limiting gene introgression.
Establishment and maintenance of local adaptations can also be strongly influenced by genetic drift. For the European grayling (box 1), extensive microsatellite data will be generated and used for modelling of different scenarios for colonization and bottleneck events. Combined with data on demography and adaptive differences among populations, we have an outstanding setup for exploring the role of stochasticity in adaptive evolution.
For the Plague system the high degree of genetic variability observed in Central Asia (presumably the cradle of all the big plague epidemics, including the Black Death of the middle ages) might be linked to the ecology of the main rodent host (gerbils) that exhibits extensive density variation in both time and space. We will obtain specimen samples (collected as part of a time-series study) of infected gerbils and of the microbes’ flea vectors. DNA isolated directly from infected individuals, as well as from isolates, will be screened for polymorphisms. Population genetic modelling, incorporating the ecological dynamics of the host-vector-plague system and taking into account the genetics of Yersinia pestis, will provide important insight into the biology of this deadly bacterium.
Competitors may be a crucial part of a species’ environment. Climatic variation has been shown to affect the competitive interactions between pied flycatcher and collared flycatcher and between great tit and blue tit differentially. Since these birds are hole-nesters, experimental studies in the wild are feasible, and fitness as well as microevolutionary responses to varying and conflicting selection pressures induced by climate and/or competition can be estimated and compared between the pairs of competing species.
Dispersal patterns within a species often vary on a geographic scale. For instance, in pied flycatcher, there is a north-south cline with respect to sex-biased dispersal, with malebiased sex ratios in northern populations. Comparative analyses of populations along geographic transects will be undertaken in order to study potential adaptive responses (e.g., in sexual selection and in mating system evolution) to differences in dispersal and sex ratio.
Habitat-specific genes may be found in bacteria from different phylogenetic lineages strongly indicating an adaptive value in that environment. For strains of Thermotogales and Cyanobacteria isolated from certain environments (e.g., oil reservoirs) habitatspecific genes will be identified through subtractive hybridization. These genes may be obtained or lost through homologous recombination (cf. Theme 3). In order to gain insight into how local adaptation affects gene flow in these populations, rates of recombination between regions flanking habitatspecific genes and the rest of the genome will be compared.