Eco-evolutionary responses of natural populations to spatial environmental variation strongly depend on the relative strength of environmental differences/natural selection and dispersal/gene flow. In absence of geographic barriers, as often is the case in lake ecosystems, gene flow is expected to constrain adaptive divergence between environments – favoring phenotypic plasticity or high trait variability. However, if divergent natural selection is sufficiently strong, adaptive divergence can occur in face of gene flow. The extent of divergence is most often studied between two contrasting environments, whereas potential for multimodal divergence is little explored. We investigated phenotypic (body size, defensive structures, and feeding morphology) and genetic (microsatellites) structure in threespine stickleback (Gasterosteus aculeatus) across five habitat types and two basins (North and South) within the geologically young and highly heterogeneous Lake Mývatn, North East Iceland. We found that (1) North basin stickleback were, on average, larger and had relatively longer spines than South basin stickleback, whereas (2) feeding morphology (gill raker number and gill raker gap width) differed among three of five habitat types, and (3) there was only subtle genetic differentiation across the lake. Overall, our results indicate predator and prey mediated phenotypic divergence across multiple habitats in the lake, in face of gene flow.
Temporally replicated studies are essential to describe and understand selection in natural populations. Selection patterns can differ among life stages representing different fitness components. Despite the increasing number of long-term studies, yearly estimates of fluctuation in strength and direction are mostly available from studies conducted on a limited number of years. Based on a population of Tree swallows (Tachycineta bicolor) monitored over 10,200 km 2 in Southern Québec, Canada, since 2004, we investigated how patterns of selection may change across breeding stages by dividing the overall selection at the nesting stage (number of fledglings produced) into hatchling (number of hatchlings produced) and fledgling (number of hatchlings having successfully fledged) selection stages. We assessed fluctuation in selection gradients on two morphological (body mass and wing length) and two reproductive (laying date and clutch size) traits in females. We found significant positive selection gradients for body mass and clutch size and negative selection gradients for laying date, though the latter only during the fledgling selection stage. We also found that selection gradients on reproductive traits significantly fluctuated in direction and/or strength among years but only during the hatchling breeding stage. Our results thus emphasize the need to consider how selection events may be fluctuating in time and among breeding stages and the importance of these patterns for the maintenance of phenotypic variation in wild populations.
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