When environments change, populations may adapt surprisingly fast, repeatedly and even at microgeographic scales. There is increasing evidence that such cases of rapid parallel evolution are fueled by standing genetic variation, but the source of this genetic variation remains poorly understood. In the saltmarsh beetle Pogonus chalceus, short-winged ‘tidal’ and long-winged ‘seasonal’ ecotypes have diverged in response to contrasting hydrological regimes and can be repeatedly found along the Atlantic European coast. By analyzing genomic variation across the beetles’ distribution, we reveal that alleles selected in the tidal ecotype are spread across the genome and evolved during a singular and, likely, geographically isolated divergence event, within the last 190 Kya. Due to subsequent admixture, the ancient and differentially selected alleles are currently polymorphic in most populations across its range, which could potentially allow for the fast evolution of one ecotype from a small number of random individuals, as low as 5 to 15, from a population of the other ecotype. Our results suggest that cases of fast parallel ecological divergence can be the result of evolution at two different time frames: divergence in the past, followed by repeated selection on the same divergently evolved alleles after admixture. These findings highlight the importance of an ancient and, likely, allopatric divergence event for driving the rate and direction of contemporary fast evolution under gene flow. This mechanism is potentially driven by periods of geographic isolation imposed by large-scale environmental changes such as glacial cycles.

No abstract

In human-dominated landscapes (semi)natural habitats are typically embedded in tracts of unsuitable habitat. Under such conditions, habitat characteristics and grain size of the surrounding landscape may affect how much food, and at what cost, is available for sedentary species with low home-range plasticity. Here we combine behavioural radio-tracking, feather ptilochronology, and landscape analysis to test how nutritional condition varies with home range size in 13 house sparrow [Passer domesticus (Linnaeus, 1758)] populations along an urban gradient. Urban individuals occupied smaller home ranges than conspecifics from rural areas, most distinctly if key cover was highly scattered. In urban plots, patch connectivity, home range sizes, and activity areas were positively correlated, indicating that individual ranging behaviour was related to the spatial distribution of suitable habitat. Urban House sparrows also showed the smallest feather growth bars, which were positively related to home range size at plot level. In contrast, growth bar widths and home range sizes were negatively related in rural populations, whereas in suburban populations, both variables varied independently. We conclude that individuals from progressively more built-up areas show a restricted ability to adjust their daily ranging behaviour to the scattered distribution of critical resources. This may complement other putative causes of the widespread population decline of urban house sparrows.

Frugivorous species heavily depend on patchy food resources and are believed to track these in space and time, thereby providing an important seed dispersal function that might be critical toward the regeneration of fruiting plants. However, isolation of suitable food patches due to habitat fragmentation or changes in landscape connectivity may hamper food tracking behaviour and adversely affect populations of both frugivores (through starvation) and food plants (through interruption of seed dispersal). We here test whether density fluctuations in four frugivorous Afrotropical bird species were larger and/or matched fluctuations in ripe fruit densities better in study plots embedded in large tracts of indigenous forest than in equally-sized plots embedded in cultivated lands. We compared these results with those of four non-frugivorous species (out-group) which were not expected to track fruit resources. Whereas densities of both frugivores and fruit crops strongly fluctuated in space and time, these fluctuations were not synchronised, nor did the level of synchrony differ in relation to matrix type. For some but not all bird species, lower densities and smaller temporal fluctuations in forest plots surrounded by cultivation may reflect decreased mobility. The observed fluctuations in bird densities most likely reflect exchange with the surrounding landscape matrix, suggesting that small pockets of fruiting trees in farmland may comprise critical food resources for frugivores inhabiting highly fragmented landscapes, apart from increasing connectivity for both bird and seed dispersal