Polygenic Selection within a Single Generation Leads to Subtle Divergence among Ecological NichesINc

'Chimes of resilience': what makes forest trees genetically resilient?

Forest trees are foundation species of many ecosystems and are challenged by global environmental changes. We assemble genetic facts and arguments supporting or undermining resilient responses of forest trees to those changes. Genetic resilience is understood here as the capacity of a species to restore its adaptive potential following environmental changes and disturbances. Importantly, the data come primarily from European temperate tree species with large distributions and consider only marginally species with small distributions. We first examine historical trajectories of trees during repeated climatic changes. Species that survived the Pliocene-Pleistocene transition and underwent the oscillations of glacial and interglacial periods were equipped with life history traits enhancing persistence and resilience. Evidence of their resilience also comes from the maintenance of large effective population sizes across time and rapid microevolutionary responses to recent climatic events. We then review genetic mechanisms and attributes shaping resilient responses. Usually, invoked constraints to resilience, such as genetic load or generation time and overlap, have limited consequences or are offset by positive impacts. Conversely, genetic plasticity, gene flow, introgression, genetic architecture of fitness-related traits and demographic dynamics strengthen resilience by accelerating adaptive responses. Finally, we address the limitations of this review and highlight critical research gaps.

Isotope geolocation and population genomics in Vanessa cardui: Short- and long-distance migrants are genetically undifferentiated

The painted lady butterfly Vanessa cardui is renowned for its virtually cosmopolitan distribution and the remarkable long-distance migrations as part of its annual, multigenerational migratory cycle. In winter, V. cardui individuals inhabit breeding grounds north and south of the Sahara, suggesting distinct migratory behaviors within the species as individuals migrate southward from Europe in the autumn. However, the evolutionary and ecological factors shaping these differences in migratory behavior remain largely unexplored. Here, we performed whole-genome resequencing and analyzed the hydrogen and strontium isotopes of 40 V. cardui individuals simultaneously collected in the autumn from regions both north and south of the Sahara. Our investigation revealed two main migratory groups: (i) short-distance migrants, journeying from temperate Europe to the circum-Mediterranean region and (ii) long-distance migrants, originating from Europe, crossing the Mediterranean Sea and Sahara, and reaching West Africa, covering up to over 4,000 km. Despite these stark differences in migration distance, a genome-wide analysis revealed that short- and long-distance migrants belong to a single intercontinental panmictic population extending from northern Europe to sub-Saharan Africa. Contrary to common biogeographic patterns, the Sahara is not a catalyst for population structuring in this species. No significant genetic differentiation or signs of adaptation and selection were observed between the two migratory phenotypes. Nonetheless, two individuals, who were early arrivals to West Africa covering longer migration distances, exhibited some genetic differentiation. The lack of genetic structure between short- and long-distance migrants suggests that migration distance in V. cardui is a plastic response to environmental conditions.

Direct observation of adaptive tracking on ecological time scales in Drosophila

Direct observation of evolution in response to natural environmental change can resolve fundamental questions about adaptation, including its pace, temporal dynamics, and underlying phenotypic and genomic architecture. We tracked the evolution of fitness-associated phenotypes and allele frequencies genome-wide in 10 replicate field populations of Drosophila melanogaster over 10 generations from summer to late fall. Adaptation was evident over each sampling interval (one to four generations), with exceptionally rapid phenotypic adaptation and large allele frequency shifts at many independent loci. The direction and basis of the adaptive response shifted repeatedly over time, consistent with the action of strong and rapidly fluctuating selection. Overall, we found clear phenotypic and genomic evidence of adaptive tracking occurring contemporaneously with environmental change, thus demonstrating the temporally dynamic nature of adaptation.

Using singleton densities to detect recent selection in Bos taurus

Many quantitative traits are subject to polygenic selection, where several genomic regions undergo small, simultaneous changes in allele frequency that collectively alter a phenotype. The widespread availability of genome data, along with novel statistical techniques, has made it easier to detect these changes. We apply one such method, the "Singleton Density Score" (SDS), to the Holstein breed of Bos taurus to detect recent selection (arising up to around 740 years ago). We identify several genes as candidates for targets of recent selection, including some relating to cell regulation, catabolic processes, neural-cell adhesion and immunity. We do not find strong evidence that three traits that are important to humans-milk protein content, milk fat content, and stature-have been subject to directional selection. Simulations demonstrate that because B. taurus recently experienced a population bottleneck, singletons are depleted so the power of SDS methods is reduced. These results inform on which genes underlie recent genetic change in B. taurus, while providing information on how polygenic selection can be best investigated in future studies.