Collaborative Research: Testing Mechanisms of Microgeographic Adaptation

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One goal of evolutionary ecology is to understand the processes that lead to species becoming better adapted to their local environment. Studies of local adaptation have historically focused on geographically separated populations, where adaptive differences are maintained by limited dispersal of individuals between populations. Growing evidence, however, suggests that adaptive evolution can occur over small spatial distances. How this fine scale, adaptation arises and is maintained remains unresolved, despite applications for human health, agriculture, and biodiversity. On Santa Cruz Island, the Island Scrub Jay has two morphologies. Birds in pine habitat have long and thin beaks where as those in oak habitats have short and deep beaks. These differences are known to facilitate feeding on pine cones versus acorns, but how such adaptive genetic differences are maintained over very small geographic distances is unknown. The researchers will collect data on the genomes of individuals from both types of habitats to test if the same set of genes are differentiated between pine and oak populations, and how other regions of the genome change spatially across the island. These measurements will be coupled with telemetry studies that track how birds move and use oak and pine habitats. In addition, vocal experiments will test if males prefer females that originate within their habitat. The data obtained from this work will then be used to develop general models that can inform the conditions under with adaptive differences in can occur. Such models would be of interest to conservation managers that manage this and other unique or rare species. Additionally, the project will produce a learning module for high school classroom use, and will provide research opportunities for both undergraduate and graduate students. Cases of microgeographic adaptation challenge classical evolutionary theory that posits adaptive divergence should be constrained by gene flow, leading to a reassessment of the spatial scale at which evolutionary processes occur. The research team has recently discovered replicated, adaptive divergence in jay bill shape in adjacent oak and pine habitats, despite gene flow between these adjacent habitats. This pattern occurs across habitats separated by less than one kilometer. The researchers will test alternative hypotheses about how divergent selection, dispersal, and non-random mating shape adaptive divergence in bill morphology. In Aim 1, they develop population genetic models that test the interacting effects of selection, gene flow, assortative mating, and genetic architecture at different spatial scales. In Aim 2, they will test for divergent selection by measuring selection gradients, examining patterns of genomic divergence, and by identifying candidate genes associated with variation in bill morphology. In Aim 3, they will use a combination of population genomics, telemetry, and transplant experiments to test for population genomic structure and if dispersal is non-random In Aim 4, they will test if divergence in bill morphology is correlated with vocalizations and facilitates assortative mating. Collectively, the proposed work takes an integrative approach in a tractable study system. This research has broad implications for understanding both the spatial scale at which adaptive divergence occurs, and the conditions that allow individuals within a single population to diverge across heterogeneous habitats.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.