Podarcis siculus Aeolian ddRADseq

Repeated adaptation provides valuable insights into the predictability of evolution. Population history, selection and stochastic processes can concur to generate a continuum from distinct to highly parallel evolutionary trajectories across replicate populations. Yet, the role of genetic and environmental factors in shaping this continuum remains underexplored. We quantified repeated genetic adaptation in lizards that colonized multiple islands with comparable environmental gradients, investigating whether environmental-dependent and divergence-dependent processes can explain the degree of repeated adaptation (genetic reuse and trajectory similarity). We found 149 genes exhibiting repeated adaptation in multiple islands, some of which are likely involved in thermal physiology and developmental processes. Genetic reuse was stronger at the functional level than at the mutation level and exceeded random expectations, highlighting that different genetic combinations can generate similar functional outcomes. Adaptive trajectories were more similar between islands with low genetic differentiation and similar environmental conditions, but the effects of genetic and environmental factors varied across the diverse facets of repeatability. Overall, our findings reveal the extent and conditions under which local adaptation is, in part, predictable.