Climatic similarity and genomic background shape the extent of parallel adaptation in Timema stick insects

Evolution can repeat itself, resulting in parallel adaptations in
independent lineages occupying similar environments. Moreover, parallel
evolution sometimes, but not always, uses the same genes. Two main
hypotheses have been put forth to explain the probability and extent of
parallel evolution. First, parallel evolution is more likely when shared
ecologies result in similar patterns of natural selection in different
taxa. Second, parallelism is more likely when genomes are similar, because
of shared standing variation and similar mutational effects in closely
related genomes. Here we combine ecological, genomic, experimental, and
phenotypic data with Bayesian modeling and randomization tests to quantify
the degree of parallelism and its relationship with ecology and genetics.
Our results show that the extent to which genomic regions associated with
climate are parallel among species of Timema stick insects is shaped
collectively by shared ecology and genomic background. Specifically, the
extent of genomic parallelism decays with divergence in climatic
conditions (i.e., habitat or ecological similarity) and genomic
similarity. Moreover, we find that climate-associated loci are likely
subject to selection in a field experiment, overlap with genetic regions
associated with cuticular hydrocarbon traits, and are not strongly shaped
by introgression between species. Our findings shed light on when
evolution is most expected to repeat itself.