Climate-driven habitat change causes evolution in Threespine Stickleback

Climate change can shape evolution directly by altering abiotic conditions
or indirectly by modifying habitats, yet few studies have investigated the
effects of climate-driven habitat change on contemporary evolution. We
resampled populations of Threespine Stickleback (Gasterosteus aculeatus)
along a latitudinal gradient in California bar-built estuaries to examine
their evolution in response to changing climate and habitat. We took
advantage of the strong association between stickleback lateral plate
phenotypes and Ectodysplasin A (Eda) genotypes to infer changes in allele
frequencies over time. Results show that the frequency of low-plated
alleles has generally increased and heterozygosity has decreased over
time. Latitudinal patterns in stickleback plate phenotypes suggest that
evolution at Eda is a response to climate-driven habitat transformation
rather than a direct consequence of climate. As climate change has reduced
precipitation and increased temperature and drought, bar-built estuaries
have transitioned from lotic (flowing-water) to lentic (still-water)
habitats, where the low-plated allele is favored. The low-plated allele
has achieved fixation at the driest, hottest southernmost sites, a trend
that is progressing northward with climate change. Climate-driven habitat
change is therefore causing a reduction in genetic variation that may
hinder future adaptation for populations facing multiple threats.