Evolutionary abilities and climate change in the sea
Species have four fundamental ways in which they can respond to climate and other rapid environmental change: acclimatize, evolve, colonize, or become extinct. Of these, evolutionary abilities may be most poorly understood, but large population sizes and high rates of gene flow suggest that marine species have particularly large stores of genetic diversity that could allow rapid evolution. However, many marine species have fallen to low abundance as a result of fishing, which might create widespread genetic bottlenecks. In addition, marine species may be locally adapted to a greater extent than has generally been realized, which could create specialization that impedes adaptation to changing climates. Here, started to examine these issues in two ways. First, we took a comparative approach to test for genetic bottlenecks from fishing by compiling data across 140 species and comparing genetic diversity between overfished populations and close relatives that were not overfished. In addition, we used RNAseq to test for signs of local adaptation in a common reef fish, the yellowtail clownfish (Amphiprion clarkii), across a latitudinal gradient from the equator to Japan.
Across the diversity of marine fishes, we found that allelic richness was on average 12% lower in overfished populations, after accounting for the effects of body size, latitude, and other factors. Heterozygosity was on average 2% lower (p = 0.030). Simulations confirmed that these patterns are consistent with a recent bottleneck in abundant species. In addition, RNAseq in the yellowtail clownfish revealed seven loci potentially involved in local adaptation (FST outliers). Annotation suggested that these loci were involved with hematopoiesis, structural integrity, and protein folding, which may be consistent with the oxygen limitation hypothesis of thermal tolerance for marine animals. Taken together, these findings help to provide a more nuanced view of the potential for marine species to evolve in response to climate change. Gene flow between sites is likely to be an important process, both for moving “pre-adapted” alleles to populations experiencing warmer conditions for the first time and for replenishing genetic diversity in populations where it has been lost.