Understanding the precise molecular mechanisms underlying changes in animal morphology is a tricky problem–usually two species which have diverged morphologically (say, mice and humans) are now so unrelated as to make genetic study exceedingly difficult, if not impossible. For years, a group led by David Kingsley has been addressing this problem in a cleverly-chosen model–three-spined sticklebacks. Importantly for the question of morphological evolution, freshwater populations of this fish have lost many of the spines and pelvic girdle carried by the saltwater populations (there are a number of hypotheses, probably not all mutually exclusive, for why this has been under selection).
In a new paper, this group demonstrates the precise genetic alteration underlying this change in a number of freshwater populations. Perhaps surprisingly, it appears to be due to the recurrent deletion (in different freshwater populations) of an enhancer of an important developmental gene. Strikingly, creating a transgenic freshwater fish with a copy of this enhancer (which normally is missing) leads to freshwater fish with a pelvis like the saltwater fish.
In fact, this enchancer seem to fall in a “fragile” (read: repeat-laden) region of the genome, which presumably increases the rate of deletion at this site. If one imagines there are a number of genetic paths to get to the reduced pelvis size favored in freshwater environments, the probability of each path depends on the mutation rate of each genetic change. In this case, many (though not all) freshwater populations have independently taken the same path, likely due to the increased mutation rate at this fragile site.
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Citation: Chan et al. (2009) Adaptive Evolution of Pelvic Reduction in Sticklebacks by Recurrent Deletion of a Pitx1 Enhancer. Science. Published Online December 10, 2009 [DOI: 10.1126/science.1182213]
