A species of fish currently dwelling within the salty lakes of San Salvador Island may one day rival Darwin's finches in the evolutionary story.
Within the shallow salty lakes of the Bahama's San Salvador Island lives the Bahaman pupfish. Evolutionary biologists Joseph McGirr and Christopher Martin have been studying three closely related pupfish species peacefully co-existing within the warm waters. They appear to be like any other fish–except that each species, through subtle differences in jaw size, has redefined its own food niche, and all within the last 10,000 years.
These pupfish specialise via specific adaptions in their jaw size and food choice, be it algae, snails or quick-striking scale-eaters. This is not unlike how the finches of Charles Darwin reflect differences in seed choices by the size and shape of their beaks.
Take, for instance, the specialised lepidophage, or scale-eater. Fifty percent of its diet comprises the scales of other fishes. To facilitate this, it has adapted to having an elongated body and a jaw mechanism that allows a fast biting action.
Another species is the cyprinodon brontotheroides, which is a speciaised durophage (predator of hard-shelled invertebrates. The physical adaptations to this behaviour include its modified jaw morphology which allows it to close its mouth with greater force and crush shells
Seeking to investigate the vast diversity, the biologists identified 12 million single DNA mutations (called single nucleotide polymorphisms, or SNPs) from 37 genomes and sequenced across nine populations of the three species caught on San Salvador Island and one nearby island. Then they matched the DNA changes with differences in jaw size to find out the candidate genes responsible for large jaws and small jaws.
However, these results bring up an intriguing question.
"If scale-eating traits were shaped by hard sweeps acting on genetic variation within ancestral Caribbean-populations, why is this trophic specialization absent from neighboring islands?" asked Martin. From previous ecological and genetic studies, the scientists were unable to detect any striking environmental differences or differences in genetic diversity between San Salvador Island and neighbouring Bahamian islands.
"Answering this question will require continued exploration of the ecological and genetic factors shaping this exceptional case of rapid ecological specialisation. So far, the usual suspects (lake area, ecological and genetic diversity) do not seem to provide the trigger of adaptive radiation as is commonly assumed. Instead, the answer seems to be far more complex and interesting than we ever imagined."