Bats and dolphins, despite their distance on the evolutionary tree of life, both possess an uncommon ability among mammals: echolocation. Now, scientists have discovered that the similarities go down to the genetic level as well as the physical -- bats and dolphins independently underwent many of the same genetic changes that resulted in their special biological sonar systems.
Animals use echolocation to locate prey and navigate by emitting sounds, then listening for the echoes of those calls. Subtle differences in the echoes, caused by sound waves bouncing off of objects, allow animals to construct an image of the world around them. Bats and dolphins aren’t that closely related, but both developed the very specialized trait of echolocation independently; it’s a classic example of what scientists call “convergent evolution” (another example would be the physical similarities between bat wings and bird wings: analogous shapes, arrived at independently).
Researchers led by a team from Queen Mary University of London wondered if some of the genetic steps that bats and dolphins took on their separate evolutionary paths might have been similar. They compared at the complete genetic sequences of 22 different kinds of mammals, including both echolocating and nonecholocating bats, bottlenose dolphins, horses, dogs, mice and humans.
“Strong and significant support for convergence among bats and the bottlenose dolphin was seen in numerous genes linked to hearing or deafness, consistent with an involvement in echolocation,” the authors wrote in Nature. “Unexpectedly, we also found convergence in many genes linked to vision: the convergent signal of many sensory genes was robustly correlated with the strength of natural selection.”
All in all, the scientists found nearly 200 genomic regions where the bat and dolphin adaptations related to echolocation overlapped.
“We had expected to find identical changes in maybe a dozen or so genes but to see [changes in] nearly 200 is incredible," lead author Joe Parker said in a statement. "We know natural selection is a potent driver of gene sequence evolution, but identifying so many examples where it produces nearly identical results in the genetic sequences of totally unrelated animals is astonishing."
Though dolphins and bats share many genetic changes related to echolocation, the processes aren’t identical. Dolphins produce their clicks in a special structure in their heads called phonic lips, and are focused and modulated by a fatty organ in their foreheads called the melon. Bats, on the other hand, use their vocal cords to create sounds, as most other mammals, including humans, do.
Despite their differences, the new work shows that both bats and dolphins clearly needed to make fundamental genetic changes to make the radical shift from relying on their eyes to relying on sound.
"These results could be the tip of the iceberg,” senior author Stephen Rossiter said in a statement. “As the genomes of more species are sequenced and studied, we may well see other striking cases of convergent adaptations being driven by identical genetic changes."
SOURCE: Parker et al. “Genome-wide signatures of convergent evolution in echolocating mammals.” Nature published 4 September 2013.