A nice example of convergent evolution has been published in the latest issue of Current Biology, with two studies showing that echolocation abilities of bats and dolphins have the same underlying genetic basis.
Convergent evolution is where two species independently acquire the same trait in response to similar evolutionary pressures. There are a huge number of examples – venom production in reptiles and mammals, bioluminscence in glow-worms and jellyfish, flightless in numerous different bird species, to name a few. Most examples of convergent evolution are the result of a different means producing a very similar looking end – different structures may be co-opted for the same purpose, or different changes at the genetic level might produce similar phenotypes.
Whats really unusual about the bat/dolphin story is that, not only have they independently acquired echolocation, but that the underlying molecular basis for how they acquired it is almost exactly the same – ie the same changes in the same protein look to have occurred independently in the two lineages. The protein involved is Prestin, which is involved in detection of high-frequency echos rather than production of the sonar. Obviously bats and dolphins are not particularly closely related as mammals go, but if you draw an evolutionary tree of their Prestin sequences, they look like each others closest relatives. The odds of this many identical changes to a protein happening independently in two separate lineages by chance are extremely slim, suggesting there are a very limited number of ways that Prestin can be changed to detect high-frequency sounds, and that selection has played an important role.
The papers are:
Liu et al. Convergent sequence evolution between echolocating bats and dolphins. Current Biology vol. 20, p 53.
Li et al. The Hearing Gene Prestin Unites Echolocating Bats and Whales. Current Biology vol 20, p 55.