In Bats and Whales, Convergence in Echolocation Ability Runs Deep:
..."However, it is generally assumed that most of these so-called convergent traits have arisen by different genes or different mutations. Our study shows that a complex trait -- echolocation -- has in fact evolved by identical genetic changes in bats and dolphins." A hearing gene known as prestin in both bats and dolphins (a toothed whale) has picked up many of the same mutations over time, the studies show. As a result, if you draw a phylogenetic tree of bats, whales, and a few other mammals based on similarities in the prestin sequence alone, the echolocating bats and whales come out together rather than with their rightful evolutionary cousins. Both research teams also have evidence showing that those changes to prestin were selected for, suggesting that they must be critical for the animals' echolocation for reasons the researchers don't yet fully understand. "The results imply that there are very limited ways, if not only one way, for a mammal to hear high-frequency sounds," said Jianzhi Zhang of the University of Michigan, who led the other study. "The sequence convergence occurred because the amino acid changes in prestin that result in high-frequency selection and sensitivity were strongly favored in echolocating mammals and because there are [apparently] very limited ways in which prestin can acquire this ability." Prestin is found in outer hair cells that serve as an amplifier in the inner ear, refining the sensitivity and frequency selectivity of the mechanical vibrations of the cochlea, Zhang explained.
This obviously plays into the arguments about contingency and inevitability. On the one hand the convergence across these two taxa suggest that this trait seems to result in an inevitable genetic architecture. But perhaps only for mammals. So inevitability might be a contingent aspect of evolution here. Reminds me a bit of opsins and vision.
Yang Liu, James A. Cotton, Bin Shen, Xiuqun Han, Stephen J. Rossiter, Shuyi Zhang. Convergent sequence evolution between echolocating bats and dolphins. Current Biology, 2010; DOI: 10.1016/j.cub.2009.11.058
Ying Li, Zhen Liu, Peng Shi, Jianzhi Zhang. The hearing gene Prestin unites echolocating bats and whales. Current Biology, 2010; DOI: 10.1016/j.cub.2009.11.042