Scientists have long cited echolocation in bats and whales as a classic example of convergent evolution. Yet conventional, non-teleological thinking expects convergence to involve different genes, different mutations, and different pathways. As Stephen Rossiter of the University of London notes “it is generally assumed that most of these so-called convergent traits have arisen by different genes or different mutations.”
Yet Rossiter’s research helped to show that the convergent phenotype of echolocation was driven by convergent amino acid changes in the same gene:
“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.
“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.
Despite these gross differences, our findings suggest that the high-frequency acoustic sensitivities and selectivities of bat and whale echolocation appear to rely on a common molecular design of prestin.
It’s this type of finding that I expected from the hypothesis of front-loading:
If both metazoans and Monosiga have independently come up with similar or identical mechanisms and architectures in the tyrosine kinase circuits, it would seem we might trace this to an intrinsic, rather than environmental, cause. That is, there was some kind of inherent molecular inertia built into the basic design plan of the TK circuit to cause it to evolve similarly in different creatures experiencing different niches.
If the two systems are indeed convergent, and if they became so similar for intrinsic, rather than niche-related reasons, we would have an excellent example where two expectations of front-loading, intrinsic control and deep homology, fuse. And this wold lead us to ask just how many examples of convergence are due to intrinsic, rather than environmental reasons?
In the case of echolocation, it appears to be the molecular design of prestin itself that has been guiding the evolution of echolocation and this protein would certainly qualify as an intrinsic factor behind this example of convergent evolution.