Meeting Abstract
Convergent evolution, independent acquisition of similar traits in distant lineages, is often regarded as a clue to the predictability of evolution. However, there is little understanding on what constitute a predictive framework for trait evolution in individual lineages. In this study, I conducted an anatomical analysis of the musculoskeletal system of wing-propelled diving birds, a classic example of locomotion-associated convergent evolution in birds. Specifically, I reconstructed the wing musculature in two extinct lineages of flightless, wing-propelled auks (Pinguinus and Mancallinae, Charadriiformes), and compared it with that of the best-known example of wing-propelled divers, modern penguins (Sphenisciformes). The wings of the flightless auks had numbers of independently derived similarities to those in penguins, including relatively small size, a dorsoventrally flattened profile, reduction of the propatagium, and notably reduced joint mobility. These features have obvious functional advantages for the use of the wings in underwater flight, where the medium is much denser than in aerial flight. However, the stiffening of the elbow joint in the two auk lineages is accomplished by different anatomical configurations from penguins, and even from each other auk lineage to a lesser extent, utilizing unique anatomical structures which were ancestrally present in their own lineages. This is a previously unnoticed instance of incomplete convergence of phenotypic characters in a functionally convergent system, which is seemingly a consequence of differences in ancestral states in different lineages. A potential implication of ancestral-state dependent incomplete convergence would be that lineage-specific ancestral states, and perhaps their variability, may need to be incorporated in the predictive framework of convergent evolution, along with selective regimes.
