When two organisms evolve in similar ways independently, we call it convergent evolution. Classical examples include the fish-like shape of whales and the separate evolution of flight by both bats and birds. Now, in this week’s PNAS, a (huge) group of scientists report that ratites, the group of flightless birds including emus, ostriches, rheas, cassowaries, and kiwis, lost the ability to fly at least three separate times in their evolutionary history [$-a].
Photo by Morti Riuuallon.
The key question this paper addresses is whether ratites are all the descendants of a single common ancestor (a “monophyletic” grouping) – if they are, then chances are that flightlessness only evolved once, and in that ancestor. The new paper’s authors use a large DNA sequence data set to show that that tinamous, the group of flying birds most closely related to ratites, actually arose within the monophyletic group of the ratites. This makes the ratites polyphyletic, not monophyletic. Since the next-most-closely related birds fly, and it’s probably easier to lose the ability to fly than it is to regain it, this suggests that the common ancestor of the ratite-tinamou group could fly, and that ratites probably lost the ability to fly multiple times.
J. Harshman, E.L. Braun, M.J. Braun, C.J. Huddleston, R.C.K. Bowie, J.L. Chojnowski, S.J. Hackett, K.-L. Han, R.T. Kimball, B.D. Marks, K.J. Miglia, W.S. Moore, S. Reddy, F.H. Sheldon, D.W. Steadman, S.J. Steppan, C.C. Witt, T. Yuri (2008). Phylogenomic evidence for multiple losses of flight in ratite birds PNAS, 105 (36), 13462-7 DOI: 10.1073/pnas.0803242105