Tuesday, 9 September 2008

Another Ugly Fact?

Review of:
Phylogenomic evidence for multiple losses of flight in ratite birds.
Harshman, J., Braun, E.L., Braun, M.J., et al. September 9th, 2008, Proceedings of the National Academy of Sciences Vol. 105, no. 36
Exhaustive analyses of DNA sequence data from 20 unlinked nuclear genes provide strong evidence that ratites are polyphyletic. We have discovered a robust genome-wide signal that is not associated with any known phylogenetic artefact. We believe this phylogeny resolves a debate on ratite origins that began in the time of Huxley and Owen. Our phylogeny implies that the numerous striking similarities associated with flightlessness had independent origins in various ratite lineages. Thus, the flightless ratites are living evidence of parallel evolutionary trajectories from flighted ancestors. The possibility that multiple, unique developmental genetic pathways underlie the ratite form should be tested in light of this new phylogenetic hypothesis. Finally, our phylogeny removes the need to postulate vicariance by continental drift to explain ratite distribution.
Although that theory seemed to represent a consilience between evolutionary biology and geology, it was never completely consistent either with any published phylogeny or the existence of paleognath fossils in the Northern hemisphere.
Perhaps the impact of our phylogeny should be viewed as yet another example of the phenomenon that Huxley called “the great tragedy of science – the slaying of a beautiful theory by an ugly fact.”
Another ugly fact?

The postulated phylogeny differs particularly from other ratite phylogenies in that it includes Tinamous as part of the clade rather than a sister group. This focus on Tinamous is noteworthy as the authors included four Tinamou and two Rhea genera in their analysis, but excluded all Moas and Elephant Birds. Since the six South American species constitute 30% of the genetic material studied some doubt due to the breadth underlying the conclusions will remain.
It is already fairly uncontroversial to suggest that ratites (extant and extinct, but minus Tinamous) lost the ability to fly sometime in the past and may have done so at different times. The conclusion that ratites are polyphyletic is rather more contentious if viewed as a basal fact, but not incompatible with monophyly when an ancestral taxon to them all (perhaps including Tinamous) existed.
It is naive of the authors to imply that a ratite ancestral family took to the air and flew in different directions to settle new habitats on various continents. The very phylogeny they suggest demands staggered departure times of many millions of years which can be adequately accommodated by plate tectonics and pre-Gondwana dispersal.
Tinamous may indeed be the best modern representatives of the ancient Paleognath ancestor, which would have been volant and small. This common ancestor would have existed about 90 MA and have dispersed into ecological niches before the break-up of Gondwana, perhaps long before. The hypothesis of vicariance biogeography concerning ratites is not refuted by the phylogenomic evidence presented.

Richard Perron

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