Resolving the avian tree of life: Part I

Since an early age, one of my major passions (some might call it an obsession!) has been birding. I watch birds, I study them, I plan holidays around them, I think and talk about them ad infinitum. But I often get the feeling that birders, myself included, often engage with them in a somewhat more superficial manner than we could be doing. That's a sparrow, that's a lark, that's an eremomela... As with anything, I suppose, we like to group things according to their obvious features. Thus, small, dull, hard-to-identify species are often called 'LBJs' - Little Brown Jobs. Likewise, birds like falcons and eagles are called 'raptors' or 'birds of prey'. But what isn't always obvious is the evolutionary history of birds that reveals the true links between them, which are often quite surprising - it's the stuff that's going on 'behind the scenes', if you will.  Yet it's the stuff that really matters, in a biological sense. Figuring out the true appearance of the tree of life provides us with invaluable insight into the evolution of the species we currently share the planet with, and with new molecular techniques and growing DNA sequence databases, it is now possible to do so not just for a few meagre twigs of the tree but for great, hulking branches of it.

So it was that Hackett et al. (2008) took on the challenge of reconstructing the phylogeny of living birds. Not all of the nearly 10 000 species of them, mind you. That task was taken up more recently, and will be the topic of my next post. Rather, Hackett and colleagues chose a select group of birds to represent all of the major orders within class Aves, including all bar three of the non-passerine families and all of the major passerine clades. The Passeriformes are a huge order containing over half of all bird species and at least 110 families - their taxonomy is ever-shifting. So it's understandable that Hackett et al. didn't include them all. As it was they analysed a total of 32kb of DNA code from 19 nuclear loci and 171 species (including two crocodilians as outgroups). DNA sequencing is generally held as the most reliable means of reconstructing phylogenies. Previous attempts at doing so for birds had been based on techniques now regarded as being unreliable. In addition, the birds themselves aren't playing along. Results from previous studies, corroborated by that of Hackett et al., suggest that birds underwent a rapid divergence very early on in their history. Because there's been so much time for change, telling who's related to who, and how, is a real challenge: there's very little to link the various highly distinct groups.

Yet despite this, Hackett et al. recovered a remarkably robust phylogeny that revealed some unexpected relationships, and supported others that had been suggested but regarded by most as pretty wacky. Notably, they showed that passerines are sister to parrots (Psittaciformes), and that this clade is sister to falcons (Falconidae). Previously falcons were thought to be sister to eagles, hawks and other raptors in Accipitridae. They also showed that flamingos and grebes are sister to each other, a fact that I remember being very surprised about when I first heard of it. They uncovered some important clues about the evolution of some real enigmas too, such as the fact that the Kagu (Rhynochetos jubatus) of New Caledonia and Sunbittern  (Eurypyga helias) of South America, both bizzare and unique birds of monotypic genera, are sister species. And buttonquails (Turnix), of which our own Turnix hottentotta occurs in the Cape of Good Hope Nature Reserve, and which have been variously thought to be part of Gruiformes and Turniciformes, are in fact nestled within Charadriiformes, sister to gulls.

However, some mysteries couldn't be solved. The totally bizarre Hoatzin (Opisthocomus hoazin), familiar to many a seasoned wildlife documentary watcher and renowned for its possession of claws in the juvenile stage, and the somewhat less familiar but equally strange Cuckoo Roller (Leptosomus discolor) of Madagascar, remain incertae sedis. Interestingly, fossil relatives of the Hoatzin have been found in Namibia (Mayr et al. 2011) from long after the break-up of Gondwana, and further evidence suggests that the lineage evolved in Africa, and then dispersed to South America. I wonder what that might imply about its evolution? The fact that these birds still defy classification is, I think, quite amazing, and it remains an exciting challenge for avian systematists.

Some of the bizarre and enigmatic birds dealt with by Hackett et al. (2008). Clockwise from top left: Sunbittern, Kagu, Hoatzin, Cuckoo Roller.

The phylogeny also shows some very interesting ecological pattern, providing us with food for thought about bird macroevolution. For example, the tinamous are nested within the ratites (Struthioniformes - ostriches, emus, kiwis, etc.), rather than being sister to them, as previously thought. This suggests that either that they regained the power of flight from flightless ancestors, or that their ancestors were flighted, meaning that flight was lost at least three times in ratites, which contradicts the traditional vicariance hypothesis of global ratite distribution (Harshman et al. 2008). They also found that large clades correspond to broad ecological niches: there are well-supported 'water bird' and 'land bird' clades, although there are some exceptions (of course!). So for the most part diversification has proceeded within the bounds of these broad niches, but some groups have bucked the trend, such as flamingos and grebes, which are aquatic but not part of the water bird clade, and the uniquely African turacos, which fall well outside the land birds and are in fact sister to water birds.

The list of interesting insights goes on, and will continue to stimulate research into avian macroevolution. As we'll see in Part II of this post, a recent study attempted to reconstruct a complete species-level bird phylogeny, building directly from Hackett et al. It provided some fascinating results, but did the authors go too far, too soon?

References

Hackett SJ et al. 2008. A Phylogenetic Study of Birds Reveals Their Evolutionary History. Science. 320: 1763-1768.

Mayr G et al. 2011. Out of Africa: Fossils shed light on the origin of the hoatzin, an iconic Neotropic bird. Naturwissenschaften. 98: 961-966.

Harshman J et al. 2008. Phylogenomic evidence for multiple losses of flight in ratite birds. PNAS. 105 (36): 13462–13467.