Birds’ evolution on the march

RICHARD the Third, according to Shakespeare, was “not shaped for courtly tricks, nor made to court an amorous looking glass”.

“Rudely stamped” and “cheated of feature”, he was “sent unformed unfinished ... before my time into this breathing world, scarce half made up.. that dogs do bark at me”.

Did the king, accused of murdering his young nephews in the Tower, suffer from curvature of the spine or was this an invention of Henry Tudor who defeated him on Bosworth Field (‘a horse, a horse, my kingdom for a horse’)? Richard’s burial place was said to be a Leicester church, the site of which is now a car park. A skeleton unearthed there last year has a severely deformed spine and marks of violence. Carbon dating showed that it’s about 500 years old. In the ultimate “who do you think you are” piece of detective work, the skeleton’s DNA was compared with that of a living descendent of the king’s sister, Anne of York. The remains, it turns out, are indeed those of the notorious monarch and so Shakespeare’s account of his appearance rings true. How extraordinary that people separated by 17 generations, spanning half a millennium, are now shown, definitively, to be related.

Origins are a preoccupation of taxonomists, the genealogists of the natural world. The relationships between most back-boned animal species are relatively easy to determine; bones survive. However, flying creatures must remain as light as possible.

Bird bones, therefore, are hollow and seldom preserved. The fossil record of our feathered friends is sparse and uncovering the history of bird evolution is difficult. Now, thanks to technologies such as those deployed on Richard’s remains, progress is being made and scientists in the USA, Canada, Britain and Australia have uncovered the family tree of the 9,993 bird species alive today. An account of their findings appeared in the November edition of Nature.

Conventional thinking about the development of new species has been turned on its head. When new habitats are created, species evolve to exploit them. Then, as niches fill up, expansion slows. Soon, no further varieties are possible, or so it was thought.

Birds evolved from the dinosaurs about 150m years ago. After such a long period, we should have a ‘house full, with few new forms developing. This, however, turns out not to be the case. According to the authors, the number of bird species began to increase about 50m years ago and it’s still rising.

The evolutionary explosion is largely confined to particular groups. The little warblers are difficult to tell apart in the field; there are so many of them and they look so alike. This group has ‘radiated’, and continues to do so.

The white-eyes, little greenish garden birds found from South Africa to Japan, are also expanding. With new varieties evolving on islands and mountain ranges, there are about 88 white-eye species today.

The finches Darwin encountered on the Galapagos are the most celebrated example of “radiation”. Descended from a few individuals, which arrived accidentally on the islands in the remote past, they split into 15 varieties, each one exploiting a particular niche. It was thought that such radiation occurred mainly in tropical regions, like equatorial South America, as these have the greatest diversity of species. However, according to the new research, radiation happens just as frequently in cool temperate areas. Gulls, for example, are a group on the evolutionary march, as the confusing relationships between our familiar herring gull and its cousins demonstrate.

But, although distance from the equator isn’t a factor in species creation, east-west orientation is. Radiations have tended to occur in Asia, North America and southern South America but not elsewhere. Just why this should be so is an even greater mystery.

* The Global Diversity of Birds in Space and Time; W. Jetz, G H Thomas, J B Joy, K Hartmann and A O Mooers; Nature 11631.