Evolucion

Flapping through the bottleneck
by Rosemary Sullivant
Bones of ancient birds speak of a past mass extinction, but modern genes tell of a large flock of survivors.
Most birds alive during the time of the dinosaurs seem to have suffered the same fate as those doomed beasts. Or so say their bones. Few, if any fossils of modern birds pre-date the mass extinction sixty-five million years ago thatwiped out sixty to ninety percent of Earth's creatures, including the dinosaurs. Only afterward do fossils of modern birds begin to appear. The bones say this extinction was an evolutionary bottleneck, and the lucky few birds who squeezed through spread their wings and flew off to populate the avian world, quickly diversifying into today's nine thousand or so species.
But fossils may not tell thewhole bird story. By using the DNA of living birds as a "molecular clock" that counts backwards in time to the beginning of a species, molecular evolutionist Alan Cooper of Oxford University and theoretical biologist David Penny of Massey University in New Zealand have found that at least twenty-two bird lineages flapped right through the great extinction. Actually, most had their beginningsmillions of years before the great die-off, known as the Cretaceous-Tertiary, or K-T, extinction. So many birds made it past the event, they say, that it was more like a mass survival. "Because this is such a common phrase -- the K-T mass extinction -- we're playing devil's advocate a bit by saying it is the other way around," Cooper notes. Indeed, the claim has caused a bit of flap among some birdexperts.
Molecular clocks work because, after two species split from a common ancestor, each takes its own genetic path. As the years pass, the DNA of each species steadily accumulates mutations. These mutations are the ticks of the clock, and if the rate at which they occur can be determined, researchers can calculate how long ago the two lines diverged. For instance, if there are ten mutationsthat differ between two species, and one mutation occurs every thousand years, then the two lineages must have split about ten thousand years ago.
Cooper and Penny looked at the differences in DNA sequences in small sections of genes from sixteen modern orders of birds. First they chose a pair of closely related birds, such as rheas and ostriches, and counted the number of differences in a genefragment found in both birds. Next, the researchers used the oldest known fossil of the pair to date when the two diverged and established a rate of change. For example, the oldest rhea fossil is sixty million years old, much older than the oldest ostrich fossil. Cooper and Penny established sixty million years ago as the date for when the two lineages split. They then calculated the mutation ratefor each pair of birds. In the case of the rhea and ostrich, the birds had acquired fifty-seven genetic differences over the sixty-million-year period. Cooper and Penny repeated the process with loons and shear-waters, another pair of birds that are distantly related to the first group. Then they averaged the mutation rates of the two pairs to come up with a frequency of about one mutation permillion years.
With the rate set, the researchers then simply took the genes within the fragment that the individuals of a pair shared and counted the number of genetic differences that actually separated the two pairs. There were thirty-eight such mutations, which meant about thirty-eight million years of change. Since those differences did not exist between the individuals of the pair, they musthave occured before each pair diverged. So Cooper and Penny added those thirty-eight million years onto the oldest fossil date of sixty million years, ending up with a grand total age for the lineage of ninety-eight million years. And that's about thirty million years before the K-T extinction.
Using the same method for other related bird groups, Cooper and Penny concluded that most modern birds...