DNA and Passerine Classification
A new laboratory technique is beginning to solve persistent mysteries of the evolutionary relationships of various groups of birds. Evolutionists Charles Sibley and Jon Ahlquist have been comparing the DNA (the molecules that encode the genetic information) of different birds. In outline the technique is simple. DNA from one species is boiled briefly, which causes it to "melt" -- that is, to separate into its two complementary component strands. Then the single strands of one species are labeled with a radioactive isotope, mixed with unlabeled strands from a different species, and incubated at 140 degrees Fahrenheit for 120 hours. At that temperature complementary DNA strands bond together chemically. Sibley and Ahlquist then determine how much they must heat the DNA to melt the hybrid molecules that have formed. The more similar the genetic information coded into the DNA from the two species, the more tightly the two halves of their hybrid DNA molecules stick together (nonhybrid DNA, both halves from the same species, has the highest melting temperature). The lower the melting temperature, the more dissimilar the two DNA strands and more distantly related the two birds supplying the strands.
Fascinating results have emerged from these DNA-DNA hybridization studies. Sibley and Ahlquist's tens of thousands of DNA-DNA hybrid comparisons have revealed that each of two close relatives, say, a mockingbird and a thrasher, show the same genetic distance from a third less-related bird. For example, a mockingbird and a thrasher each form hybrid DNA with a finch that melts at roughly the same temperature. This means that the genetic distance between evolving bird species with the same generation time (e.g., passerines, which all mature at one year) always increases at roughly the same rate. Both the mocker and thrasher must have been separated for the same amount of time from the last common ancestor the two shared with the finch.

Below we give Sibley and Ahlquist's proposed classification of the passerine birds of North America, based on their DNA-DNA hybridization studies. It has features that will strike those familiar with older classifications as quite unusual. For instance, vireos and wood warblers have usually been thought to be closely related, but here they are not. Mockingbirds, thrashers, and catbirds are placed in the starling family, and wagtails and pipits are shown as relatives of African weaver finches and the House Sparrow group. On the other hand, the Yellow-breasted Chat is confirmed as a close relative of the wood warblers.

We believe that the Sibley-Ahlquist classification is much closer to biological reality than that recognized in the 1983 AOU Checklist. Since the Sibley-Ahlquist proposal is somewhat controversial and unfamiliar to most bird enthusiasts, however, the higher classification (arrangement of groups above the generic level-subfamilies, families, etc.) used elsewhere in this book follows the AOU treatment.

SEE: Species and Speciation; Birds, DNA, and Evolutionary Convergence.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.