Charles G. Sibley advanced a very controversial methodology to compare the DNA of differing species. Sibley initially attempted to determine the extent of genetic differences between bird species--but his work was ultimately expanded to characterize the differences between chimps, gorillas, and humans. In contrast to previous studies that indicated that chimps, gorillas, and humans were all equally distant from each other (each was a genetically distinct line from a common ancestor) within phylogenic classifications. Sibley's methodology indicated that humans were closer to chimps in terms of DNA or gene content.
Sibley initially developed a genetically based system of bird taxonomy. In conjunction with Jon Ahlquist, he pioneered a new technique to create DNA-DNA hybridizations in order to determine the evolutionary relationships among birds. Sibley's objective was ultimately to devise a methodology to determine how similar the DNA of one species is to that of another species. The more similar the DNA, the more evolutionary related are the two species (i.e, more closely related they are to a common ancestor).
Sibley obtained DNA from a species and separated the double helix into its complementary strands by applying heat. The strands were then labeled with a radioactive isotope and combined with strands from another species. The mixture was incubated at 140° Fahrenheit for 120 hours causing complementary strands to bond forming DNA hybrid molecules. Sibley's hypothesis depended on measuring how much heat was necessary to separate the DNA hybrids and hypothesized that that measure of heat determined how closely related the two species of birds were. DNA molecules from the same species had the highest melting temperature. In addition, dissimilar strands melt at a lower temperature because the less bonding occurs due to lack of complementary nitrogenous bases. The lower the melting temperature required to separate the hybrid, the less similarity existed between the DNA of the two species, and the greater the measure of genetic distance.
Sibley and Ahlquist compared 26,000 DNA hybrids from 1700 species of birds and constructed a genetically based phylogeny from their findings.
Sibley's theories were ultimately disproved because he assumed equal rates of change among lineages when he constructed the phylogeny; however, time between generations of birds varies from species to species and thus, affects the rate of DNA evolution. In addition, the distance measures from the data were pooled together and the calculated statistics did not account for variation in the data. In effect, his classification system was argued to be mathematically unreliable.
Sibley was born in Fresno, California. He earned his Ph.D. in zoology from the University of California, Berkeley.
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