On Evolution

5. Nature’s Family Archives

Since the late s, mitochondrial DNA (mtDNA) has been an important tool for genealogical research in evolutionary biology. Key features of the molecule in most animals are its rapid evolution and its asexual mode of transmission through female lines. Indeed, some remarkable genealogical parallels exist between maternally inherited mitochondrial lineages and paternally inherited human family surnames. This chapter comes from a paper written for a general audience in which Avise draws explicit connections between these seemingly different kinds of historical information. Family names were first used in China during the Han Dynasty (about the time of Christ), but the widespread practice of assigning hereditary surnames to family lines came relatively recently to most parts of the world. In England, surnames were not customary until at least the fourteenth century, and in Japan, only the governing classes were allowed surnames until , when a cabinet decree mandated their adoption by the entire populace. Hereditary surnames help to organize family records in complex societies. They also provide a chronicle of recent human history—a record that can be read to reveal patterns of human dispersal and settlement. Evolutionary biologists have a similar record for other animal species, and can trace the lineages of a species back through time to reveal the origin and subsequent dispersal of family groups. This record resides in especially clear form in the molecular makeup of a piece of genetic material called mitochondrial DNA (or mtDNA). Although both sons and daughters take the surname of their father , only sons pass it on to the next generation in most families. Analogously, sons and daughters both inherit the mtDNA of their Nature’s Family Archives 5 mother, but only daughters pass mtDNA to their progeny. This is because mitochondria—cell components sometimes called the “power plants of a cell”—are found not in the cellular nucleus, but outside it in the cell’s cytoplasm, away from the chromosomes. When sperm and egg unite to form a new organism, they contribute equally to the genetic material in the new cell’s nucleus, but for the most part, the cytoplasm is the contribution of the egg alone. Males are therefore as irrelevant to mitochondrial heredity as females are to traditional human surname heredity. This single parent transmission greatly simplifies evolutionary bookkeeping. As mtDNA makes copies of itself, sometimes independently of cell division, mutations occur. By counting the number of mutations that distinguish populations of known age, one can determine the approximate rate at which mtDNA differences accumulate. With the rate of evolution calibrated, one can then estimate the length of time that any other lineages have been separated from one another by comparing their mutational differences. In vertebrates, mtDNA typically evolves up to ten times faster than nuclear DNA. Many mtDNA mutations appear neither to harm nor help their carriers, but much like surnames—Smith, Smyth, Smithers, for example—they simply distinguish one group from another. Most species have a great number of these idiosyncratic genotypes, which can give clues to the place of a species’ origin just as a surname may indicate nationality. For example, based on comparisons of mtDNA among desert tortoises of the North American southwest, biologist Trip Lamb and I found evidence for three population subdivisions. One group lives in southern Sonora, Mexico; the second lives in desert and subtropical scrublands throughout western and central Arizona and northern Sonora; and the third apparently lives only in southern California, Nevada , and Utah. The Colorado River valley separates the last two. The “clock” calibrations suggest that the mtDNAs of these three groups have been separated from one another for several million years. The timing of the split between two major branches in the tortoise ’s mtDNA phylogenetic tree can be related to the geology of the  On Evolution Colorado River basin. About six million years ago, a -to -milewide embayment of brackish waters, the Bouse Sea, reached northward from the Gulf of California to the current Lake Mohave area in southern Nevada. The sea may have split the ancestral population of the desert tortoise. Subsequent geological uplifting resulted in the sea’s retreat and the formation of the modern Colorado River, which, until dammed in this century, probably continued to separate the tortoise populations. Freshwater fishes in the southeastern United States exhibit similar genetic breaks. Eldredge Bermingham, while a graduate student in my laboratory, surveyed the spotted sunfish in rivers along the coastal plain...