Genetic variation drives adaptation and evolution. The early eugenicists and modern proponents of "ethnic purity" ignore the fact that heterozygous organisms tend to be more vigorous. Many alleles are responsible for lethality, sterility, or other defects when they are homozygous. Thus, inbreeding in populations with diverse genotypes usually results in a loss of fitness, defined as the probability that an organism will produce offspring. In one species of fruit fly, almost 70% of chromosomes 2 and 3 contain alleles that cause male sterility when homozygous.
Many domesticated plants and animals have been inbred for centuries, with nonvigor-ous offspring eliminated through artificial selection. Nevertheless, the vigor of some of these can be increased by crossing them to form hybrids. Hybrid crops are often more productive. However, the vigor commonly declines with subsequent generations in hybrids, so the hybrids must be regenerated from the original breeding stocks for each planting.
In natural populations, inbreeding is more often deleterious. About 6.7% of human genes are heterozygous. Inbreeding in human populations increases the rates of spontaneous abortions, birth defects, and genetic disease. Many endangered species have very low heterozygosity. The cheetah is only 0.07% heterozygous, giving it a low capacity to adapt to changes such as to challenges from new disease organisms. Within humans, the variation between racial subgroups is accounted for by about 10% of the genome. The difference in skin color between Europeans and Africans is controlled by only two to five genes.
Genetic variation between species can be studied by looking at the number of differences in (1) the amino acid sequence of one of their proteins, or (2) the nucleotide sequence of a particular gene. At the genetic level, the distinction between similar species can be very small. Two species of fruit fly, for example, differ in DNA sequence by only about 0.55% located at about 15 to 19 genes. Yet their morphology and breeding behaviors are drastically different.
The amount of time that has elapsed since two species diverged from a common ancestor is sometimes known from paleontological data. By examining the number of protein or nucleotide sequence differences that separate two such species, it is possible to calibrate a molecular clock, to measure the time since other species diverged by evolutionary change. These data can be used to construct genetic ''family trees,'' called phylogenetic trees, to show the degree of relationships. For example, the phylogenetic tree in Figure 6.9 was developed on the basis of a measure of DNA hybridization. This value was calibrated by comparison with fossil evidence to determine the rate at which mutations accumulate in the genome. The figure shows that chimpanzees and humans are more closely related to each other than either is to the gorilla. This type of analysis has led to revision of many taxonomic classifications formerly based on measures that are more ambiguous.
Chimpanzee (Pan troglodytes) Pigmy Chimpanzee (Pan paniscus) Human (Homo sapiens) Gorilla (G. gorilla) Orangutan (Pongo pygmaeus) Common Gibbon (Hylobates lar) Siamang (H. syndactylus) Old World Monkeys (Cercopithecidae)
30 20 10 0
Millions of years ago
Figure 6.9 Phylogenetic tree for some primates. (Based on Klug and Cummings, 1997.)
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