Selection in Natural Populations

By Jeffry B. Mitton | Go to book overview

4
The Impact of a Single Gene

Human judgment is notoriously fallible and perhaps seldom more so than in facile decisions that a character has no adaptive significance because we do not know the use of it.

G. G. Simpson, The Major Features of Evolution ( 1953)

Some theoretical population geneticists have been reluctant to ascribe an important role to balancing selection in the maintenance of protein polymorphisms. Part of this reluctance is due to concern about genetic load. But much of it springs from the difficulty of knowing how a single polymorphism--buried deep in the metabolism of a liver, muscle, or leaf--could substantially contribute to variation in fitness. Many population geneticists have said that the only locus for which we have convincing evidence for balancing selection is the gene for sickle cell hemoglobin in humans, but this statement is no longer true.

Although many studies have explored the adaptive significance of protein polymorphisms, most of them have been indirect and inferential. Koehn conducted the first study that described the geographic variation of enzyme allelic frequencies as a consequence of enzyme kinetics and environmental variation ( Koehn 1969; Koehn and Rasmussen 1967). The locus coding for serum esterase in the Gila mountain sucker, Catostomus clarkii, segregates two alleles, a and b, which differ in their rates of migration through a starch gel. Allelic frequencies were estimated from samples taken from natural populations in Arizona, New Mexico, Colorado, Utah, Nevada, Wyoming, and Idaho. Over a distance of 525 miles, the frequency of the b allele varied from 0.18 in Preston Springs, Nevada, to 1.00 in the San Pedro River in Arizona, and it decreased linearly with the latitude of the collection locality.

The pattern of geographic variation of serum esterases of the Gila mountain sucker is consistent with the variation in enzyme activity among the three genotypes. Enzyme activities were measured in a spectrophotometer at 0, 8, 25, and 37°C. At 37°C, the bb genotype had more than ten times the activity of the aa genotype. At 10°C, the pattern reversed; the aa genotype had nearly ten times the activity of the bb genotype. At both 0 and 37°C, the activity of the heterozygote was intermediate between that of the homozygotes, but in the temperature range of 8 to 25°C, the heterozygote had the highest activity. The bb homozygote, which had by far the highest activity at the high tempera

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