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Early population geneticists were forced to rely on the phenotypic traits that had a simple genetic basis. Variation in the spotting patterns of the butterfly Panaxia dominula is an example.

Measures of genetic variation The amount of genetic variation in populations is commonly measured by two parameters. The proportion of polymorphic loci is the proportion of examined loci in which more than one al-lele is present in a population. If we examined 30 different loci and found two or more alleles present at 15 of these loci, the percentage of polymorphic loci would be 15/30 = 0.5. The expected heterozygosity is the proportion of individuals that are expected to be heterozygous at a locus under the Hardy-Weinberg conditions, which is 2pq when there are two alleles present in the population. The expected heterozygosity is often preferred to the observed heterozygosity because expected heterozygosity is independent of the breeding system of an organism. For example, if a species self-fertilizes, it may have little or no heterozygosity but still have considerable genetic variation, which will be detected by the expected heterozygosity. Expected heterozygosity is typically calculated for a number of loci and is then averaged over all the loci examined.

23.19 Molecular variation in proteins is revealed by electrophoresis. Tissue samples from three fruit flies have been subjected to electrophoresis and stained for malate dehydrogenase. Homozygotes are represented as single bands; heterozygotes as triple bands. The genotype of each fly is given below each sample.

The percentage of polymorphic loci and the expected heterozygosity have been determined by protein elec-trophoresis for a number of species (Table 23.8). About one-third of all protein loci are polymorphic, and expected heterozygosity averages about 10%, although there is considerable diversity among species. These measures actually underestimate the true amount of genetic variation, though, because protein electrophoresis does not detect some amino acid substitutions; nor does it detect genetic variation in DNA that does not alter the amino acids of a protein (synonymous codons and variation in noncoding regions of the DNA).

Explanations for protein variation By the late 1970s, geneticists recognized that most populations possess large amounts of genetic variation, although the evolutionary significance of this fact was not at all clear. Two opposing hypotheses arose to account for the presence of the extensive molecular variation in proteins. The neutral-mutation hypothesis proposed that the molecular variation revealed by protein electrophoresis is adaptively neutral; that is, individuals with different molecular variants have equal fitness. This hypothesis does not propose that the proteins are func-tionless; rather, it suggests that most variants revealed by protein electrophoresis are functionally equivalent. Because these variants are functionally equivalent, natural selection does not differentiate between them, and their evolution is shaped largely by the random processes of genetic drift and mutation. The neutral-mutation hypothesis accepts that

Table 23.8 Proportion of polymorphic loci and heterozygosity for different organisms, as determined by protein electrophoresis

Proportion of

Polymorphic Loci Heterozygosity

Table 23.8 Proportion of polymorphic loci and heterozygosity for different organisms, as determined by protein electrophoresis

Group

Number of Species

Mean

SD*

Mean

SD*

Plants

0 0

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