Large, isolated populations whose members mate randomly and do not experience any selection pressure will tend to maintain a frequency of genotypes predicted by a simple equation called the Hardy-Weinberg Theorem. For example, if b has a frequency of 20 percent and B has a frequency of 80 percent, we can predict the frequency of the three genotypes (bb, Bb, and BB). The total of all the genotype frequencies is 100 percent (b + B), and the frequencies of each are given by (b + B)2 100 percent. This can be restated as the following equation:
100% = b2 + 2(bB + B2).
And we can calculate the genotype frequencies as:
100% = (20%)2 + 2(20% × 80%) + (80%)2 = 4% + 32% + 64%.
So even though 20 percent of all the genes in this imaginary population are b alleles, only 4 percent of the population is homozygous for b and actually has blue eyes. Furthermore, this same distribution will be maintained over time, as long as the conditions of the Hardy-Weinberg Theorem are met.
However, few, if any, natural populations (including human ones) actually conform to the assumptions of Hardy-Weinberg, so both genotype frequencies and allele frequencies can and do change from generation to generation. For example, humans do not mate randomly. Instead, they tend to take partners of similar height and intelligence. And even in modern human populations, genetic diseases such as Tay-Sachs kill children long before they grow up and reproduce. A difference in survival and reproduction due to differences in genotype is called selection. Even subtle selection can change gene frequencies over long periods of time.
Another assumption of the Hardy-Weinberg theorem is that individuals from different populations do not mate, so that gene flow, the passage of new genetic information from one gene pool into another, is zero. Such isolation does characterize many animal and plant populations, but almost no modern human populations are isolated from all other populations. Instead, humans travel to different countries, intermarrying and producing children who reflect the novel intermingling of unusual alleles.
- Population Genetics - Genetic Drift
- Population Genetics - Gene Pool And Genetic Structure
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