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I.9.3 Hardy- Weinberg law

Darwin’s theory of evolution by natural selection requires heritable variation for selection to work on. Heritability centered on the idea of “blending inheritance”—the hypothesis that the offspring receive some average mix of the parental characters. If this were true, each generation would be more average than the last and variation would steadily decline. To make natural selection work, Darwin had to suppose that the frequency of mutations— was high.

Mendel had proposed that each character is controlled by two genetic factors (genes), one from each parent. Most characters have a dominant form and a recessive form (phenotype). The different phenotypes result from different versions of the gene, called alleles, whether they are the same or different in the two chromosomes. The dominant phenotype is expressed when one or both alleles are dominant; when both alleles are recessive, the recessivephenotype is seen. It was not immediately obvious that Mendel’s model had anything to say about evolution; biologists assumed that dominant alleles would slowly displace their recessive counterparts, thus inexorably reducing variation. In 1908, however, G. H. Hardy and W. Weinberg independently realized that Mendelian inheritance would, in fact, maintain variation at current levels under most circumstances.

The Hardy–Weinberg principle, also known as the Hardy–Weinberg equilibrium, model, theorem, or law, states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences. These influences include genetic driftmate choiceassortative matingnatural selectionsexual selectionmutationgene flowmeiotic drivegenetic hitchhikingpopulation bottleneckfounder effect and inbreeding.

The conditions necessary for this Hardy-Weinberg equilibrium in a sexually reproducing population are:

  1. The population needs to be large; otherwise, chance effects could lead to genetic drift—a random change in allele frequencies that usually leads to the local extinction of one of the two alleles.
  2. Mutations must not be common, since mutations produce new alleles.
  3. There must be no immigration or emigration that alters allele frequency.
  4. Mating must be random with respect to alleles.
  5. Reproductive success must be random with respect to alleles. If any of these conditions is violated, evolution (a change in allele frequency) will occur.

Violation of condition 5, of course, is the essence of natural selection. (Condition 4,which Darwin called “sexual selection,” is only a special case of condition 5.)

Five factors can change genotype frequencies – nonrandom mating, gene flow, genetic drift, mutation, and natural selection.

The Hardy-Weinberg formulation not only accounts for how variation is maintained, it identifies alternative mechanisms for evolution—genetic drift, for instance. What Hardy and Weinberg could not know at the time is that most genes are pleotropic; that is, each has multiple effects of the phenotype. Moreover, most phenotypic characters are controlled by a combination of many genes. Each of these genotype/ phenotype interactions helps preserve variation. In addition, sexual recombination operates to produce a nearly infinite number of novel combinations of alleles in each generation.

Applications of Hardy-Weinberg Law

The formula provides a standard against which genetic change in a population may be measured and predicted. The formula serves as a basic theorem which can be expanded and elaborated by other mathematical models that deal with changes in populations.

The Hardy-Weinberg formula may be applied to large populations to provide an estimate of gene frequencies at a single point in time.

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