EvolutionMechanisms160 - Evolutionary Mechanisms - 1 The...

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Evolutionary Mechanisms - 1 The Gene Pool and Genetic Equilibrium As we stated at the beginning of our discussion on evolutionary principles, evolution involves changes that occur in the frequency of a gene's alleles in a population from generation to generation. Each individual member of a population inherits a set of genes, and for most of these genes, two alleles. He or she cannot evolve or change the alleles inherited. But the contribution he or she makes to the population's gene pool through reproduction, relative to the contribution other members of the population make, can change the population's genetic composition from generation to generation. The collection of genes (alleles) in a population is called the gene pool. For a population's gene pool to change, there must be some mechanism that promotes differential reproduction or differential survival of one allele that is reflected in reproduction. When such change occurs, we have evolution. A review of the inheritance of a single gene illustrates this. In a population of cats, assume there are two pre-existing alleles for coat color: black and white. Black is dominant. The two coat colors have been reproduced year after year. Then one year, a new nocturnal predator enters the environment. This predator sees the white cats at night and eats them. The white cats rarely reproduce, since they are eaten when young and are more visible to predators. Within a few generations, the frequency of the white allele diminishes significantly. The predator has been the selection force behind the change in coat color allele frequency. Initial Frequency of coat color alleles We can see this and explain this today, because we know how genes and alleles are inherited. In the 1800's, they did not know this, and for about 50 years after Darwin's publications, scientists and others searched for mechanisms of evolution. It took several years to bring together genetics, population biology and natural selection as means of evolution. It started in 1908. The HARDY-WEINBERG principle for genetic equilibrium Hardy and Weinberg demonstrated that the equation for a binomial expansion (p 2 + 2 pq + q 2 = 1) could be used to calculate gene frequencies within a population. They first showed the gene frequency needed for genetic equilibrium, the condition in which gene frequency would not change from generation to generation, hence no evolution.
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Evolutionary Mechanisms - 2 Genetic Equilibrium Formula Where: p = frequency of 1 st allele for gene (A) q = frequency of the 2 nd allele for gene (a) and: p + q = 1 Where: p 2 = homozygous (AA) q 2 = homozygous alternative (aa) 2pq = heterozygous (Aa) and: p 2 + 2 pq + q 2 = 1 The allele frequencies and genotype frequencies will be stable (genetic equilibrium) from generation to generation as shown by this equation. In an ideal population there would be genetic equilibrium (no change in gene
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This note was uploaded on 01/06/2012 for the course BIOLOGY 106 taught by Professor Rosemaryrichardson during the Fall '08 term at Bellevue College.

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EvolutionMechanisms160 - Evolutionary Mechanisms - 1 The...

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