BIOL 1001 Reading 3.docx

Directional selection type of natural selection where

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Directional Selection – type of natural selection where the average phenotype of a population changes in one direction o Usually decreases genetic variation o If this selection increases, favoured alleles will have a frequency of 1.0, meaning it will be fixed Non-favoured alleles will have a frequency of 0.0, meaning it will be lost o Purifying selection – when disadvantageous alleles decline in frequency Stabilizing Selection – reduction of extreme phenotypes in a population o There is no change in average value of a trait over time o Genetic variation is reduced Disruptive Selection – reduction of intermediate phenotypes; favours extremes o Genetic variation maintained o Can lead to formation of new species Balancing selection – no single allele has an advantage nor increases in frequency, instead there is a balance of alleles based on their fitness and frequency o Dependent on the time and change in environment o Certain alleles are favored when they are rare ie. Rare coloured guppies to fool predators in a process known as frequency-dependent selection Genetic Drift o Allele frequencies change due to blind luck in a process known as sampling error Gene flow
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o Sometimes a high fitness allele for one environment may be introduced to a new population of a new environment. The gene flow of this allele may be of low fitness for the new environment, having negative effects for the bird population of the new environment.
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