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Hardy Weinberg Principle (equilibrium)

Hardy Weinberg Principle (equilibrium). A llele frequencies will remain constant unless one or more factors cause the frequencies to change. If there is no change, there is no evolving. When will evolution not occur? What conditions must exist under which evolution will not occur?

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Hardy Weinberg Principle (equilibrium)

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  1. Hardy Weinberg Principle (equilibrium) • Allele frequencies will remain constant unless one or more factors cause the frequencies to change. • If there is no change, there is no evolving. • When will evolution not occur? • What conditions must exist under which evolution will not occur? • How can we tell if evolution won’t take place?

  2. Can Individuals Evolve? • Genes determine most of an individual’s features. • Individuals cannot evolve a new phenotype in response to their environment. • Populations, not individuals, evolve.

  3. Evolution of a Population • Natural selection acts on the range of phenotypes in a population. • Evolution occurs as a population’s genes and their frequencies change over time.

  4. How can a population’s genes change over time? • All of the alleles of a population’s genes together make up a gene pool. • Allele frequency - % of any specific allele in the gene pool. • Genetic equilibrium – a population in which the frequency of alleles remains the same over generations.

  5. Conditions that cause changes in Genetic Equilibrium • A population in genetic equilibrium is not evolving. • Mutations are one cause of genetic change. • Lethal mutations disappear quickly, but mutations that cause a useful variation become part of the gene pool

  6. Conditions that cause changes in Genetic Equilibrium • Genetic drift – the alteration of allele frequencies by chance events. • Gene flow – transport of genes into or out of a population by migrating individuals. • Genetic drift, gene flow, and mutations can greatly affect small populations. • Natural selection is usually the most significant cause of changes in any gene pool.

  7. Natural Selection Acts on Variation • Some variations increase or decrease an organism’s chance of survival in an environment. • Variations are controlled by alleles. • Allelic frequencies in a gene pool will change due to natural selection of variations.

  8. If conditions are not met the genetic equilibrium is disrupted – *population evolves and changes* • If conditions are met the genetic equilibrium stays the same– *population does not evolve*

  9. Genotype Proportions • Genotype proportions remain constant – calculated from allele frequencies P+Q = 1 or 100 percent (phenotype frequencies) P2 + 2PQ + Q2 = 1 or 100 percent (genotype frequencies)

  10. Key Information • P = frequency of the dominant allele in the population • Q = frequency of the recessive allele in the population • P2 = percentage of homozygous dominant individuals • Q2 = percentage of homozygous recessive individuals • 2PQ = percentage of heterozygous individuals

  11. Question • If 98 out of 200 individuals in a population express the recessive phenotype, what percent of the population are homozygous dominant? Answer: Recessive phenotype = homozygous genotype P + Q = 1 Q2 = 98/200 = 0.49 or Q = 0.7 P + .7 = 1 P + 1 - 0.7 = 0.3 or P = 0.3 P2 = 0.32 = 0.09

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