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Evolution of Populations

Explore the concept of microevolution and variations in gene pools, including the importance of relative frequencies of alleles and sources of genetic variation. Learn about single-gene and polygenic traits, natural selection, genetic drift, and the Hardy-Weinberg principle. Discover how natural selection affects polygenic traits through directional, stabilizing, and disruptive selection.

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Evolution of Populations

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  1. Evolution of Populations Microevolution

  2. Variations in Gene Pools • Microevolution: evolution on the smallest scale – generation to generation change in the frequencies of alleles in a population • Gene Pool: consists of all the genes that are present in a population • Relative Frequency: The number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene appear

  3. Relative Frequencies of Alleles Sample Population Frequency of Alleles allele for brown fur allele for black fur 48% heterozygous black 16% homozygous black 36% homozygous brown

  4. Sources of Genetic Variation • Mutations: a change in the genetic sequence. Some may produce changes in the phenotype that result in better fitness. • Gene Shuffling: which results from the shuffling of gametes and sexual reproduction; produces many different combinations of genes.

  5. Single Gene & Polygenic Traits • A single gene trait is controlled by 1 gene with 2 alleles. Ex. Widow’s peak. • Polygenic Traits: Each gene has two or more alleles which results in many possible genotypes and phenotypes. Frequency of phenotype No widow’s peak Widow’s peak Frequency Phenotype Height

  6. Natural Selection of Single-gene Traits • Can lead to change in allele frequencies and thus evolution.

  7. Genetic Drift • The random change in allele frequency. • In a small population, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences of this type can cause an allele to become more common.

  8. Genetic Drift Sample of Original Population Descendants Founding Population A Founding Population B Founder’s Effect: Allele frequencies change as a result of the migration of a small subgroup.

  9. Evolution Versus Genetic Equilibrium • Hardy-Weinberg Principle (1908) states that allele frequency in a population will remain constant unless one or more factors cause those frequencies to change. • Genetic Equilibrium: allele frequencies remain the same.

  10. Five Conditions of Hardy-Weinberg • Random mating – no sexual selection • Large population – no genetic drift • No migrations – immigration or emigration • No mutations • No natural selection • STOP

  11. Natural Selection on Polygenic traits • Can affect the distribution of phenotypes in three ways: direction, stabilizing, or disruptive.

  12. Graph of Directional Selection Key Directional Selection Low mortality, high fitness High mortality, low fitness Food becomes scarce. The environment favors one extreme

  13. Graph of Stabilizing Selection Stabilizing Selection Key Low mortality, high fitness The average Organism is More fit High mortality, low fitness Selection against both extremes keep curve narrow and in same place. Percentage of Population Birth Weight

  14. Graph of Disruptive Selection Disruptive Selection Largest and smallest seeds become more common. Key Population splits into two subgroups specializing in different seeds. Low mortality, high fitness Number of Birdsin Population Number of Birdsin Population High mortality, low fitness Beak Size Beak Size Organisms with either of the two Extremes are better fit.

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