16.2 Evolution as Genetic Change

Transcription

1 16.2 Evolution as Genetic Change 1 of 40

2 16-2 Evolution as Genetic Change 16-2 Evolution as Genetic Change If an individual dies without reproducing, it does not contribute to the gene pool. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. 2 of 40

3 Natural Selection on Single-Gene Traits Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Ex: hypothetical lizard population 3 of 40

4 Natural Selection on Single-Gene Traits 4 of 40

5 Natural Selection on Polygenic Traits On polygenic traits, natural selection can affect distribution of phenotypes in 3 ways: 1 directional selection 2 stabilizing selection 3 disruptive selection 5 of 40

6 16-2 Evolution as Genetic Change Directional Selection 6 of 40

7 Directional Selection Directional selection when individuals at one end of the curve have higher fitness (than the rest) 7 of 40

8 Stabilizing Selection Stabilizing selection when individuals near the center of the curve have higher fitness than individuals at either end of the curve Ex: average birth weight 8 of 40

9 Natural Selection on Polygenic Traits Disruptive selection when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle Ex: average-sized seeds become scarce 9 of 40

10 Genetic Drift Natural selection gives direction to changes in population adaptive change Genetic drift a change in allele frequency due to random events 10 of 40

11 Natural Selection on Single-Gene Traits Bozeman on Genetic Drift feature=player_embedded&v=mjq_yn5znyk 11 of 40

12 16-2 Evolution as Genetic Change Bottleneck 12 of 40

13 Bottleneck Bottleneck an event in which a population s size is greatly reduced (like squeezing through a bottleneck) may have a substantial effect on the population. Why? many genes may be lost from the population, reducing the population s genetic variation 13 of 40

14 Bottleneck Founder Effect when allele frequencies change due to migration of a small subgroup of a population 14 of 40

15 Evolution Versus Genetic Equilibrium The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. When allele frequencies remain constant it is called genetic equilibrium. 15 of 40

16 Evolution Versus Genetic Equilibrium Five conditions are required to maintain genetic (Hardy-Weinberg) equilibrium 1 random mating 2 large population 3 no movement into or out of the population 4 no mutations 5 no natural selection 16 of 40

17 Evolution Versus Genetic Equilibrium Random mating ensures that each individual has an equal chance of passing on its alleles to offspring. Genetic drift has less effect on large populations than on small ones. Immigration or emigration can bring alleles in or out of the gene pool. Mutations will introduce new alleles into the gene pool. All genotypes in the population must have equal probabilities of survival and reproduction. No phenotype can have a selective advantage over another. 17 of 40