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Transcript
Chapter 16 What Darwin didn’t know…. ◦ ◦ How traits were inherited What caused variations Mutations – change in the DNA Sexual reproduction ◦ ◦ “gene shuffling” crossing-over Sexual reproduction by itself does not cause evolution to occur….. ◦ Gene pool - All the genes (alleles) in a given population. Population - A group of individuals of the same species that interbreed. ◦ Relative Frequency - The number of times an allele appears in a population relative to other alleles. Sample Population 48% heterozygous black 36% homozygous brown 16% homozygous black Frequency of Alleles allele for brown fur allele for black fur Evolution happens when the relative frequency of alleles in a population changes. ◦ What can cause relative frequencies of alleles to change? Single-gene traits have 2 or 3 phenotypes. ◦ Natural selection can lead to changes in allele frequencies. Frequency of Phenotype (%) 100 80 60 40 20 0 Widow’s peak Phenotype No widow’s peak Polygenic traits are more complicated; there are many phenotypes. ◦ Natural selection affects the distributions of phenotypes. Frequency of Phenotype Phenotype (height) “normal distribution” Key Directional Selection Food becomes scarce. Low mortality, high fitness High mortality, low fitness Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness Birth Weight Selection against both extremes keep curve narrow and in same place. Disruptive Selection High mortality, low fitness Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Low mortality, high fitness Number of Birds in Population Key Largest and smallest seeds become more common. Beak Size Natural selection is not the only source of evolutionary change. ◦ Small populations migrating to a new habitat can quickly cause a change in allele frequencies. This results in a small number of individuals having a profound effect on gene frequencies. The is called genetic drift or the “founder effect” Darwin’s finches are an example of genetic drift. Sample of Original Population Founding Population A Founding Population B Descendants 1. 2. 3. 4. 5. Five conditions for genetic equilibrium (HardyWeinberg principle): Random mating Very large population No migration in or out No mutations No natural selection All five conditions can’t exist at once, so evolution must happen. Formation of a new species. 1. Isolating mechanisms separate a founding population Geographic isolation Behavioral isolation Temporal isolation 2. Natural selection and genetic drift causes changes in the gene pool 3. Members of the two populations can no longer interbreed and are reproductively isolated Ex: Darwin’s finches Reproductive Isolation results from Isolating mechanisms which include Behavioral isolation Geographic isolation Temporal isolation produced by produced by produced by Behavioral differences Physical separation Different mating times which result in Independently evolving populations which result in Formation of new species