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Transcript
Evolution as Genetic Change Evolution as Genetic Change • Natural selection acts on phenotypes, survival and reproduction determine which alleles are inherited, changing relative frequencies of alleles in a population over time. • Thus evolution is any change in the relative frequencies of alleles in a population’s gene pool and acts on populations, not individuals. Evolution of Single-Gene Traits • Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. – One of the two phenotypes may make an organisms better fit, thus under pressure from natural selection and its relative frequency will increase Single Allele Selection • Which phenotype has higher fitness? – Orange • Which allele’s frequency will decrease? – Green Evolution of Polygenic Traits • Natural selection can affect the distributions of phenotypes in any of three ways: – Directional selection – Stabilizing selection – Disruptive selection Graph of Directional Selection Section 16-2 • Directional Selection: When the entire bell moves left/right because there’s a higher fitness and increase in Directional Selection the number of individuals with the trait at one end of the curve. – Traits at one end or the other are selected for Key Low mortality, high fitness Food becomes scarce. High mortality, low fitness Directional Selection Section 16-2 Stabilizing Selection • Stabilizing selection: When the bell becomes more narrow, because there’s a higher fitness and increase in the number of individuals with the trait in the center of the curve Stabilizing Selection Key Low mortality, high fitness High mortality, low fitness • The average trait is selected for Birth Weight Selection against both extremes keep curve narrow and in same place. Evolution of Clutch Size Section 16-2 Disruptive Selection • Disruptive selection: The bell can split into two, because there’s a higher fitness and increase in the number of individuals at both ends of the curve • Traits at both “extremes” are selected for Disruptive Selection Low mortality, high fitness High mortality, low fitness Population splits into two subgroups specializing in different seeds. Beak Size Number of Birds in Population Key Number of Birds in Population Largest and smallest seeds become more common. Beak Size Disruptive Selection Genetic Drift • Populations can also evolve without selection pressure through the process of genetic drift. • Genetic drift = random change in allele frequencies • In small populations, individuals that carry a particular allele may leave more descendants than other individual, just by chance. Over time, a series of chance occurrences can cause an allele to become common in a population. • Genetic drift can happen when a small group of individuals colonize a new habitat carrying different relative frequencies that the larger population. • 2 special Cases: • Founder effect = allele frequencies change as a result of the migration of a small subgroup of a population • Bottle Neck= a population experiences a great reduction in the gene pool, leaving only a small subset of alleles behind. Results in inbreeding. Genetic Drift- Chance Genetic Drift- Founder Effect Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B Genetic Drift- Founder Effect Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B Genetic Drift- Founder Effect Section 16-2 Sample of Original Population Descendants Founding Population A Founding Population B Genetic Drift Bottleneck Genetic Equilibrium • Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more of a set of factors causes the population to change. • The following conditions that must be met to avoid evolution: – Random mating- no mate preferences, or choice (rare) – Large population- lots of diversity (less chance of genetic drift) – No movement into or out of the population- individuals don’t move between populations, carrying new alleles – No mutations- mutations change the DNA – No natural selection- all individuals have an equal chance of surviving and reproducing Hardy-Weinberg (p2) + (2pq) + (q2) = 1