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 the number of individuals with the trait at one end of the curve. Traits at one end or the other are selected for Directional Selection Key Low mortality, high fitness High mortality, low fitness Food becomes scarce.

Directional Selection

Stabilizing Selection Section 16-2 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 The average trait is selected for Key Percentage of Population Birth Weight Selection against both extremes keep curve narrow and in same place. Low mortality, high fitness High mortality, low fitness Stabilizing Selection

Evolution of Clutch Size

Disruptive Selection Section 16-2 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 Largest and smallest seeds become more common. Number of Birds in Population Beak Size Population splits into two subgroups specializing in different seeds. Key Low mortality, high fitness High mortality, low fitness

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