BIOL 111: Genetic Drift and Natural Selection

Darwin’s key observations

Members of a population vary in their traits, variation inherited from parents to offspring, all species capable of producing more offspring than environment can support, most offspring don’t survive (lack of resources)

Darwin’s inferences

Organisms that possess traits that increase probability that they will survive and reproduce leave greater numbers of offspring that are likely to possess traits, result in accumulation of favorable traits in population over generations

Evolution

Changes in allele frequency

Changes in allele frequency

Result from either genetic drift or natural selection

Requirements for evolution

Requires variation in the form of mutations: random with respect to the environment and in the germ line (heritable)

Meiosis and sexual reproduction

Crossing over increases the range of genotypic variation and the phenotypic variation that results

Hardy-Weinberg equilibrium conditions

Random mating, no mutations, no gene flow, large population, no selection

Conditions for evolution

Non-random mating, mutations, migrations, small population, selection

Genetic drift

“Sampling error”: individuals that are able to contribute to the next generation are not representative of the larger population, potent force in small populations but has little effect in large populations

Effect of genetic drift

Can counteract the effect of natural selection in small populations, natural selection is most effective in large populations where the effect of drift is minimized

Probability that neutral allele will go to fixation

1/(2N), N equals to population size, decreases with increasing population size

Bottlenecks

Lower genetic diversity for a long time

Natural selection

Results in adaptation

Relative fitness

Fitness of a genotype standardized by comparison to other genotypes

Directional selection

Extreme phenotype favored over other phenotypes, shift of allele frequency in direction of phenotype over time

Stabilizing selection

Middle phenotype favored, decline in variation over time

Disruptive selection

Both extreme phenotypes favored