AP Biology Notes: Natural Selection, Gene Pools, and Speciation

Natural Selection and Gene Pools

Natural selection is the process by which organisms better adapted to their environment survive and reproduce, leading to changes in allele frequencies within a population. Key concepts include:

  • Variation in Populations: There is variation among individuals in a population, and this variation is heritable.
  • Gene Pool: The gene pool consists of all the alleles present in a population, described by allele frequencies.
  • Selective Pressures: Limited resources create selective pressures such as competition for mates and escape from predators.
Hardy-Weinberg Principle

The Hardy-Weinberg principle describes allele frequencies in a population when no evolution is occurring. Key formulas include:

  • p2+2pq+q2=1p^2 + 2pq + q^2 = 1 where:
    • pp = frequency of dominant allele
    • qq = frequency of recessive allele
      This principle requires certain conditions to be met, including no mutations, random mating, no selection, and large population sizes.
Factors Changing Allele Frequencies

Several mechanisms can lead to changes in allele frequencies:

  1. Natural Selection: Favoring certain genotypes leads to adaptive changes.
  2. Genetic Drift: Random events can have significant impacts, especially on small populations.
  3. Gene Flow: Movement of alleles between populations can homogenize genetic differences.
  4. Mutation: Introduces new alleles into a population, contributing to genetic variation.
  5. Nonrandom Mating (Sexual Selection): Mating preferences can influence allele frequencies by favoring certain traits over others.
Speciation Process

Speciation occurs when populations are geographically isolated, leading to divergent evolution. The steps include:

  1. Geographic Isolation: Physical barriers separate populations, preventing gene flow.
  2. Diverse Selective Pressures: Different mutations can arise and be favored in each population.
  3. Reproductive Barriers: When two populations diverge sufficiently, reproductive barriers (such as temporal or behavioral isolation) arise, leading to the formation of new species.