• Genetic Variation: difference in DNA bases or sequences
    • Genetic variation enables evolutionary responses to environmental change
    • Evidence of genetic variation: diff genotypes or phenotypes
  • Natural selection acts on variation among individuals in the population and arises by…
  1. Mutations: are original source of new variation; invent alleles that didn’t exist in the gene pool
  • Natural selection and other mechanisms increase variation by rearranging existing alleles & mutations into new combinations
    • All new alleles are the result of nucleotide variability
  1. Sexual Reproduction creates individuals with new combinations of alleles; Genetic recombinations comes from: 
  • Crossing Over; Independent assortment of homologous chromosomes; Random joining of gametes
  1. Diploidy: presence of two copies of each chromosome in a cell
  • In heterozygous condition, recessive allele is hidden from natural selection → allows variation to be “stored” for future generations → maintains variation in gene pool
  1. Outbreeding: mating with unrelated partners increases the possibility of mixing different alleles and creating new combinations
  2. Balanced Selection selection itself may preserve variation at some loci →  helps maintain multiple phenotypic forms in a population
  • Often a single phenotype provides the best adaptations while others are less advantageous → favorable allele increases in frequency
  • Examples of polymorphism (2 or more diff phenotypes) can be maintained by
  1. Hybrid Vigor (Heterosis): superior quality of offspring resulting from crosses between two diff inbred strains of plants
  • Results from the reduction of loci with harmful homozygous recessive conditions and increase with heteroz advantage
  • Ex: a hybrid of corn in more resistant than either inbred strains
  1. Heterozygous Advantage
  • When heterozygotes have a greater fitness than either homozygous type→ both alleles and three phenotypes maintained in the population by selection
  • Ex: heterozygotes for sickle cell disease are ~healthy but oxygen-carrying impaired; provides resistance to malaria = higher % in Africa → both alleles are maintained in gene pool
  1. Frequency-Dependent Selection (minority advantage): the fitness of a phenotype depends on how common it is in the population.
  • Rare phenotypes are selected → become common → are selected against
    • Ex: rarer prey escape predators
  • Maintain multiple phenotypes (and their alleles) that alternate between high and low frequencies

What Decreases Variation

  • Stabilizing selection, sexual selection/nonrandom mating, genetic drift

Neutral Variation

Not all variation has selective variation → some (especially at molecular level in DNA and proteins) is neutral variation

    • Ex: differences in fingerprint patterns
  • Environment determines whether variation is neutral or has selective value

Humans Impacting Evolution

  1. Monocultures: only grow one type of crop → reduce genetic variation bcuz only a few varieties of many wild varieties of plants are used
  • Monocultures have no genetic variation and are susceptible to changing environmental conditions
  1. Overuse of Antibiotics reduces variation in bacteria population by eliminating certain individuals
  • Absence of susceptible individuals decrease competition and allows pathogenic bacteria to increase in number and dominate population
  1. Artificial Selection/Selective Breeding: breed individuals to produce desired traits
  • Similar to Natural Selection: Needs genetic variation

              Different to Natural Selection: Humans (not environment) does the selecting