4-5

• Hardy-Weinberg Law


  States that in an infinitely large, interbreeding population in which mating is random, in which there is no selection, migration, or mutation, gene and genotype frequencies will remain constant from generation to generation
• These conditions are rarely strictly present, but unless any departure is a marked one, there is no statistically significant movement away from equilibrium
• Allele frequencies


  p - frequency of the "A" allele, q - frequency of the "a" allele in the population
• Genotype frequencies


  p^2 - frequency of the homozygous AA, q^2 - frequency of the homozygous aa, 2pq - frequency of the heterozygous genotype Aa
• The sum of the allele frequencies for all the alleles at the locus must be 1, so p + q = 1
• If the p and q allele frequencies are known, then the frequencies of the three genotypes may be calculated using the Hardy-Weinberg equation
• Mechanisms that cause changes in allele frequencies over time

  • Natural selection
  • Gene flow
  • Genetic drift
• Natural selection


  • Individuals with certain genotypes are more likely than individuals with other genotypes to survive and reproduce, and thus pass on their alleles to the next generation
  • Occurs when one allele (or combination of alleles of different genes) makes an organism more or less fit, that is, able to survive and reproduce in a given environment
• If the following conditions are met, natural selection must occur: 1) There is variation among individuals within a population in some trait, 2) This variation is heritable, 3) Variation in this trait is associated with variation in fitness
• Mutation is the original source of all genetic variation, but the mutation rate for most organisms is pretty low
• Gene flow


  Involves the movement of genes into or out of a population, due to either the movement of individual organisms or their gametes
• Genetic drift


  Involves changes in allele frequency due to chance events - literally, "sampling error" in selecting alleles for the next generation
• Species


  In biology, a classification comprising related organisms that share common characteristics and are capable of interbreeding
• Reproductive isolating mechanisms


  Prevent fertilization and zygote formation (pre-zygotic) or allow fertilization but produce nonviable, weak, or sterile hybrids (post-zygotic)
• Pre-zygotic isolation mechanisms


  • Geographic/ecological/habitat isolation
  • Temporal or seasonal isolation
  • Behavioral isolation
  • Mechanical isolation
  • Gametic isolation
• Post-zygotic isolation mechanisms


  • Hybrid inviability
  • Hybrid sterility
  • Hybrid breakdown
• Allopatric or geographic speciation


  Occurs when some members of a population become geographically separated from the other members, thereby preventing gene flow
• Sympatric speciation


  Occurs when members of a population that initially occupy the same habitat within the same range diverge into two or more species
• Parapatric speciation


  Occurs when the groups that evolved to be separate species are geographic neighbors, with reduced gene flow due to the distance