Chapter 23- Microevolution and population genetics 

Microevolution: change in allele frequencies in a population over generations; be able to explain the mechanism of evolution by natural selection

Natural Selection: the only mechanism causing adaptive evolution (improving the match between organisms and their environment). 

  • Requires variation in heritable traits to be present in a population; understand the types of genetic variation (mutations and such); natural selection can only act on phenotypic variation that has a genetic component. 

  • Adaptive evolution is a continuous, dynamic process for many organisms. Phenotypes come in variety, often a continuous spectrum; recognize the selective mechanisms of natural selection:

  • Directional selection: favors individuals at one end of a phenotypic range

  • Disruptive selection: favors individuals at both extremes of a phenotypic range

  • Stabilizing selection: favors intermediate phenotypes and acts against extreme phenotypes

Genetic drift: chance events altering survival and reproduction, and thus allele frequency; tends to reduce genetic variation

through losses of alleles, especially in small populations

Define the Founder effect: how can a few individuals become isolated from a larger population? How do the new allele frequencies not represent the original larger population?

Define Bottleneck effect: how does it result in a gene pool that no longer reflections the larger population’s gene pool? Why is it more significant in small populations?  How are some alleles disproportionately represented?; why does this cause allele frequencies to change at random?; what happens to genetic variation?; can this eventually influence adaptation of a specie to an environment?; can cause harmful alleles to become fixed!


Define Gene Glow: how can it increase (example: mosquito resistance spreading) or decrease the fitness of a population? 

Know what a Population is and what we mean when we refer to the ‘gene pool’ 

Know how to apply the Hardy-Weinberg principles – counting allele frequencies and genotype frequencies to determine if a population is evolving

  • If a population meets Hardy-Weinberg criteria, it is not evolving


Know how to calculate the number and frequencies of alleles in a population; sum of 2 alleles for one genetic locus (example A and a in a population) is p + q = 1; remember to count both dominant and recessive alleles in both homozygous (AA and

aa) and heterozygous individuals (aa) when considering allele frequencies.


Know how to calculate genotype frequency: p2 + 2pq + q2 = 1; (AA = p x p; Aa = 2 x p x q; and aa = q x q) in a population given p or q.


UNDERSTAND WHY there are five conditions for populations to be non-evolving (if any of these things occurs, the population can evolve – even at individual genes)

  1. no mutations

  2. random mating

  3. no natural selection

  4. very large population size

  5. no gene Glow


In real populations, allele frequencies do change over time. Some genes in a population can be in Hardy-Weinberg equilibrium, while other genes in the same population are undergoing selection and evolving.