• When the allele frequencies in a population remain constant from generation to generation, the population is said to be in genetic equilibrium or Hardy-Weinberg Equilibrium
    • At genetic equilibrium, there is no evolution
      • But allele representation in generations might differ
    • In most natural populations, the conditions of hardy-Weinberg equilibrium are not obeyed
      • But calculations serve as starting point that reveals how allele frequencies are changing, which equilibrium conditions are being violated, and what mechanisms are driving the evolution of a population
    • Population: a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
    • Gene Pool: consists of all copies of every type of allele at every locus in all members of the population.

Conditions for Hardy-Weinberg Equilibrium

  • If allele frequencies are changing then one of these are likely being violated
  1. No natural selection (all traits are selectively neutral)
  2. No mutations
  • Gene pool modified by mutations
  1. No gene flow (the population must be isolated from other populations)
  • Moving alleles in and out of a population can alter allele frequency
  1. No genetic drift (The population is large)
  • In small populations, allele frequencies fluctuate by chance
  1. No sexual selection (mating must be random)

Values of Genetic Equilibrium

  1. Allele frequencies for each allele (p,q)
  2. Frequency of homozygous dominant (p^2) and homozygous recessive ( q^2)
  3. Frequency of heterozygous [2 diff alleles] (2pq)

2 Equations

  1. p + q = 1 (all alleles sum to 100%)
  2. p^2 + 2pq + q^2 = 1 (all individuals sum to 100%)


  1. Find percentage of homozygous recessive
  2. Square root q% (as decimal) → q
  3. 1 – q = p
  4. Can use q and p to find homozygotes and heterozygotes

Causes for Changes in Allele Frequencies  

  • Multiple factors, together with natural selection, cause evolution
  1. Natural Selection: increases or decreases allele frequencies bcuz of impact of environment
  2. Mutations introduce new alleles that may provide selective advantage
  • WEAK force for changing allele frequencies; STRONG force for creating new alleles
  1. Gene Flow: the transfer of alleles between populations
  • Movement of individuals between populations resulting in the removal of alleles when they leave (emigration) or introduction of new alleles when they enter (immigration) the population
    • Ex: pollen transferred from one population to another
    • Tends to reduce the genetic differences between populations
  1. Genetic Drift: random increase or decrease of alleles
  • Especially in small (usually <100) chance events can cause allele frequencies to fluctuate and an allele to be disproportionately over or underrepresented in the next generation
    • Decreases genetic variation & evolutionary adaptability and increases homozygosity
  1. Founder Effect: “When a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population”
  • Ex: founding fathers contain mutated allele and established community → reproductive isolation cause mutation to be concentrated in that area
    • Population tends to have reduced genetic diversity
  1. Bottleneck: when the population undergoes dramatic decrease in size (predation, catastrophe, disease) → becomes susceptible to genetic drift
  1. Nonrandom Mating: when individuals choose mates based upon their particular traits
  • Ex: always choose mates with traits similar to their own or different from their own; only nearby individuals
  1. Inbreeding: individuals mate with relatives
  2. Sexual Selection: process in which individuals with certain inherited characteristics are preferred as mates
  1. Intrasexual Selection: Individuals of one sex compe2te directly for mates of the opposite sex.
  2. Intersexual Selection (mate choice): females choose males based on attractive appearance or behaviour
  • Leads to sexual dimorphosim:

Extra Notes

  • Fixation: when one allele goes extinct and only one remains (becomes fixed)

                → all individuals will be homozygous for allele