Five Conditions of Hardy–Weinberg Equilibrium

Hardy–Weinberg Equilibrium

• If Hardy–Weinberg Equilibrium is achieved then no evolution occurs.
• To achieve the equilibrium five conditions must be met:
  1. Population must be very large.
  2. Population must be isolated from other populations (no immigration or emigration).
  3. No mutations
  4. Random mating
  5. No natural selection (i.e. every individual has an equal chance of survival)
• If the five conditions are not met then evolution occurs:
  • There is a change in allele frequency in the population.
  • Hardy–Weinberg Equilibrium is not present.

Consequences of Violations of the Five Conditions

1. A small population causes genetic drift
   • Changes in allele frequency due to chance
   • Example: a small number of coin tosses versus large number of coin tosses
   • Special cases of genetic drift:
     • Bottleneck Effect
       • Occurs when some event reduces the population to a really small size.
       • Example: Northern Elephant Seal population was almost hunted to extinction—only about a dozen individuals survived to repopulate the current large population.
     • Founder Effect
       • Occurs when a few individuals colonize a new area.
       • Example #1: immigration of religious groups (in small numbers) to the United States that will not reproduce with other individuals. They maintain a very small isolated population.
       • Example #2: Darwin's finches
2. Non-isolated population causes gene flow
   • Movement of individuals from one population to another, resulting in interbreeding of what were once isolated populations.
   • Reduces genetic differences between populations.
3. Mutations cause new alleles to be introduced into the population.
4. Nonrandom mating causes more expression of recessive phenotypes
   • Inbreeding: mating between closely related partners
   • Assortative mating: individuals choose partners like themselves.
   • In both cases, two organisms carrying a recessive gene are more likely to pair up, so a recessive phenotype (e.g. resulting from genotype aa) is more likely to be expressed.
5. Natural selection causes the expression of successful alleles
   • Certain alleles provide traits that create a better ability to survive and reproduce than the traits of other alleles do.