The Hardy-Weinberg Equation Can Be Used to Test Whether a Population Is Evolving

Rucete ✏ Campbell Biology In a Nutshell

Unit 4 MECHANISMS OF EVOLUTION — Concept 23.2 The Hardy-Weinberg Equation Can Be Used to Test Whether a Population Is Evolving

The Hardy-Weinberg equation is a fundamental tool used to assess whether evolutionary change is occurring within populations. By comparing observed genotype frequencies to those predicted under conditions of no evolution, biologists can determine if allele frequencies are changing.

Gene Pools and Allele Frequencies

• Gene pool: All copies of every type of allele at every locus in a population.

• Fixed allele: Only one allele exists at a locus, all individuals are homozygous.

• Allele frequency: Proportion of a specific allele within the gene pool.

  • Example: If a wildflower population has 500 individuals with two alleles (Cʀ and Cᴡ) and 800 of 1000 total alleles are Cʀ, then frequency Cʀ = 0.8.

Hardy-Weinberg Equilibrium (HWE)

• HWE: A state where allele and genotype frequencies remain constant generation after generation, assuming no evolutionary influences.

• The equilibrium condition is expressed by the Hardy-Weinberg equation:

  $$p^2+2pq+q^2=1$$
  

  • $p^2$: frequency of homozygous dominant genotype.

  • $2pq$: frequency of heterozygous genotype.

  • $q^2$: frequency of homozygous recessive genotype.

• Populations in HWE are not evolving.

Conditions for Hardy-Weinberg Equilibrium

Five conditions must be met for a population to remain in equilibrium:

1. No mutations: Gene pool remains unchanged.

2. Random mating: No selective mating or inbreeding.

3. No natural selection: Equal survival and reproductive success.

4. Extremely large population size: Prevents genetic drift.

5. No gene flow: No movement of alleles between populations.

Violation of any of these conditions usually leads to evolution.

Applying the Hardy-Weinberg Equation

• The equation helps test if a population is evolving.

• Example: Phenylketonuria (PKU), a recessive genetic disorder, occurs in 1 of every 10,000 U.S. births.

  • Using HWE, scientists estimate about 2% of the population are PKU carriers (heterozygotes), though they show no symptoms.

  • Practical medical use: Estimating genetic disease carriers within populations.

Importance of Hardy-Weinberg Equilibrium

• Serves as a baseline model to test evolutionary hypotheses.

• Helps identify which evolutionary forces (mutation, selection, drift, gene flow) act on populations.

In a Nutshell

The Hardy-Weinberg equation is a valuable tool to determine whether a population is evolving. Deviations from equilibrium indicate evolutionary forces like selection, drift, or gene flow, driving changes in allele and genotype frequencies across generations.