Introduction

Welcome back to Unit 7 of AP Biology. So far, we learned about the factors that drive evolution. But, how do we know if a population is evolving? Well in this article, we will be learning about topic 7.5: Hardy-Weiberg Equilibrium to determine this.

Hardy-Weinberg Overview

As we’ve learned thus far, evolution is measured by genomic changes and yet, we do not have a way to assess if evolution is occurring. But now, we do! 

The Hardy-Weinberg Equilibrium is a model used to assess whether natural selection or other factors are causing evolution at a particular locus by describing and predicting allele frequencies in a non-evolving population. The Hardy-Weinberg principle states that the frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.

Remember, this is a hypothetical situation where no evolution would take place. In real populations, the allele and genotype frequencies do change over time.

Five conditions must be met to be in the Hardy-Weinberg Equilibrium:

If any of these conditions are not met, then microevolution occurs (i.e. mutation, gene flow, genetic drift, natural selection, and non-random mating). As a reminder, these conditions are never met, but they provide a valuable null hypothesis.

Calculating and Applying Hardy-Weinberg

There are two formulas for Hardy-Weinberg:

The formula you use or start with depends on the information you are given. If a problem gives “allele frequencies”, it is referring to $p+q=1$. If it gives information about individual organisms or a population, then it’s referring to $p^2+2pq+q^2=1$. Most of the time, you’ll use both formulas to complete the problem.

Concluding Hardy-Weinberg: Interpretation

The Hardy-Weinberg Equation determines the genetic makeup of a population if it were not evolving. We determine whether or not it’s actually changing by comparing the equation to the actual data.

Practice Questions