In the beginning of the last century, scientists began to understand genetics and how offspring inherited traits from their parents. Figuring out how offspring inherited traits from their parents had stumped Darwin, and twentieth-century biologists soon recognized the importance of genetic theory to natural selection and evolution. By the 1920s, a new field of science had emerged called population genetics. These scientists expanded the scope of genetics, which had focused previously on how individuals inherited and passed on genetic information, by studying how genetic change occurs in populations. As scientists combined ideas from population genetics with Darwin's ideas about evolution, a modern synthesis theory of evolution emerged in the 1940s. This synthesis, which includes most of Darwin's ideas but focuses on populations, combines population genetics with natural selection and creates a powerful tool for examining evolution in action in the natural world.

One of the most important tools population genetics gave to the study of evolution was the principle of Hardy-Weinberg equilibrium. This principle states that there is nothing in gene replication, meiosis, fertilization, or reproduction that changes the frequency of gene alleles over time. As long as no other forces act upon the population, a gene in that today has 25% allele "A" and 75% allele "a" will still have 25% A and 75% in a million years.

The Hardy-Weinberg equilibrium is based in five assumptions, which, if they held true in nature, would create a situation where no other forces were acting upon a population and no change in gene frequencies (evolution) would occur. These assumptions are:

1. The population is infinite, isolated, and panmictic, where all individuals have an equal chance of mating with any other individual in the population.
2. There is no mutation in the genetic material of the population.
3. There is no gene flow in the population (no individuals leave or join the population).
4. There is no genetic drift.
5. There is no difference in reproductive success between members in the population (natural selection is not acting on the population).

Since these are the assumptions that must hold true for the Hardy-Weinberg equilibrium to be maintained, their opposites are the causes of evolution, or the change of allele frequencies in populations. For example, mutation can cause changes in allele frequency by creating new, altered genetic material while gene flow can change allele frequencies by introducing new alleles to the population through immigration or removing alleles through emigration. Of these five causes of evolution, only the last one involves natural selection, or directional change imposed by survival of the fittest in a harsh environment. The first four, all of which can play critical roles in evolution, involve chance events.

By studying the allele frequency in a population, and doing some math, ecologists can determine whether allele frequencies occur as predicted by the Hardy-Weinberg equilibrium principle. Hardy-Wienberg allele frequencies are likely with large populations that have lots of gene flow and random mating, and are unlikely if populations are small, isolated, or have non-random mating patterns. If a study reveals allele frequencies that do not match Hardy-Weinberg predictions, the population is probably structured in some way (e.g., is made up of more than one isolated subpopulation or has non-random mating). This is what two teams of researchers have done with monarch butterflies.