The island is suffering from a drought, and in this dry environment, brown beetles are better camouflaged than green beetles. Birds are more likely to see and eat green beetles. Over time, brown beetles survive and reproduce more than green beetles. This is an example of natural selection. Individuals with inherited traits better suited to their environment survive and produce more offspring than other individuals. Here, natural selection causes the frequency of the brown allele to increase. Through natural selection, the population has become better adapted to its new environment. Genetic drift occurs when populations evolve due to chance events. Let's see how genetic drift affects this small population of beetles. Chance constantly affects which beetles actually reproduce, and which of their alleles they pass on. In a small population, chance events produce large fluctuations in allele frequencies from one generation to the next. Here, the frequency of the brown allele has decreased to zero, and the population is left with only green beetles. Genetic drift also occurs in large populations. But in a large population, genetic drift has less effect because it causes much smaller fluctuations in allele frequencies. Other random events can also lead to genetic drift. By chance, this volcano kills more brown beetles than green beetles, causing the frequency of the green allele to increase. Gene flow describes evolution due to individuals moving into or out of a population. When green and brown beetles migrate into the all-green population, the frequency of the brown allele increases. Natural selection, genetic drift, and gene flow are the major mechanisms responsible for the evolution of all populations. Together, they shape populations of beetles as well as humans.