We will use a hypothetical insect population to illustrate four causes of evolutionary change: Genetic drift, Gene flow, mutation and natural selection. Our insect population consists of 100 individuals, some red and some green. The green allele is dominant (G), the red allele recessive (g). The bar graphs show: the frequencies of green (G) and red (g) alleles the frequencies of homozygous dominant (GG), heterozygous (Gg), and homozygous recessive (gg) individuals the numbers of green and red individuals in the population Assume that our starting population is at Hardy-Weinberg equilibrium. Now let's look at some hypothetical scenarios that demonstrate four causes of evolutionary change. A windstorm has dramatically reduced our insect population. Such a small population is subject to genetic drift, a random change that occurs in a small gene pool due to sampling errors in the propagation of alleles. It is difficult to predict the allele frequencies from one generation to the next in a small population because chance plays a greater role in allele distribution. By chance, only red alleles were passed on to the next generation. Note that the increase in the frequency of the red allele and red insects had nothing to do with any particular advantage of being that color. The increase was purely due to chance-hence the term genetic drift. Chance occurrences like this are much more likely to affect a small population, like this one. Gene flow is change in the gene pool due to migration of fertile individuals into or out of a population. Most populations are not completely isolated, and migration of individuals (or gametes, as in plant pollen) can alter the gene pool. Observe these insects and then see how the allele frequencies, genotype frequencies, and phenotype numbers will change in the next generation. Mostly green insects left the population, and mostly red insects migrated in from elsewhere, increasing the frequency of the g allele. As a result, the next generation has more red individuals. Mutations are random changes in an organism's DNA. A new mutation that is transmitted in gametes can immediately change the gene pool by substituting one allele for another. Imagine that we can see mutations happening, shown here by yellow flashes. Let us see how these mutations will affect the allele frequencies, genotype frequencies, and phenotype numbers of our population in the next generation. Mutation can change a red allele into a green one, or a green allele into a red one. In this scenario, mutations slightly increased the proportion of red alleles in the gene pool and thus red phenotypes in the next generation. Hardy-Weinberg equilibrium requires that all individuals in a population be equal in their abilities to survive and reproduce. This condition is probably never met-because individuals vary, and some variants leave more offspring than others. This differential reproductive success is called natural selection. Natural selection increases the frequencies of some alleles and phenotypes and decreases the frequencies of others. Watch the birds preying on the insects. How will predation affect the genetic structure of the next generation and the evolution of the insects? The birds could see the red insects more easily against the background, so they caught a disproportionate number of the red bugs. This reduced the frequency of the red alleles in the gene pool, increased the frequency of green alleles, and similarly altered the relative numbers of green and red individuals in the population. Over time, this process of natural selection adapts a population to its environment. Under pressure of predation, the insects are becoming more and more green.