Variation is a major feature of the natural world. We humans are particularly good at noticing phenotypic variation among individuals of our own species. As we discussed in Chapter 16, a phenotype is an observable trait, such as human height or wing color in butterflies. As we saw in Chapter 18, two factors contribute to phenotype: an individual’s genotype, which is the set of alleles possessed by the individual, and the environment in which the individual lives. We can take the environment out of the equation by looking directly at genotypic differences through sequencing DNA regions in multiple individuals. We now explore genetic variation directly, in terms of differences at the DNA sequence level.