Natural selection is the fundamental mechanism through which evolution occurs, but for selection to be possible there must be some underlying variability in genetic makeup within a species. Since selection usually acts to reduce variability, there must also be a source of new genetic variation. This is introduced at the molecular level, in the DNA of individuals, through what are viewed as random changes as the molecules are copied into new generations.

Depending on the nature of these changes in the DNA, offspring may be more, less, or equally viable than the parents. Many of the molecular changes are believed to be selectively neutral, and so are passed on to further descendents and preserved. The DNA within a particular gene may continue to mutate from generation to generation, gradually accumulating more differences from its ancestral form. Thus, several species arising from a common ancestor will have similar, but often not identical, DNA forming a particular gene. The similarities hint at the common ancestor, while the differences point to the evolutionary divergence of the descendents.

Since we can now “read” the structure of DNA with relative ease, a natural and compelling question arises: Can we reconstruct evolutionary relationships between several modern species by comparing the DNA sequences of their versions of a certain gene?