Evolutionary Study
The use of genetics in evolutionary studies is based on one basic premise: if two species are closely related, then they will have similar DNA sequences. Thus, many evolutionary studies now include DNA sequence data to support their conclusions. Some of the most common types of DNA studies are being performed on mitochondrial DNA, Y-chromosome DNA, and ribosomal RNA (rRNA) genes.
Mitochondrial DNA is an incredibly interesting resource for animal evolutionary studies. Since it is inherited almost solely from the maternal gamete, most changes in the mitochondrial DNA will be due to mutations. If we assume that mutations in mitochondrial DNA occur at a fixed rate in all animals, then the number of mutational differences between the mitochondrial DNA of two animals can be used to calculate how closely related they are. It is also possible to use this 'molecular clock' to deduce how long ago two species diverged. While such molecular clocks are not an exact way to measure species divergence, they can still provide valuable insights.
Using a combination of PCR amplification and DNA sequencing, scientists can study the mitochondrial DNA of skeletons over 50,000 years old. Ultimately, these tests will allow us to trace our evolutionary roots back to our oldest ancestors. Most genetic and fossil data indicate that these ancestors originated in Africa, as shown in the animation to the right, but arguments still exist as to whether these African ancestors replaced indigenous species as they migrated worldwide, or whether they interbred with them, thus producing modern humans.
Since Y-chromosomes are paternally inherited, evolutionary studies have also been performed using this DNA source. In fact, a recent study shows that a group of people in Colombia has Native American mitochondrial DNA and Spanish Y-chromosome DNA. Thus, the genetic data confirms one of history's familiar but tragic stories: the colonists killed the native men and married the native women.
Ribosomal RNA genes have provided tremendous insight into the evolutionary relationships among all species because, unlike mitochondria, ribosomes are found in bacteria. The data generated from rRNA sequences suggest that there may only be three kingdoms of life, and that two of these kingdoms consist of prokaryotes (species that have no nucleus or organelles in their cells). In other words, your dog and your favorite houseplant may have more in common, genetically, than two different species of bacteria!