Evolution

The process of classifying and reconstructing the evolutionary history, or phylogeny, of organisms is known as phylogenetic systematics. Its goal is to group species in ways that reflect a common ancestry. The members of each group, or taxon, share uniquely derived characteristics that have arisen only once. For instance, the taxon Amniota includes amphibians, reptiles, birds and mammals, all of which arose from a single common ancestor that possessed an amnion in the egg stage. Classifying species involves only one apect of phylogenetic systematics. Understanding the evolutionary interrelationships of organisms by investigating the mechanisms leading to diversification of life and the changes that take place over time encompasses phylogenetic systematics. Systematics exceeds taxonomy or naming groups within species by attempting to develop new theories to describe the possible mechanisms of evolution. Historical remnants leave residual clues that allow phylogeneticists to piece together using hypotheses and models to describe history and how organisms evolve.

Systematists gather as much evidence as they can concerning the physical, developmental, ecological, and behavioral traits of the species they wish to group, and the results of their analysis are one (or more) branching "tree" diagrams, representing the hypothetical relationships of these taxa. Analysis of the genetic material itself has become an increasingly valuable tool in deducing phylogenetic relationships. The approach is to determine the nucleotide sequence of one or more genes in the species and use this to contribute to determining the appropriate phylogeny. Comparison of the differences in these DNA sequences provides an estimate of the time elapsed since the divergence of the two lineages. With the ability to sequence an entire organisms genome, the remnants of various species DNA, extinct (depending on the quality of the DNA) and living, can be compared and contrasted. This has exciting implications in the analysis of evolutionary modeling and applications to phylogenetic systematics.