Biologists represent relationships among organisms using phylogenetic trees. The tips, or leaves, of a phylogenetic tree can represent a variety of biological forms, such as DNA sequences, individual organisms, or species and other taxonomic groupings of organisms. In the image to the right, a hypothetical set of organismal forms is represented at the tips of a phylogenetic tree. Branches in the tree reflect lines of descent, which are connected at nodes that represent common ancestry of descendant forms. Subsets of more closely related organisms are connected at nodes reflecting unique common ancestors of that group. For example organisms C and D are connected through a unique common ancestor, thus C and D are more closely related to each other than to A, B and E. At the very base of the tree on the left-hand side, the single common ancestor is represented for all organisms at the tips of the tree. Chronological time in this diagram of a phylogenetic tree moves from left to right, with the most ancient split between lineages on the left and the representation of living organisms on the right.
Pedigrees and family trees are specialized types of phylogenetic trees commonly used to illustrate relationships in human families. Since humans reproduce sexually, each node within a pedigree consists of a pair of ancestors through which descendant lineages are produced. For example, your siblings are connected to you through your parents, your 1st cousins are connected to you through your grandparents, your 2nd cousins through your great grandparents, etc.
Relationships within a phylogenetic tree represent a hierarchical ordering of common ancestry. Each node connects lines of descent from a common ancestor. This series of historical relationships can be reconstructed from the characteristics of organisms. Presence of shared features indicates common ancestry, because evolutionary changes that accumulate in a lineage are passed along to descendant lineages. For example, the presence of a mid-section in two of the representative hypothetical organisms indicates that they share a common ancestor which evolved this feature. Likewise, the other grouping reflects descent from a common ancestor that evolved legs. Note that in each of these cases the evolutionary changes have undergone modifications in descendant lineages. In essence the characteristics of living organisms provide a record of evolutionary history, and a variety of methods have been developed for deciphering phylogenetic relationships by comparison of characteristics among living organisms. Similar methods are used in the analysis of personal genetic data to estimate relationships among humans.
Since the DNA present in each cell provides the recipe for cellular life, and this recipe is copied and transmitted as cells divide and organisms reproduce, DNA is an excellent source of characteristics for reconstructing historical relationships. The sequence of bases in DNA molecules change through the process of mutation. Mutation is rare, but persistent, so that DNA sequences slowly accumulate changes over generations. Shared changes in DNA sequence provide evidence for reconstructing patterns of common ancestry. Ultimately, however, phylogenetic reconstruction is an inference (or hypothesis) describing biological relationships.