“Phylogenetic relationship” refers to the relative times in the past that species shared common ancestors. The information about relationships is not in where the species sit relative to one another at the tips of the branches; we don’t read trees across the top from left to right.

The phylogenetic tree is a branching diagram or “tree” showing the evolutionary relationships among various species or organisms.

Knowing the evolutionary relationships among species allows scientists to choose appropriate organisms for the study of diseases, such as HIV. Scientists are even using the principles of natural selection to identify new drugs for detecting and treating diseases such as cancer.

In trees, two species are more related if they have a more recent common ancestor and less related if they have a less recent common ancestor. Phylogenetic trees can be drawn in various equivalent styles. Rotating a tree about its branch points doesn’t change the information it carries.

Phylogeny, the history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms. …

Phylogenetics is the study of evolutionary relationships among biological entities – often species, individuals or genes (which may be referred to as taxa). The major elements of phylogenetics are summarised in Figure 1 below.

The most widely used phylogenetic metric is Faith’s phylogenetic diversity (PD) (Faith 1992) which is defined as the sum of the branch lengths of a phylogenetic tree connecting all species in the target assemblage.

A phylogenetic classification system attempts to arrange species into groups based on their evolutionary origins and relationships. It uses a hierarchy in which smaller groups are placed within larger groups, with no overlap between groups. Each group is called a taxon (plural taxa).

Paraphyletic taxa include Pisces and Reptilia, the former comprising all ray-finned fish but excluding terrestrial descendants of fleshy-finned fish, and the latter comprising all scaly tetrapods but excluding mammals and birds with their modified scales.

Phylogenetic comparative methods (PCMs) use information on the historical relationships of lineages (phylogenies) to test evolutionary hypotheses. Although most studies that employ PCMs focus on extant organisms, many methods can also be applied to extinct taxa and can incorporate information from the fossil record.