Phylogenetic Tree: Definition & Types

Instructor: Sarah Phenix
We'll explore the ways taxa are categorized into a visual diagram, known as a phylogenetic tree, as well as what information these 'trees' tell us about the evolutionary relationship between organisms.

What Is Phylogenetic Systematics?

Most of you are likely familiar with the term taxonomy, which concerns classifying organisms into various level of taxa (Kingdom, Phylum, Class, Order, Family, Genus and Species). However, many of you may not have heard of a closely related field called phylogenetic systematics.

Phylogenetic systematics is the study of the evolutionary relationships between groups of organisms (known as taxa) in an effort to understand the way life has diversified over time. It was founded by a German taxologist named Willi Hennig who, in the 1950s, derived a way of visualizing evolutionary relationships by categorizing taxa based on their shared physical traits.

Hennig classified these traits as either derived traits (shared and expressed by that particular group of organisms) or ancestral traits (a trait that can be traced back to a common ancestral organism and shared by multiple taxa). For instance, primates express the shared derived trait of opposable thumbs, which only that particular order express, yet, all primates express the shared ancestral trait of having hair, which can be traced back to a common ancestor shared by mammalian taxa.

What Is the Tree of Life?

Hennig's method of visualizing these relationships resulted in what we loosely refer to as a genealogical tree of life. The tree is constructed using a system of nodes and branches.

Anatomical description of the parts of a phylogenetic tree
Anatomy of a Phylogenetic Tree

The term node refers to any terminating end of a branch (a line). External nodes represent the final taxon (singular form of taxa) while internal nodes represent a common ancestor that underwent some speciation event (where organisms within that taxon stop interbreeding due to reasons like physical isolation, such as the formation of an island, or the preference of a particular physical trait that a subset of the population begins to favor through the process of sexual selection). As a result, speciation events give rise to divergent lineages of taxa and are represented by horizontal branches.

These diverging lines of taxa stem from a common ancestor, resulting in a relationship called sister taxa (such as taxon A and taxon B), meaning that they share the closest evolutionary relationship because they stem from the same common ancestor. In this way chimpanzees are our sister taxon, as we are more evolutionarily related to them than we are with, say, gorillas.

Taxa outside of that common ancestor are referred to as outgroups as they are more evolutionarily distant in relation than sister taxa are to one another, due to a more distant common ancestor. With each successive speciation event, a new clade is formed within the tree, allowing scientists to identify common ancestors between evolutionarily distant taxa.

Example clades highlighted by color
Highlighted Clades of a Phylogenetic Tree

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