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.
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.
What Is a Phylogram and How Is It Different than a Cladogram?
There are two main types of trees used: cladograms and phylograms. Just like a street map uses a scale to notify the user that, for instance, 1 inch equals 20 miles, phylogenetic trees may also use similar scales where the measurement isn't distance but time. A scaled phylogenetic tree is a phylogram whereas an unscaled tree is referred to as a cladogram.
Cladograms are more readily used in instances where scientists are hypothesizing a relationship, thus they needn't be concerned with when a taxon diverged but where on the tree. Furthermore, these cladograms and phylograms may either be rooted, thereby indicating a common shared ancestor for the entire tree, or unrooted (without reference to a common shared ancestor).
Phylograms and cladograms are both forms of phylogenetic trees, which differ based on whether the branches are scaled (phylogram) or unscaled (cladogram). These genealogical diagrams are extremely useful tools for hypothesizing and depicting relationships between taxa. They are based on the presence or absence of certain physical traits known as derived traits (physical characteristics specific to that taxon) and ancestral traits (traits that first appeared or are specific to a shared ancestral taxon). Phylogenetic trees allow scientists to create a visual representation of not only the evolutionary relationship between taxa but also indicate when (in evolutionary history) taxa developed certain traits as well as how those traits have changed or remained consistent through time.
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