Lecture 16 - Introduction to Phylogenetic Systematics
The first principle of The Theory of Evolution is that all life on earth is descended from a
single common ancestral species.
If we could go back in time we could observe the
process by which descendent taxa coalesced into a common ancestral species, all the
way back to the origin of life.
The historical relationships between organisms can be depicted as a tree, called a
There is only one true phylogeny, which we cannot know.
However, we can
attempt to infer
phylogeny from properties of organisms, living and extinct.
Systematics is the science of classification, which is an old endeavor.
Prior to Darwin,
biologists attempted to classify organisms into categories with the goal of uncovering
the "plan of the Creator".
Modern classification dates to Linnaeus (1735), from which comes the modern
taxonomic hierarchy (species, genus, family, order, class, phylum, kingdom).
Classical systematics often had two criteria, genealogy and phenotypic divergence.
Thus, for example, humans were given our own family, Hominidae, in the superfamily
Hominoidea, even though it was generally recognized that humans and chimps shared
a common ancestor more recently that either humans or chimps do with orangs
In the 1950s, two methods were proposed to attempt to make systematics systematic.
(Michener and Sokal 1957) proposed to group organisms based on overall
similarity, based on many characters.
The method was based on the idea that
, the closer the relationship between taxa, the more similar they are.
called "numerical taxonomy".
A "character" is a feature of an organism that varies (in principle) independently of other
features and is (in principle) homologous among the taxa in question.
characters descend from the same structure in the common ancestor
(i.e., the character is shared because it was present in the common ancestor).
"Character states" are alternative conditions of the character.
E.g., "six" and "eight" are
character states possessed by insects and arachnids of the character "number of legs".
Or, "A, C, G, and T" are possible states of the character "position 273 in the alcohol
dehydrogenase B1 gene".
, or Cladistics (Hennig 1950) - Hennig argued that
classification should reflect only phylogenetic relationships, not phenotypic divergence
Hennig pointed out there are three reasons for species to share similar character states: