What is phylogenetics? Wikipedia says "the study of evolutionary relatedness among various groups of organisms which is discovered through molecular sequencing data and morphological data matrices". A less ridiculous way of saying this would be "the study of relations between organisms through comparison of genetic or morphological (form and structural) information. Phylogenetics assumes evolution is a branching process whereby a population of organisms which have become separated for some reason, be it geographical or behavioural, undergo evolutionary change through mutation and natural selection.
We can build phylogenetic trees to help understand the order of branching, representing the pattern of evolutionary diversification, within a particular group of organisms. Phylogenetic trees are constructed using a method called cladistics, which assigns organisms with shared derived characters (synapomorphies) to groups called clades. As palaeontologists cannot access genetic information for extinct organisms, we use morphological characters as synapomorphies.
Fig 1. Click to expand.
This is a phylogenetic tree of extant vertebrates which serves as a good example of which sort of groups are useful and informative and which aren't. Clades are, by definition, monophyletic. This means that the group contains all descendants of a common ancestor. Birds are a clade united by the presence of synapomorphies such as feathers. Node no. 1 on the diagram is also a clade. This is Osteichthyes - the bony fish, which are united by synapomorphies such as a bony skeleton. Although the name suggests that the group contains just fish, Osteichthyes is a monophylectic group therefore must contain all the descendents, including amphibians, mammals, lizards, crocodiles and birds. In fact, each node on the tree defines the base of a monophylectic group (if you’re interested the clades are, in ascending order, osteichthyans, tetrapods, amniotes, diapsids, archosaurs).
Traditionally, the class ‘reptiles’ excludes birds. On our tree we could group lizards and crocodiles together as 'reptiles'. This however is a paraphyletic group as it excludes one or more descendent. Paraphylys are thought to be of limited use, primarily because they are more difficult to define than monophylys. However, they are sometimes useful when discussing evolutionary transitions, as we shall explore at a later date. The worst of the worst are polyphyletic groups. The most obvious one on the tree above is 'warm blooded'; a grouping of mammals and birds which has no evolutionary significance whatsoever. In fact the ability to maintain thermal homeostasis evolved independently within each group. Polyphyletic groups are nonsense groups with no relevance to evolutionary relationships.
Through phylogenetics we can begin to build a picture of how vertebrates evolved. For example, we know that tetrapods (node 2) are defined by synapomorphies such as limbs with digits and sepertation of the head and forelimb. Therefore we can infer that these major evolutionary changes must have occurred between nodes 1 and 2 on our tree. This might seem a lot to take in, with lots of difficult terminology, but a working knowledge of phylogenetics will be essential for future posts. In the immediate future however, I hope I can consolidate phylogenetics by teaching you how to build your own phylogenetic trees. I always learn best when doing.
I enjoyed it.
ReplyDeleteI did. Really.