Session 2: Cladistics Flashcards
What is Cladistics?
A method of classifying organisms into groups of species called clades. Each clade consists of an ancestral organism and all of its evolutionary descendants. Members of a clade will possess common characteristics as a result of their shared evolutionary lineage. Clades can be organised according to branching diagrams (cladograms) in order to show evolutionary relationships
Describe cladograms.
Cladograms are tree diagrams where each branch point represents the splitting of two new groups from a common ancestor. Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution. The fewer the number of nodes between two groups the more closely related they are expected to be, and the less time has passed. Cladograms can be structured based on structural features (morphological) or molecular evidence (biochemical), in modern science we typically use molecular evidence.
What are the four main features of cladograms?
Root – The initial ancestor common to all organisms within the cladogram (incoming line shows it originates from a larger clade). Nodes – Each node corresponds to a hypothetical common ancestor that speciated to give rise to two (or more) daughter species. Overall, a node is a speciation event. Outgroup – The most distantly related species in the cladogram which functions as a point of comparison and reference group. Clades – A common ancestor and all of its descendants (i.e. a node and all of its connected branches)
Describe the use of molecular evidence in relation to cladograms.
When comparing molecular sequences, scientists may use non-coding DNA, gene sequences or amino acid sequences. Non-coding DNA provides the best means of comparison as mutations will occur more readily in these sequences. Gene sequences mutate at a slower rate, as changes to base sequence may potentially affect protein structure and function. Amino acid sequences may also be used for comparison, but will have the slowest rate of change due to codon degeneracy. Amino acid sequences are typically used to compare distantly related species (i.e. different taxa), while DNA or RNA base sequences are often used to compare closely related organisms
What is a molecular clock?
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years). If this rate of change is reliable, scientists can calculate the time that has passed according to the number of differences. Limits to the molecular clock include: Different genes or proteins may change at different rates, The rate of change for a particular gene may differ between different groups of organisms, and over long periods, earlier changes may be reversed by later changes, potentially confounding the accuracy of predictions. The longer something takes to accumulate mutations, the further back in time we can look.
