Review of cladistic technique Shared derived (apomorphic) traits are useful in understanding evolutionary relationships Shared primitive (plesiomorphic) traits are not useful

Constructing a cladogram  We use the principle of parsimony (Occam’s razor)  We assume the cladogram requiring the least number of evolutionary changes is the correct one  It is much more likely that a particular structure evolved once in an ancestral species rather than evolving separately  It is possible that the true phylogeny may not be the tree with the fewest changes

Molecular data in cladistics  The advent of DNA sequencing has revolutionized cladistics  Evolutionary biologists can now compare DNA sequences for homologous regions of DNA in two organisms to determine shared derived characteristics

Molecular data for phylogeny reconstruction  Different types of DNA change at different rates  Mutations can be considered derived characteristics  The DNA coding for ribosomal RNA mutates very slowly - hundreds of millions of years may be required to see significant differences in nucleotides  Mitochondrial DNA evolves relatively rapidly - mtDNA is useful for examining the relationships of closely related species or populations within a species

Molecular clock Based on the observation that at least some portions of the genome mutate at constant rates For taxa known to have diverged from common ancestors in the past, the number of nucleotide substitutions is proportional to the time that has elapsed since the lineages branched For example, the bats and dolphins have fewer differences of homologous gene sequences than sharks and tuna This result is consistent with the fossil record: sharks and tunas have been on separate evolutionary paths much longer than bats and dolphins

Modern systematics is flourishing with lively debate Biologists have been accumulating morphological information about many species for centuries The development of cladistics provides an objective method for comparing morphology and incorporating that information into phylogenetic hypotheses (cladograms) The addition of molecular systematics is bringing us closer to understanding the history of life on earth