•Taxonomists now use evolutionary relationships when classifying organisms (so called ‘natural’ classification).

•Diagrams can be drawn that show these evolutionary relationships, known as phylogenetic trees or cladograms.

•A clade is a group of organisms that has evolved from a common ancestor.

•Taxonomists must decide which features are the more significant in a phylogenetic taxonomy

•i.e. those that should receive the greater emphasis in devising a scheme.

•In cladistics, classification is based on an analysis of relatedness and the product is a cladogram

A Cladogram

•Show patterns of shared characteristics is a diagram that shows the evolutionary relationships among a group of organisms

•Classify organisms according to the order in time at which branches arise along their phylogenetic tree

•Have 2 important features:

• branch points in the tree – representing the time at which a divide between two taxa occurred

• the degree of divergence between branches – representing the differences that have developed between the two taxa since they diverged

Construction of Cladogram

•The more derived structures that are shared by two organisms, the closer their evolutionary relationship (i.e. the more recently their common ancestor lived).

•Points at which two branches form are called nodes. They represent speciation events

•Close relationships are shown by a recent fork – the closer the fork in the branch between two organisms, the closer their relationship.

•Cladograms provide strong evidence for evolutionary relationships, although they cannot be regarded as absolute proof. They assume that the smallest number of mutations (changes in the genetic code) possible account for differences between species. If such assumptions are incorrect, errors may occur in cladograms

•Using several different cladograms, derived independently from different data, can overcome such difficulties.

•It is important to distinguish similarities that are based on:

•shared ancestry i.e homologous structure) from those based on structures that have evolved independently but under similar selective pressures and so appear the same

•similar (so called analogous structures, such as human and octopus eyes).

•Species that are similar in appearance may not be closely related – their resemblance is due to analogous adaptations to very similar environments.

•Morphology (form and structure) of organisms can lead to mistakes in classification, due to misinterpreting whether structures are analogous or homologous. Base or amino acid sequences are more accurate ways of determining members of a clade because they represent true homology.

•Evidence from cladistics has shown that classification of some groups based on structure does not correspond with the evolutionary origins of a group or species.

•The use of base and amino acid sequences has made the study of phylogenetic trees more accurate. Traditional classification based on morphology does not always match the evolutionary origin of groups of species.