5.4 Cladistics Study Guide  A clade is a group of organisms that have evolved from a common ancestor.  Clades include all species alive today, together with the common ancestral species and any species that evolved from it and then became extinct.  Evidence for which species are part of a clade can be obtained from the base sequences of a gene or the corresponding amino acid sequence of a protein  Species with a recent common ancestry have fewer differences in base or amino acid sequences than species that diverged from a common ancestor tens of millions of years ago.  Sequence differences accumulate gradually so there is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor.  differences in base sequences of DNA and amino acid sequences of proteins result from mutations which occur over long periods of time at a roughly constant rate and can therefore be used as a molecular clock.  Traits can be analogous or homologous.  homologous traits are similar due to similar ancestry; pentadactyl limbs  analogous structures are similar due to convergent evolution. Ex: human eye and octopus eye.  Cladograms are tree diagram that show the most probable sequence of divergence in clades.  The branching points on cladograms are called node.  A node is a hypothetical common ancestor that split to form two or more species.  Evidence from cladistics has shown that classifications of some groups based on structure did not correspond with the evolutionary origins of a group of species.

Define clade. Explain why morphology (form and structure) is seldom used for identifying clade members. What is used in its place? Discuss the reclassification of groups and cladograms.

Using examples, distinguish between analogous characteristics and homologous characteristics.

Outline the value of classifying organisms.

Outline how variations in specific molecules can lead to phylogeny.

Be able to use data to create a cladogram. Discuss how variations in a specific protein can be used as an evolutionary clock.