from: http://taxonomy.zoology.gla.ac.uk/~rdmp1c/teaching/L4/Evolution/Session2/cam.html

Phylogeny and Cladistics

I. Introduction

phylogenetic systematics

cladistics

II.. Elements of Cladistics

A. Introduction

-a cladogram is a testable hypothesis about the evolutionary relationships between organisms

-a complete clade is monophyletic

B. How do you construct a cladogram?

1. Identify a group of related organisms

-gather data on many aspects of phenotype

-species must vary in character states, present in some and absent in others

-hypothesize if characters are ancestral or derived

-without fossils, comparison is with a related out-group

-try to sort homology from analogy (if possible)

-homology

-analogy

 convergent evolution

the rules about homology

2. Construct a character table

3. Construct a cladogram

-all extant taxa are organized at the top of the cladogram

-all cladogram nodes must have a list of shared derived characteristics (synapomorphies) common to taxa above the node (a synapomorphy should only appear on a cladogram once)

- species that share synapomorphies belong to a clade

-each node represents a hypothetical ancestral species

-organisms in the same clade are assumed to be closely related because they share a common ancestor

-species that occur next to each other at the top of the cladogram are not necessarily closely related

C. What does the cladogram mean?

1. It is a hypothesis about the evolutionary history of an organism

-molecular data (protein, DNA sequences); fossil evidence, etc.

2. chronology implied by cladogram is relative, not absolute

3. cladograms are used to place species in a taxonomic hierarchy

4. Cladistics may replace Linnaean classification (phylocode)