PHYLOGENY AND SYSTEMATICS

Systematics - The goal of systematics is to have classification reflect the evolutionary relationships of species.

Phylogeny

Phylogeny leads to classification (Fig 25.8)

Taxonomy

1) binomial nomenclature

2) hierarchical classification (Fig 25.7)

3) taxon (taxa)

4) only species exist in nature as biologically cohesive units

Phylogenetic systematics (Cladistics)

Cladistics classifies organisms according to the order in time that branches arise along a phylogenetic tree, without considering the degree of divergence (how much difference).

- cladogram (Fig 25.12)

- ancestral or primitive characters

- derived or shared derived characters (synapomorphies)

Groups subordinate to other groups in the taxonomic hierarchy should represent finer and finer branching of phylogenetic trees.

- A monophyletic taxon = a clade (Fig. 25.9)

- A polyphyletic taxon

- A paraphyletic taxon

Morphological characters used in determining the phylogeny of a group:

   - Analogous structures result from convergent evolution (Fig. 25.10)

   - Analogy vs. homology (Fig. 25.16)

   - Homologous structures evolve from the same ancestral structure though the function may differ.

Molecular clocks - In the past used morphological characters (still do), but now can also use molecular techniques:

- protein comparisons = amino acids sequences

- molecular clock of cytochrome c

- rate of mutation important in Molecular clocks

- DNA sequences (Fig. 25.13)

- mitochondrial DNA

CONSTRUCTING PHYLOGENIES (Fig 25.11)

A phylogenetic tree should have an outgroup that is a closely related taxon recently ancestral to the organisms for which the phylogeny is being constructed. Ingroups share derived characteristics (synapomorphies) that the outgroup does not have. To determine characters to use, must determine if it is analogous or homologous. If homologous, must then decide if character is ancestral or derived.

Parsimony refers to a rule used to choose among possible cladograms, which states that the cladogram implying the least number of changes in character states is the best.

Fig. 26.16 - several systems of classification. Also Fig. 26.15

Classification schemes - the Movie!

Visit the Tree of Life to view the systematics of all living organisms.

Berkeley's Introduction to Systematics.

Berkeley's Introduction to Cladistics.