1) How do we grapple with the bewildering diversity of animal life?
will emphasize common elements for each taxon: a) cladogram, b) key characters, c) habitat/habits, d) general 'body plan' (size, symmetry, regionalization, limb number & form, skeleton, body cavities), e) locomotory & feeding structures, f) diversity of form
2) Two kinds of information help us infer the history of animal life:
the fossil record, unfortunately it is very incomplete
distribution of features (structures, genes) among living spp.
3) "Characters" are sometimes divided into two groups:
homologous (present in a common ancestor) and
analogous (similar in form but not present in common ancestor)
although easy in principle, the distinction can be very difficult
4) Cladistic analysis is a formal method for inferring a phylogenetic tree (cladogram) based on distribution of character states among taxa
it uses a matrix of data (rows= taxa, column= characters)
character: some observable feature that varies among taxa
the character state (e.g., presence/absence) is noted for each character
it assumes: 1) all organisms evolved from pre-existing ones by a simple branching process and 2) convergence is rare
'best' phylogenetic tree (still a hypothesis) requires the fewest total character state changes (= shortest tree): criterion of parsimony
cladograms are very helpful for remembering character states
5) DNA sequence data & computers have triggered a surge of interest in phylogenetic relations unseen since the turn of the century
6) "Sister group" refers to a branch of a cladogram that lies adjacent to another branch (e.g., the Porifera is the sister group to all the remaining Metazoa); sister group is not synonymous with ancestor
7) Organisms are grouped into a hierarchical (nested) classification: kingdom, phylum, class, order, family, genus, species, variety
ideally(!) a classification should not contradict a cladogram: valid taxa (group names) should be monophyletic, not polyphyletic
paraphyly may be common: it causes classification problems BUT can yield valuable insights about character evolution