Zool 250: DIVERSITY, PHYLOGENY, CLASSIFICATION
(Read Ch. 30)
- 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) 'body plan' (symmetry, size, 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:
- distribution of characters (structures, genes) among living spp.
- fossil record (not emphasized much in Z250), unfortunately it is very incomplete
- 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 characters among taxa
- it uses a matrix of data (rows= taxa, column= characters)
- the character state (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
- it determines the 'best' phylogenetic tree by finding the one that requires the fewest total character state changes
- 5) Organisms are placed into a heirarchical (nested) classification: kingdom, phylum, class, order, family, genus, species, variety
- ideally(!), a classification should not contradict a cladogram
- 6) "Sister group" refers to a branch of a cladogram that lies adjacent to another branch (e.g., the Porifera are the sister group to all the remaining Metazoa)
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Copyright © 1999 by A. Richard Palmer. All rights reserved.
(revised Jan. 6, 1999)