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Lecture 3

BIO153 Lecture 3.pdf

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University of Toronto Mississauga
Christoph Richter

BIO153: Lecture 3 Cladistics January 12, 2009 You may find it helpful to refer to the Glossary of terms used in Phylogenetic analysis (in the folder entitled “Supporting Material for Lectures”). Topics: ▯ comparing phenetics, Linnaean taxonomy and cladistics ▯ steps in conducting a cladistic analysis ▯ the principle of parsimony ▯ some practical applications of phylogenetic analysis Recall from lecture 2 that characters can be shared through convergence (two unrelated organisms resemble each other because they have been shaped in the same way by natural selection); characters can be shared by descent (two organisms resemble each other because they are closely related). (Example: horses and zebras share many characters because they are closely related (the ancestor of zebras and horses was around not long ago, and there has been relatively little time for horses and zebras to diverge from each other). On the other hand, zebras and tigers share the character of having striped fur, but this does NOT indicate a close evolutionary relationship – the ancestor that gave rise to both zebras and tigers was not striped, and stripes were acquired independently in both lineages.) Only characters shared by descent help us reconstruct evolutionary relationships. Characters shared through descent may be symplesiomorphies or synapomorphies Symplesiomorphies = ancestral homologies ▯ shared characters inherited from a distant common ancestor 1 ▯ Character is usually not found in all descendents (Why? If the common ancestor is distant, a lot of time has passed and there has been a lot of opportunity for the character to be modified in at least some of the descendents.) Synapomorphies = derived homologies ▯ Shared character is inherited from a recent common ancestor ▯ Character is usually found in all descendents (Why? If the common ancestor is recent, little time has passed and thus the character has probably not been modified in the descendents.) Because synapomorphies are usually found in all descendents, synapomorphies are reliable indicators of shared ancestry. Because symplesiomorphies are often not found in all descendents, symplesiomorphies are not reliable indicators of shared ancestry. The difference between using synapomorphies and symplesiomorphies in reconstructing phylogenies: cladistics versus phenetics (Example: classifying humans, sea stars and jellyfish) Phenetic taxonomy (also called numerical taxonomy): organisms are grouped on the basis of overall similarity (thus shared characters may be analogies, symplesiomorphies or synapomorphies) Jellyfish and sea stars are both invertebrates, jellyfish aquatic, have radial symmetry etc. – share many more characters than either shares with a human – sea star leading to the conclusion: Cladistic approach: only synapomorphies are human used to deduce evolutionary relationships. In this example, an aquatic lifestyle, radial symmetry, being invertebrate, are ancestral traits (traits present early in the evolution of animals). However, the sea star and the human share a derived trait (a synapomorphy) in the 2 pattern of embryonic growth exhibited by both species. Humans and sea stars are both deuterostomes; during embryonic development, the first opening that forms during gastrulation becomes the anus. This is a derived trait; the ancestral condition in animals is to be protostomate: in jellyfish, the first opening that forms during gastrulation becomes the mouth. human The true phylogeny: sea star jellyfish Cladistics: phylogenetic systematics ▯ system developed by German entomologist Willi Hennig ▯ Synapomorphies provide the only evidence for identifying recent common ancestry ▯ only monophyletic groups (no paraphyletic or polyphyletic groups) are permissible in a cladistic taxonomy Reconciling cladistics and Linnaean taxonomy: ▯ Linnaean taxonomy is rigid – 7 hierarchical levels (defined subjectively); number of levels in a cladistic analysis varies ▯ Linnaean taxonomy permits paraphyletic groups: “fish”; “reptiles”; cladistics doesn’t ▯ There are many more levels in a taxonomy derived through cladistics than in the Linnaean 7-rank system 3 A cladogram showing relationships among the dinosaurs. (Note where the birds are!) Cladistic analysis: ▯ figure out which traits are ancestral & which are derived traits (i.e. determine the polarity of the trait) ▯ figure out the “simplest”* tree that groups C DE organisms on the basis of shared derived outgroup A B traits ▯ simplest = most parsimonious = fewest evolutionary steps How to identify ancestral and derived characters? Ancestral traits are usually: outgroup: taxon known to have diverged from 1) traits present in fossils 2) common in many of taxa taxa under study 3) those that appear early in development speciation before they diverged 4) present in the outgroup from each other 4 Outgroup: 1. a taxonomic group that is known to have diverged from the ancestor (thenode) of all the taxa in the “ingroup”. 2. outgroup roots the tree groups that occupy 3. identifies the most basal (oldest) node adjacent branches more than 2 descendants (unresolved) – cladistic approach assumes bifurcating branching For example: to construct
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