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Lecture 2

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Simon Fraser University
Biological Sciences
BISC 316
Tammy Mc Mullan

Lecture 2 – BISC 316 Vertebrate Biology Summer 2011 Classification = grouping of organisms taxonomy = the naming and classification of species phylogeny = the evolutionary history of a species or group of related species systematics = the study of biological diversity in an evolutionary context WHY ARE WE CONCERNED WITH THE CLASSIFICATION OF ORGANISMS -museums -conservation biology and biodiversity -understanding the biology of vertebrates requires an appreciation of the diversity of the organisms that make up this group where did the science of classification start? -systematics/taxonomy -concerned with the diversity of organisms -originally designed by Carl Linneaus -organisms are grouped into taxa (singular=taxon) -originally based on similarities -now based on evolutionary relationships Hierarchy of Classification Kingdom Phylum Class Order Family Genus Species Species: the basic unit; a group of naturally interbreeding populations that are genetically isolated from other groups. Rules for Classification 1. 'Narrow' taxa - subsets of 'broader' taxa -> each higher taxon is an assemblage of evolutionary related lower taxa -> at Order level and above - shared structural features 2. Broader taxa have fewer characteristics common to all members of the taxon -> ancestral features 3. Broader taxa originated earlier in evolutionary time Phylogenetic Trees -relationship between organisms -two significant structural features 1. location of the branch point -> relative time of origin of different taxa 2. extent of divergence between the two taxa -> divergence from the common ancestor If classification is based upon the evolutionary history of the group, which property should be given the most weight? -> three different approaches to classification 1. Phenetics 2. Cladistics 3. Classical evolutionary systematics 1. Phenetics or Numerical Taxonomy - less subjective -no phylogenetic assumptions - taxonomic affinities based entirely on measurements -uses many anatomical characteristics -reduced bias -homology vs. analogy - computer analysis of multiple quantitative comparisons -important toll -molecular comparison x critics - overall phenotypic similarity is not reliable index of genotypic proximity 2. Phylogenetic Systematics or Cladistics -Classifies organisms based upon the branching pattern in the Cladogram -> each taxon evolved by the dichotomous splitting from the sister group -objective - identify a series of nesting sister groups -increasing inclusive levels of evolutionary hierarchy -> does not consider the degree of evolutionary divergence -> each branching point is a novel feature unique to that taxon -> features should establish ancestry Cladograms -each cladogram represents an evolutionary hypothesis -features called character-states -> ancestral (or plesiomorphic) or derived (apomorphic) -evolutionary sequence of character states -cladograms constructed to express probable ancestry In cladistics: - all taxa must be monophyletic -i.e. each taxon must contain all the descendants of the common ancestor -other classication systems -may have taxa that are polyphyletic -more than one ancestor for memebers of the taxon or paraphyletic -may exclude some species that share the same common ancestor 3. Classical Evolutionary Systematics oldest system -balances the extent of divergence with the branching sequence - if conflict-> subjective judgement gives one higher priority -identified characteristics indicate evolutionary relationships -comparative ana
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