L6 Species concepts3-11 - Lecture 6 Species Concepts What...

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4/14/11 1 Lecture 6: Species Concepts What is a Species? Species Concepts • Biological Species Concept • Phylogenetic Species Concept • Problems in Recognizing Species Why do species concepts matter? • permits identification and understanding of biodiversity • taxonomic status often dictates conservation priority Steps in delimiting species common to all species concepts 1. Survey morphological or molecular variation within and among populations. 2. Group individuals into groups (taxa, taxon). 3. Rank taxa in taxonomic hierarchy (subspecies, species). Species Concepts Biological Species Concept: • actually or potentially interbreeding populations that are reproductively isolated from other such groups Species Concepts Biological Species Concept (BSC) – most widespread definition – proposed by Ernst Mayr in 1942 – useful in many settings – reproductive isolation (lack of gene flow) is the criterion
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4/14/11 2 Defining Biological Species Ask the question - Do these two populations interbreed or would they if they came into contact? Species Concepts Problems with the BSC: • doesn t account for asexual organisms • can t be applied to fossil forms • not useful in plant groups where hybridization is common • evolution happens prior to reproductive isolation • if isolated populations don t overlap, can t test the BSC Testing the BSC Example: Brown Towhee Two populations that are slightly divergent. Ranges don t overlap, are they one species or two? Species Concepts Phylogenetic Species Concept (PSC) – the smallest diagnosable cluster of organisms within which there is a parental pattern of ancestry and descent (Cracraft 1989) diagnosable - characters differentiate the species ancestry and descent - share history Species Concepts diagnosable characters - synapomorphies PSC: the smallest monophyletic group of common ancestry Defining Phylogenetic Species Build a phylogeny of populations Group of populations that forms an independent branch is considered a species.
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