Bio1b Summer 2008 Evolution Lecture 6 Eric Harris Page 1 of 2 Updated on 06/17/08 EVOLUTION LECTURE 6: SPECIATION Reading: 7th ed., 472–480; 8th ed., 487–498, 523–524. A. Species Concepts i) Morphological species concept – using morphological features, problems with variation (polymorphism) and with change. ii) Biological species concept – a species is a group of potentially interbreeding natural populations that are reproductively isolated from other groups. Problems with asexual organisms, extinct organisms, geographical separation, and ring species. (***There are many more species concepts and there is controversy. e.g., 7th ed., p. 476, 8th ed. p.492) iii) Importance of species definitions for conservation B. Isolating mechanisms i) Premating (prezygotic) – mechanisms that act to prevent zygote formation Habitat isolation – species occupy different habitats Temporal isolation – reproduction occurs at different times Behavioral isolation – e.g., courtship displays are very different Mechanical isolation – physical anatomy does not correspond Gametic incompatibility – egg and sperm cannot fuse ii) Postmating (postzygotic) – mechanisms that occur after fertilization Hybrid inviability – hybrids do not survive Hybrid sterility – hybrids survive but cannot reproduce Hybrid breakdown – hybrids survive and can mate but their offspring are sterile C. Modes of speciation i) Allopatric speciation – populations are separated by a geographical barrier a) The ‘Dumb-bell model’ – divides ancestral population into two large populations b) Peripheral isolate model – isolation of small population at the periphery of range. Important because: gene pool may already be different, population is small and the environment may be different. Repeated isolation of populations may lead to adaptive radiation.Bio1b Summer 2008 Evolution Lecture 6 Eric Harris Page 2 of 2 Updated on 06/17/08 ii) Parapatric speciation – speciation across a continuous distribution; an intermediate between allopatric and sympatric iii) Sympatric speciation – new species arise without geographic isolation a) Autopolyploidy, - one species doubles its chromosome number, self-fertilization likely. b) Allopolyploidy – 2 different species form an offspring with an increased sets of chromosomes. c) Disruptive selection and assortative mating – habitat
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