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Chapter 24 Notes.pdf

3 Pages

Course Code
BIOL 226
Robert Weladji

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CHAPTER 24. THE ORIGIN OF SPECIES • Speciation, the origin of new species, is at the focal point of evolutionary theory • The process by which one species splits into two or more species • Evolutionary theory must explain how new species originate and how populations evolve • Microevolution consists of adaptations that evolve within a population, confined to one gene pool • Macroevolution refers to evolutionary change above the species level – appearance of evolutionary novelties (e.g. feathers and flight in birds) used to define higher taxa Biological Species Concept • Defined by Ernst Mayr 1942 • Species: population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring • Fig 24-2 • Criterion: potential to interbreed • Strength: addresses the mechanism of isolation • Weakness: doesn’t work for? • 1. Fossils • 2. Species without sexual reproduction • Also many sexual organisms – Lack of knowledge about their ability to mate with different kinds of organisms Therefore – Need of alternative species concepts Morphological Species Concept • Criterion: similarity of appearance • Strength: easy and objective • Weakness: – similar looking things can be different species – different looking things can be the same species Why are species discrete? • Reproductive isolating mechanisms • Prevents populations belonging to different species from interbreeding, even if their ranges overlap • 2 types of reproductive barriers? • Prezygotic barriers (before zygote) • Postzygotic barriers (after zygote) Prezygotic barriers • Habitat • Time • Behavior • Mechanical • Gametic 1 Postzygotic barriers • Reduced hybrid viability • Reduced hybrid fertility • Hybrid breakdown Speciation can occur in 2 ways (1) Allopatric speciation (2) Sympatric specieation Allopatric (“Other Country”) Speciation • In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations • One or both populations may undergo evolutionary change during the period of separation • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations • Even if contact is restored between populations, interbreeding is prevented • Why is geographic isolation so important? • Keeps the gene pools separate • Allows populations to diverge by the 5 mechanisms Sympatric (“Same Country”) Speciation • In sympatric speciation, speciation takes place in geographically overlapping populations • Gene flow may be reduced by: – Polyploidy – Habitat differentiation – Sex
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