ORIGIN OF SPECIES CH 24 • Speciation: origin of new species • Microevolution: changes in allele frequencies • Macroevolution: changes that result in formation of different species I. The Biological Species Concept • Biological species: group of populations whose members can breed and produce viable, fertile offspring A. Reproductive Isolation • biological barriers that prevent formation of viable fertile offspring • Can be prezygotic or postzygotic • Prezygotic barriers:Prevent the formation of a zygote (fertilized egg) – Habitat Isolation: if 2 populations occupy different habitats, they don’t encounter each other to mate – Behavioral Isolation: Differences in behavior prevent mating – Temporal Isolation: Different breeding times prevent mating – Mechanical Isolation: Genitalia are incompatible – Gamete isolation: sperm doesn’t fertilize egg • Postzygotic Barriers: Prevents the formation of normal fertile offspring – Embryo formed is not viable – Offspring produced are weak and die – Offspring produced are sterile II. Mechanisms of Speciation • Allopatric Speciation – Results from lack of gene flow between populations that have been geographically isolated – Populations evolve independently due to mutation, natural selection, genetic drift – Reproductive isolation can result from genetic divergence • Sympatric speciation – Evolution of different species WITHOUT geographic isolation – Polyploidy in plants is the doubling of chromosomes from one generation to the next resulting in reproductive isolation – Appearance of new ecological niches in an area can result in sympatric speciation Polyploidy III. Hybrid Zones • Regions where two species with incomplete reproductive barriers meet and produce hybrid offspring • Usually a band where 2 species meet • 3 possible outcomes in the hybrid zone: – Reinforcement of reproductive barriers: hybrid offspring are less fit so rate of hybridization decreases – Fusion: if hybrids are as fit as parents there could be enuf mating and gene flow between the populations that they fuse into 1 species – Stability: extensive gene flow prevents the continued selection of reproductive barriers