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