Mechanisms of Speciation.ppt

Mechanisms of Speciation and Macroevolution 1 Mechanisms of Speciation and Macroevolution I. The Biological Concept of Species II. Mechanisms of Speciation III. Hybrid Zones IV. Rates of Speciation 2 Learning Objectives 1. Apply the biological species concept to the study of animal diversity 2. Eval...

Mechanisms of Speciation and Macroevolution 1 Mechanisms of Speciation and Macroevolution I. The Biological Concept of Species II. Mechanisms of Speciation III. Hybrid Zones IV. Rates of Speciation 2 Learning Objectives 1. Apply the biological species concept to the study of animal diversity 2. Evaluate the usefulness of the phylogenetic, ecological and morphological species concepts 3. Distinguish between prezygotic and postzygotic reproductive barriers 4. Compare and contrast allopatric and sympatric speciation 3 Learning Objectives 5. Explain hybrid zones and the three directions hybrid zones can follow over time 6.Explain punctuated equilibrium 7.Compare punctuated equilibrium to Darwin’s gradualistic evolutionary change 4 I. The Biological Concept of Species A. The biological species concept states that a species is a group of populations whose members have the potential to interbreed and produce viable, fertile offspring. However, they do not breed successfully with other populations 5 B. This concept emphasizes that gene flow must not exist for new species to form. Gene flow is what maintains the gene pool of a species together. 6 D. Prezygotic reproductive barriers These barriers block fertilization from occurring by: 1.Impeding different species from attempting to mate 2.Preventing the successful completion of mating 3.Hindering fertilization if mating is successful 7 E. Postzygotic reproductive barriers These prevent the hybrid from developing into a viable, fertile adult: 1. Reduced hybrid viability: incomplete development 2. Reduced hybrid fertility: Hybrids are strong but sterile 3. Hybrid break down: Hybrids are fertile but the 2nd generation is sterile 8 F. Limitations of the biological species concept 1. Cannot be applied to fossils or asexual organisms (including all prokaryotes) 2. Emphasizes absence of gene flow. However, gene flow can occur between distinct species  For example, grizzly bears and polar bears can mate to produce “grolar bears” 9 F. Other species concepts include sexual and asexual species: The morphological species concept defines a species by structural features The ecological species concept views a species in terms of its ecological niche The phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree 10 II. Mechanisms of Speciation A. Allopatric (“Other Country”)Speciation: Two species may form when a geographic barrier interrupts gene flow between two populations. 1. The Grand Canyon is a barrier that separates two closely related species of Ammospermophilus antelope squirrels 11 2. Separate populations may evolve independently through mutation, natural selection, and genetic drift Reproductive isolation may arise as a result of genetic divergence Ex. Flightless cormorant 12 B. Sympatric (“Same Country”) Speciation. This occurs within populations with overlapping boundaries. 1. Meiosis can produce gametes with extra chromosome numbers and lead to speciation (Polyploidy) a) Autopolyploidy b) Allopolyploidy 13 2. Habitat Differentiation: When genetic traits enable some individuals to use a space or resource not used by the parent population. Ex. Feeding differences development in neotropical cichlids. 14 3. Intersexual selection may also lead to sympatric speciation. Ex. Female desert funnel web spiders from canyons avoid mating with aggressive dry land males 15 JiTT Question Genetic similarities suggest that anole lizards in Puerto Rico evolved from a common ancestor that colonized the island. What speciation mechanism may be responsible for the evolution of five anole species? a)Sympatric speciation b)Allopatric speciation 16 III. Hybrid Zones A. These areas produce individuals of mixed ancestry as members of two species meet and mate. 17 B. When closely related species meet in a hybrid zone, there are three possible outcomes: 1. Reinforcement: Hybridization decreases, weak hybrids 2. Fusion: Gene flow leads to two species becoming one 3. Stability: Hybrids continue to be produced 18 IV. Rates of Speciation A. The Darwinian process of descent with modification implies a gradual process of speciation that occurs over long periods of time. 19. The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria to describe periods of apparent stasis punctuated by sudden change 20