LE 24-4ab

1. LE 24-4ab Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult Hybrid breakdown Reduced hybrid fertility Reduced hybrid viability Viable, fertile offspring Fertilization REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

3. Chapter 24 Reading Quiz • What is the term for the origin of a new species? • Evolution of many species from one common ancestor is known as… • ____ is the mutant condition when there are extra sets of chromosomes. • The concept that species diverge in spurts of relatively rapid change is… • True or False: An evolutionary trend does not mean that evolution is goal oriented.

4. 1. Distinguish between anagenesis and cladogenesis. • Anagenesis  (phyletic evolution) the transformation of an unbranched lineage of organisms, sometimes to a state different enough from the ancestral population to justify renaming it as a new species • Cladogenesis  (branching evolution) the budding of one or more new species from a parent species that continues to exist - this is more common and promotes biological diversity 

6. 2. Define morphospecies and explain how this concept can be useful to biologists. • “Morphospecies” characterizes species on the basis of measurable physical features • Species  Latin term meaning “kind” or “appearance” • Linnaeus described species in terms of their physical form (morphology) and it is still the most common method used for describing species 

7. 3. Define biological species (E. Mayr). • It is a population or group of populations whose members have the potential to interbreed with one another in nature and to produce viable, fertile offspring, but cannot produce viable, fertile offspring with members of another species 

8. 4. Describe some limitations of the biological species concept. • Reproductive barriers prevent interbreeding between closely related species • A “species” is the largest unit of population in which gene flow is possible 

9. 5. Explain how gene flow between closely related species can be prevented. • Reproductive barriers do not allow gene flow to occur • Reproductive isolation occurs in natural environments 

10. 6. Distinguish between prezygotic and postzygotic isolating mechanisms. • Prezygotic  impedes mating between species or hinders fertilization of the ova should members of a different species attempt to mate • Postzygotic  prevents the hybrid zygote from developing into a viable, fertile adult 

12. 7. Describe five prezygotic isolating mechanisms and give an example of each. • Habitat isolation  two species living in different habitats within the same area may encounter each other rarely ex: two species of garter snake, one is in the water, and one is on land 

13. #7 continued… • Behavioral isolation  species-specific signals and elaborate behavior to attract mates are important reproductive barriers among closely related species ex: fireflies, moths (pheromones) 

14. #7 continued… 3. Temporal isolation  two species that breed at different times of the day, seasons, or years do not mix gametes ex: brown and rainbow trout - brown only mate in the fall - rainbow only mate in the spring 

15. # 7 continues…. 4. Mechanical isolation  anatomical incompatibility may prevent sperm transfer when closely related species attempt to mate ex: plant floral anatomy may prevent pollen from entering into ovule 

16. The end of # 7 5. Gametic isolation  gametes of different species that meet rarely fuse to form a zygote, but it may not survive in the female internal environment 

18. 8. Explain why many hybrids are sterile. • If the chromosomes of the two parent species differ in number or structure, meiosis cannot produce normal gametes in the hybrid • Ex: mule and liger 

19. 9. Explain in your own words how hybrid breakdown maintains separate species even if gene flow occurs. • If species cross-mate, the first generation hybrid may be viable and fertile, but when these hybrids mate with one another or with the parent species, offspring will be feeble or sterile 

21. 10. Distinguish between allopatric and sympatric speciation. • Allopatric  speciation that occurs when the initial block to gene flow is a geographical barrier that physically isolates the population (ocean, mountains) • Sympatric  the formation of a new species within the range of parent populations - reproductive isolation evolves without geographical isolation 

23. 11. Explain, in your own words, the allopatric speciation model and describe the role of intraspecific variation and geographical isolation. • Allopatric speciation  speciation that occurs when the initial block to gene flow is a geographical barrier that physically isolates the population • A small isolated population is more likely to change substantially enough to become a new species than a large isolated population • Small populations may become geographically isolated from the parent population 

24. 12. Explain why peripheral isolates are susceptible if geographic barriers arise. • As long as the gene pools are isolated from the parental population, peripheral isolates are good candidates for speciation for 3 reasons: • The gene pool of the peripheral isolate probably differs from that of the parent population from the outset • Genetic drift will continue to cause chance changes in the gene pool of the small peripheral isolates until a large population is formed • Evolution caused by a selection is likely to take a different direction in the peripheral isolate than in the parental population 

25. 13. Describe the adaptive radiation model and use it to describe how it might be possible to have many sympatric closely related species even if geographic isolation is necessary for them to evolve. • Adaptive radiation  the evolution of many diversely adapted species from a common ancestor • Ex: Galapagos finches  ancestral finch from South America speciated on the Galapagos Islands  can occur quickly if a genetic change results in a reproductive barrier between the mutants and the parent population 

27. 14. Define sympatric speciation and explain how polyploidy can cause reproductive isolation. • Sympatric speciation  the formation of new species within the range of parent populations • Polyploidy is a mutant condition which originated from improper cell division = extra sets of chromosomes ex: larger fruits due to polyploidy 

28. 15. Distinguish between autopolyploidy and allopolyploidy. • Autopolyploid  an organism that has more than 2 chromosome sets, all derived from a single species ex: nondisjunction, self-fertilization • Allopolyploid  a polyploid hybrid resulting from contributions by 2 different species  when two different species interbreed and a hybrid is made  these sterile hybrids are more vigorous and can propagate asexually (plants) 

30. 16. List some points of agreement and disagreement between the two schools of thought about the tempo of speciation (gradualism vs. punctuated equilibrium). • Gradualism  species gradually diverge with each new species evolving continuously over long spans of time • Punctuated equilibrium  species appear and disappear suddenly with a lack of transitional forms in the fossil record • “sudden” can refer to 1000’s of years on the geological time scale • Stabilizing selection would hold a population at one adaptive peak to produce long periods of stasis • Gradualists feel that species continue to change during these periods in ways undetectable in fossils 

32. 17. Describe the origins of evolutionary novelty. • Most evolutionary novelties are modified versions of older structures  one mechanism is the gradual refinement of existing structures for new functions  “exaptation” is a term applied to a structure that evolves in one context and becomes co-opted for another function (ex: wings & light bones on birds) • Genes that control development play a major role in evolutionary novelty  requires large modifications at many gene loci • An evolutionary trend does not mean that evolution is goal oriented  the term “progression” is misleading 