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The Origin of Species

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  1. The Origin of Species chapter 24 Campbell and Reece

  2. Speciation • process by which one species splits into 2 or more species • Speciation explains both the diversity of life and the unity of living things.

  3. Speciation : forms bridge between: MICROEVOLUTION • Evolutionary change below species level • Example: • change in allele frequencies in population over generations MACROEVOLUTION • Evolutionary change above the species level • Examples: • origin of new group of organisms • impact of mass extinctions

  4. Biological Species Concept • Species: a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring • members of a species cannot produce viable, fertile offspring with other groups • emphasizes the separateness of species due to reproductive barriers

  5. What holds the gene pool of a species together? • Gene Flow: transfer of alleles between populations of same species • exchange of alleles tends to hold populations together genetically

  6. Reproductive Isolation • existence of biological barriers that keep members of 2 populations from interbreeding over long periods of time

  7. Reproductive Isolation • block gene flow between the species & limit formation of hybrids

  8. Reproductive Isolation • hybrids: offspring that result from the mating of individuals from 2 different species or from 2 true-breeding varieties of same species

  9. Reproductive Isolation • single barrier may not prevent all gene flow • combination of several barriers can effectively isolate a species’ gene pool • 2 categories: • prezygotic barriers • postzygotic barriers

  10. Prezygotic Barriers • block fertilization from happening by: • impeding members of different species from attempting to mate • preventing attempted mating from being completed successfully • hindering fertilization if mating was completed successfully

  11. Postzygotic Barriers • reproductive barrier that prevents hybrid zygotes produced by 2 different species from developing into viable, fertile adults • lethal developmental errors • infertility in viable offspring

  12. Types of Prezygotic Reproductive Barriers • Habitat Isolation 2 species that occupy different habitats w/in same area may rarely interact example:

  13. Types of Prezygotic Reproductive Barriers 2. Temporal Isolation: • species breed during different times of day, different seasons, or different years • eastern & western spotted skunks: 1 breeds in late summer other in late winter

  14. Types of Prezygotic Reproductive Barriers 3. Behavioral Isolation: courtship rituals used to attract mates are effective barriers

  15. Types of Prezygotic Reproductive Barriers 4. Mechanical Isolation: • morphological differences prevent successful completion even if attempted

  16. Types of Prezygotic Reproductive Barriers 5. Gametic Isolation: • sperm of 1 species may not be able to fertilize egg of another: • reproductive tract hostile to sperm • sperm does not have enzymes to penetrate zonapellicida of another species

  17. Types of Postzygotic Reproductive Barriers • Reduced Hybrid Viability: hybrids development or survival is impaired

  18. Types of Postzygotic Reproductive Barriers 2. Reduced Hybrid Fertility: hybrids may develop and be healthy but they are not fertile

  19. Types of Postzygotic Reproductive Barriers • Hybrid Breakdown: Some 1st generation hybrids are fertile but those offspring are feeble or sterile

  20. Species • There is no single, universally applicable species concept that can define, explain, and identify all species. • There are multiple ways to think about & define species.

  21. Biological Species Limitations • unable to use these characteristics on fossils of extinct species • only applies to organisms that reproduce sexually • only applies where there is no gene flow

  22. Other Definitions of Species • these dfns emphasize the unity w/in a species. • morphological species concept: • characterizes a species by a structural feature • applies to species that reproduce sexually or asexually • how scientists distinguish most species • disadvantage: subjective

  23. Morphological Species Concept • Problems: • domestic dogs may look very different but are still same species • mouse lemurs look very similar but there are 18 species of them • grey mouse lemur lesser mouse lemur

  24. Ecological Species Concept • views species in terms of its niche • the sum of how members of the species interact with the nonliving & living parts of their environment • asexual or sexual species • emphasizes role of disruptive NS as organisms adapt to different environmental conditions

  25. Phylogenetic Species Concept • defines species as smallest group of individuals that share a common ancestor, forming one branch on the “tree of life” • determining degree of differences is difficult

  26. Species • There are >20 other ways to define species

  27. Speciation can take place with or w/out geographic separation • Speciation can occur in 2 main ways: • Allopatric Speciation • Sympatric Speciation

  28. ALLOPATRIC SPECIATION • “other country” • gene flow is interrupted when population is divided into geographically isolated subpopulations

  29. Allopatric Speciation Process • once geographic separation has occurred, the separated gene pools will each have their own mutations • NS & genetic drift may alter allele frequencies in different ways in each subpopulation group

  30. Allopatric Speciation

  31. Evidence of Allopatric Speciation • are many studies & examples supporting this type speciation • indirect support: regions that are isolated or highly subdivided have more species than regions w/out those features

  32. Drosophila Experiment

  33. Sympatric Speciation • “same country” • occurs in populations in same geographic area • less common than allopatric • occurs if gene flow is reduced by factors like: • polyploidy • habitat differentiation • sexual selection

  34. POLYPLOIDY • means extra sets of chromosomes • can occur in animals • gray tree frog (Hylaversicolor) • around Great Lakes

  35. Polyploidy • much more common in plants • estimate: 80% of today’s plants species have ancestors that formed by polyploid speciation • 2 forms

  36. 1. Autopolyploid • individual has >2 chromosome sets all derived from a single species • plant polyploidy

  37. Plant Polyploidy • tetraploid plant can produce fertile tetraploid offspring by self-pollinating or mating with other tetraploids

  38. 2. Allopolyploid • 2 different species interbreed making a hybrid • hybrid reproduces asexually • over generations sterile hybrid  fertile polyploid (called an allopolyploid)

  39. Allopolyploids • can breed with each other but not with either of their parents so are a new species • rare: 5 new plant species since 1850 documented • Mimulusperegrinus

  40. Allopolyploids • include many agricultural crops • Triticumaestivum(bread wheat) has 6 sets chromosomes (2 pair from each of 3 parents), an allohexaploid • 1st polyploidy event probably occurred ~8,000 yrs ago as spontaneous hybrid

  41. Top 2 parentsbottom: Triticumaestivum

  42. Allopolyploids • plant geneticists “create” new polyploids making hybrids with desired characteristics • use chemicals that induce meiotic & mitotic errors

  43. Habitat Differentiation • Sympatric speciation can occur when genetic factors enable a subpopulation to exploit a habitat or resource used by the parent population

  44. Rhagoletispomonella • North American apple maggot fly • original habitat was the native hawthorn tree

  45. Habitat Differentiation • apples mature faster than hawthorn fruit, NS has favored flies with rapid development • have an allele that benefits flies that feed off only 1 or the other not both (post-zygotic barrier to reproduction) • flies feeding on apple trees now show temporal isolation from flies still eating hawthorn fruit (prezygotic restriction to gene flow)

  46. Sexual Selection • can also drive sympatric speciation: • cichlid fish Pundamiliapundamilia