The Origin of Species

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

8. 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

9. 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

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

11. Types of Prezygotic Reproductive Barriers 2. Temporal Isolation: species breed during different times of day, different seasons, or different years

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

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

14. 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

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

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

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

18. 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.

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

20. 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

21. 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

22. 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

23. 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

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

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

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

27. 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

29. Allopatric Speciation

30. Evidence of Allopatric Speciation • There 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

31. Drosophila Experiment

32. 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

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

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

35. 1. Autopolyploid • Individual has >2 chromosome sets all derived from a single species • Plant polyploidy

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

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

38. 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

39. 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

40. Top 2 parentsbottom: Triticumaestivum

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

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

43. Rhagoletispomonella • North American apple maggot fly • Original habitat was the native hawthorn tree

44. Habitat Differentiation • As 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) • The flies feeding on apple trees now show temporal isolation from flies still eating hawthorn fruit (prezygotic restriction to gene flow)

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

46. Cichlid Fish • >600 species found in Lake Victoria • originated in past 100 000 yrs • hypothesis : subgroups of original population adapted to different food sources  genetic divergence • female preference for mates may also be a factor: 1 species breeding males have blue back another species has orange back

47. Breeding Cichlids Colors

48. Sexual Selection Study • placed the 2 subspecies in same tank • used monochromatic orange light so both appeared very similar • females bred with either • Conclusion: mate choice by females is based on male coloration so it’s the main reproductive barrier (prezygotic behavior)

49. Allopatric & Sympatric Speciation are the 2 main modes of speciation • Allopatric • geographic isolation • NS • genetic drift • sexual selection • Sympatric • requires emergence of a reproductive barrier that isolates a subgroup • less common • polyploidy • sexual selection

50. Hybrid Zones