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Speciation Where do species come from?

This text discusses the two basic patterns of evolutionary change, anagenesis and cladogenesis, in the context of speciation. It defines a species as a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but are unable to do so with members of other populations. The text explores the concept of reproductive isolation, the existence of biological factors that impede two organisms from producing viable, fertile offspring. These factors, known as reproductive isolating mechanisms or RIMs, can be prezygotic (occurring before fertilization) or postzygotic (occurring after fertilization). Prezygotic barriers include habitat isolation, temporal isolation, behavioural isolation, mechanical isolation, and gametic isolation, while postzygotic barriers include reduced hybrid viability, reduced hybrid fertility, and hybrid breakdown. The text also explains the concept of allopatric speciation, which occurs when gene flow between populations is interrupted by a geographical barrier, and sympatric speciation, which occurs when there is no geographical barrier to interrupt gene flow between populations. It explores examples and mechanisms of both types of speciation, including vicariance events, founder effects, chromosome changes, and behavioural differences. The text concludes with a discussion on the tempo of speciation over time and the concept of hybrid zones.

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Speciation Where do species come from?

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  1. SpeciationWhere do species come from?

  2. Two Basic Patterns of Evolutionary Change Anagenesis Cladogenesis

  3. Definition of a species Biological species A population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring but unable to do so with members of other populations

  4. The biological species is based on reproductive isolation Reproductive isolation - the existence of biological factors (=barriers*) the impede two organisms from producing viable, fertile offspring *these barriers are known as reproductive isolating mechanisms or RIM’s

  5. What are these ‘barriers’? 2 kinds: Prezygotic - those that inhibit reproduction before fertilization Postzygotic - those that inhibit reproduction after fertilization

  6. Prezygotic Barriers Habitat Isolation Water bugs - Trichocorixa Brackish saltmarsh ponds Freshwater ponds Never meet to breed

  7. Prezygotic Barriers Temporal Isolation Western spotted skunk Eastern spotted skunk Area where ranges overlap

  8. Prezygotic Barriers Temporal Isolation Western spotted skunk Eastern spotted skunk Area where ranges can overlap Mates in late winter Mates in late summer

  9. Prezygotic Barriers Behavioural Isolation Courtship differences Prairie grouse Sage grouse

  10. Prezygotic Barriers Mechanical Isolation - damselflies Mating in Damselflies

  11. Prezygotic Barriers Gametic Isolation -reproduction in sea urchins Red and purple urchins fertilize externally at the same time Gametes are incompatible and won’t fuse

  12. Prezygotic Barriers Summary Habitat Isolation Temporal Isolation Behavioural Isolation Mechanical Isolation Gametic Isolation All prevent production of offspring by acting before fertilization

  13. Postzygotic Barriers - those that inhibit reproduction after fertilization 1. Reduced hybrid viability X E. eschscholtzi E. klauberi Lower viability

  14. Postzygotic Barriers - those that inhibit reproduction after fertilization 1. Reduced hybrid fertility X Horse Donkey Mule - robust but sterile

  15. Postzygotic Barriers - those that inhibit reproduction after fertilization 1. Hybrid Breakdown - hybrid strains of rice Fertile hybrid Fertile hybrid Infertile offspring of the two hybrids

  16. Postzygotic Barriers Summary Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown All prevent production of offspring by acting after fertilization

  17. Postzygotic Barriers Prezygotic Barriers Habitat Isolation Temporal Isolation Behavioural Isolation Mechanical Isolation Gametic Isolation Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown Roughly reflect time since species began to differentiate

  18. Summary from your book

  19. Kinds of Speciation Allopatric (= “different country”) Speciation that occurs when the gene flow between populations is interrupted by a geographical barrier

  20. Kinds of Speciation Allopatric - via a vicariance event Geographical separation Original population Populations change independently of one another No interbreeding if barrier is removed

  21. Kinds of Speciation Allopatric - via a vicariance event A real example - antelope squirrels (Ammophilus) on the edges of the Grand Canyon A. leucurus A. harrisi

  22. Kinds of Speciation Allopatric - Founder effect (similar to a bottleneck) - at least one fertilized female invades a new habitat or geographical location Several generations Invasion of new habitat

  23. Kinds of Speciation Allopatric - Founder effect - Hawaiian Drosophila Initial invasion (6-8 million years ago)

  24. Kinds of Speciation Allopatric - Founder effect Often closely tied with ADAPTIVE RADIATION Adaptive radiation - the evolution of many diversely adapted species from a common ancestor after introduction to new environments

  25. Kinds of Speciation Allopatric - Founder effect ADAPTIVE RADIATION - Hawaiian silverswords Tarweed ancestor

  26. Kinds of Speciation 2. Sympatric (= “same country”) Speciation that occurs when there is no geographical barrier to interrupt gene flow between population.

  27. Kinds of Speciation 2. Sympatric (= “same country”) Sympatric speciation occurs most frequently by chromosome changes or through non-random mating.

  28. Kinds of Speciation 2. Sympatric (= “same country”) a. Chromosome changes Figure 24.8 in your book

  29. Kinds of Speciation 2. Sympatric (= “same country”) a. Chromosome changes Anemone

  30. Kinds of Speciation 2. Sympatric (= “same country”) b. Behavioural differences Apple maggot flies -laid eggs on hawthorn Apple maggot flies -lays eggs on hawthorn and apple Introduction of apples 200 years ago Now

  31. Kinds of Speciation 2. Sympatric (= “same country”) b. Behavioural differences “apple” flies Females prefer to lay eggs on apples “apple” males prefer “apple” females Females prefer to lay eggs on hawthorn “hawthorn” males prefer “hawthorn” females “hawthorn” flies “hawthorn” flies

  32. Tempo of Speciation How does it occur over time? T I M E

  33. Hybrid Zones Hybrid Zone Range of species A Range of species B

  34. Hybrid Zones

  35. Gene flow in hybrid zone Divergence and speciation Some barrier to gene flow is established Normal interbreeding populations Reinforcement Fusion Stability

  36. Reinforcement in Hybrid Zones

  37. Fusion in Hybrid Zones Cichlids in Lake Victoria

  38. Speciation Rates Single gene mutant Euhadra – Japanese land snail

  39. Finally, reproductive isolation by a pollinator Mimulus lewisii Mimulus cardinalis

  40. Finally, reproductive isolation by a pollinator Mimulus lewisii yuplewisii gene yupcardinalis gene Mimulus cardinalis

  41. Visits by pollinator 68x increase 74x increase M. lewisii M. lewisii with cardinalis yup gene M. cardinalis M. cardinalis with lewisii yup gene

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