Types & Rates of Speciation

2. Types & Rates of Speciation

3. Learning Objectives • Explain the processes and mechanisms that drive speciation. • Explain how speciation results in diverse lifeforms. • Describe the differences between allopatric and sympatric speciation. • Describe the rate of evolution and speciation under different ecological conditions.

4. Speciation Can Occur in Two Ways • Allopatric speciation-speciation occurs when gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations • Sympatric speciation-speciation occurs even though the two groups are still living in the same area.

5. Allopatric Speciation • Two populations become geographically isolated from one another. • If the groups are reunited, one of two things result… • They become separate species because they can no longer interbreed. • If they can still interbreed, remaining the same species.

6. Why does speciation occur after geographic isolation? • Separate populations may evolve independently through mutation, natural selection, and genetic drift • Reproductive isolation may arise as a by-product of genetic divergence • For example, isolated populations of mosquitofish have evolved reproductive isolation as a result of selection under different levels of predation

7. Talk with a Partner!!! • What is the reproductive isolation? • How is this allopatric speciation? • What are the two possible outcomes after 40 generations?

8. Allopatric Speciation • Regions with many geographic barriers typically have more species than do regions with fewer barriers • Islands produce some of the most profound examples of allopatric speciation due to geographic isolation • Reproductive isolation between populations generally increases as the distance between them increases

9. Allopatric Speciation Leads to Adaptive Radiation • Adaptive radiation occurs when many new species arise from one common ancestor. • Speciation rates can be rapid during times of adaptive radiation as new habitats become available. • Classic example involves the finches of the Galapagos Islands.

10. Allopatric Speciation Leads to Adaptive Radiation • There are 14 different species of finches found on 13 main islands and 3 smaller islands, with a variety of food sources on each island.

11. How does Adaptive Radiation Occur? • Migration to an island (Founder Effect) • Varying selection pressures • Varying mutations • Gene flow is prevented due to reproductive isolation • A new species evolves

12. Sympatric Speciation • In sympatric speciation, speciation takes place in geographically overlapping populations • Sympatric speciation can occur if gene flow is reduced by factors including • Polyploidy • Sexual selection • Habitat differentiation

13. Sympatric Speciation Due to Polyploidy • Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division • much more common in plants than in animals • Polyploidy can produce new biological within a single generation • Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids • Two types of polyploidy

14. Types of Polyploidy • An autopolyploid is an individual with more than two chromosome sets, derived from a single species • Can produce fertile offspring by self-pollinating or mating with other tetraploids • Can no longer mate with other diploids

15. Types of Polyploidy • An allopolyploid is a species with multiple sets of chromosomes derived from different species

16. Sympatric Speciation: Due to Sexual Selection • Sexual selection can drive sympatric speciation • Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria

17. Sympatric Speciation Due to Habitat Differentiation • Sympatric speciation can also result from the appearance of new ecological niches • For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees

18. Sympatric Speciation: Habitat Differentiation • Suppose that a certain species feeds on a particular host and only that host. • Next, suppose a mutation occurs that allows it to feed upon a different host. • Eventually, the species is divided into two groups that are separated from one another. Given enough time, speciation can occur.

19. Figure 24.5 Allopatric or Sympatric? B A

20. Figure 24.5 Allopatric or Sympatric?

21. Figure 24.5 Allopatric or Sympatric?

22. Figure 24.8 Allopatric or Sympatric? ATLANTIC OCEAN A. nuttingi A. formosus PACIFIC OCEAN Isthmus of Panama A. panamensis A. millsae

23. Allopatric and Sympatric Speciation: Summary • In allopatric speciation, geographic isolation restricts gene flow between populations • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations • Even if contact is restored between populations, interbreeding may be prevented

24. Allopatric and Sympatric Speciation: Summary • In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species • Sympatric speciation can result from polyploidy, natural selection, or sexual selection

25. Hybrid Zones • What happens if two species with incomplete reproductive barriers come into contact?

26. Hybrid Zones • A hybrid zone is a region in which members of different species mate and produce hybrids, where the habitat of the interbreeding species meet • Hybrids are the result of mating between species with incomplete reproductive barriers and have reduced fitness • Once a hybrid zone is formed, how does the hybrid zone change over time?

27. Hybrid Zones • When closely related species meet in a hybrid zone, there are three possible outcomes • Reinforcement • Fusion • Stability

28. Figure 24.13-4 Hybrid Zones Possible outcomes: Isolated population diverges. Hybrid zone Reinforcement Fusion Gene flow Population Barrier to gene flow Stability Hybrid individual

29. Reinforcement: Strengthening Reproductive Barriers • When hybrids are less fit than parent species, reinforcement of reproductive barriers may occur through strong selection for prezygotic or postzygoticbarriers • Over time, the rate of hybridization decreases • Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species • For example, in populations of flycatchers, males are more similar in allopatric populations than sympatric populations

30. Figure 24.14 Females choosing between these males: Females choosing between these males: Sympatric pied male Allopatric pied male Sympatric collared male Allopatric collared male 20 16 12 Number of females 8 4 (none) 0 Own species Other species Own species Other species Female mate choice Female mate choice

31. Fusion: Weakening Reproductive Barriers • If hybrids are as fit as parents, there can be substantial gene flow between species • If gene flow is great enough, reproductive barriers weaken and the parent species can fuse into a single species • For example, pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species

32. Figure 24.15 Pundamilianyererei Pundamiliapundamilia Pundamilia“turbid water,” hybrid offspring from a location with turbid water

33. Stability: Continued Formation of Hybrid Individuals • Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone • For example, parent species of Bombina routinely migrate into the narrow hybrid zone resulting in ongoing hybridization

34. Talk with a Partner!!! • Explain the three possible changes in the hybrid zone over time?

35. Talk with a Partner!!! Lions form groups or prides and live in the grasslands. Tigers are more solitary and live in the forests. Tigers and lions form a hybrid zone where their habitats merge and produce sterile ligers. What is the likely outcome should this hybrid zone persist?

36. Talk with a Partner!!! What is the likely outcome if the hybrid offspring survived and reproduced better than offspring from intraspecific mating?

37. Tempo of Speciation • Speciation can occur rapidly or slowly and can result from changes in few or many genes • Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data

38. Patterns from the Fossil Record • 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 • The punctuated equilibrium model contrasts with a model of gradualism in a species over time, where the population slowly accumulate changes and evolves.

39. Figure 24.16 (a) Punctuated model Time (b) Gradual model

40. Tempo of Speciation • Anagenesis- Over time a species may accumulate enough changes that it leads to a species that differs from the ancestral species. • Cladogenesis- is the budding of one or more new species from an ancestral species that continues to exists. This results in biological diversity.

41. From Speciation to Macroevolution • Macroevolution is the cumulative effect of many speciation and extinction events

42. Summary of Learning Objectives