Evolution

1. Evolution

2. Evolution • Evolution occurs whenever there is a change is frequency of genes in a population • Individuals do not evolve, but populations do • There are several agents of populations change; name some: • Genetic drift, gene flow, meiotic drive, & mutation Natural Selection!!

3. Evolution • Genetic Drift: a random sampling bias; generally associated with small pop(s) • Gene Flow: migration movements of plants and animals within and among pop(s) with different gene frequencies • Meiotic Drive: the over-representation of an allele • Mutation: mutation rates can change the frequencies of alleles (or create new ones)

4. Evolution • However, only NS is the only pressure that results in conformity between organisms and their environment

5. Types of NS • Under stable conditions, intermediates in a population typically leave more descendants, on average, than do the extremes (more fit) • Remember, this is a relative measure • Selection of this sort is termed ‘stabilizing selection’

6. Types of NS • In a stable environment, genetic recombination increases the population’s variance each generation whereas stabilizing selection reduces it to approximately what it was in the previous generation

7. Types of NS • Stabilizing Selection: pick a trait

8. Types of NS

9. Types of NS • Directional Selection

10. Types of NS • A third type usually occurs in distinctly heterogeneous environments with a discrete number of different ‘patches’ • Disruptive selection is one mechanism that produces and maintains polymorphisms (e.g. the green brown color morphs of many insects)

11. Types of NS • Disruptive Selection

12. Types of NS • Consider leaf butterflies • Pretty good camouflage…

13. Types of NS • Another morph resembles live leaves…however intermediate morphs do not tend to resemble either and are subsequently selected against

14. Types of NS • Another important type of NS is known as frequency-dependent selection • This occurs when the fitness of a particular trait varies with its frequency in the population • Negative frequency-dependent selection promotes genetic variability • E.g. prey switching when abundance falls

15. Ecological Genotypes • The European land snail is polymorphic (genetically based) for shell color: brown, pink and yellow • Dietary preference can be determined by shell piles of birds

16. Types of NS • Fig 7.2 As conditions green-up in spring, the % of yellow shells become rarer • Thus thrush predation will help maintain the polymorphism

17. Allopatric and Sympatric Speciation • How do new species arise? • How does two populations become two species? • The most common form of speciation is of allopatric (or geographic)

18. Allopatric and Sympatric Speciation • When a population is broken into two, sets of populations are isolated and gene exchange is prevented, thus allowing them to diverge if they are subjected to different selection pressures

19. Allopatric and Sympatric Speciation • There are many things that can act as barriers: glaciers, mountains, oceans, rivers

20. Allopatric and Sympatric Speciation

21. Allopatric and Sympatric Speciation • Black and Juniper Titmouse distributions separated by the Sierra Nevada Mts.

22. Allopatric and Sympatric Speciation • When gene flow ceases, geographically isolated populations of a species are free to adapt to local conditions • Given enough local pressure, the two isolated populations may have diverged greatly from one another • Accumulation of differences may or may not have occurred

23. Allopatric and Sympatric Speciation • Eventually, the two incipient ‘species’ interbreed and hybridize extensively, the two subsets merge together into one species again in a process known as introgression

24. Allopatric and Sympatric Speciation • However, if enough differences have accumulated and the hybrids have reduced fitness, natural selection may favor the evolution of reproductive isolating mechanisms that prevent introgression and reinforces the differences

25. Allopatric and Sympatric Speciation • Speciation can also occur more or less instantly, without geographic isolation, and in several ways • In plants, allotetraploidy can occur v. quickly • Consider AA BB CC and XX YY ZZ hybridizing (F1 ABCXYZ); the offspring is sterile, but potentially superior in intermediate habitats

26. Allopatric and Sympatric Speciation • Eventually endoduplication (relatively common in plants) or a non-disjunction event during meiosis converts the hybrid’s genome from ABCXYZ to AA BB CC XX YY ZZ and restoring fertility

27. Allopatric and Sympatric Speciation • Another example of sympatric speciation occurs in tephritid fruit flies (oviposit on hawthorn and apple trees) • It appears that they have expanded their host range to include several other members of the family Rosaceae • The young tend to return to the same host species the ‘devoured’ as young larvae

28. Reproductive Isolating Mechanisms • Many closely related species that do not interbreed in nature have been hybridized in capavity • E.g. all pairs of Falcocan produce young; however, this does not occur in the wild

29. Reproductive Isolating Mechanisms • Isolating mechanisms can be prezygoticor postzygotic • Hybrid sterility is an example of postzygotic isolating mechanism • Prezygotic behavioral isolating mechanisms can involve courtship behavior, pheromones, vocalizations, and/or color patterns that promote species recognition and prevent mismatching

30. Galapagos Finches • The Galapagos Islands support a remarkable group of finches that illustrate many evolutionary principles • Only 26 species of land birds occurred in the archipelago naturally and 13 are finches

31. Galapagos Finches • The archipelago (16 major islands) were formed from volcanic activity and their entire biota is from mainland sources • However, because they are relatively remote, few things get out to them

32. Galapagos Finches • The 13 finch species are thought to have evolved from a single mainland ancestor • Archipelagos are great for speciation events • With effectively isolated populations, different selective pressures can act upon each population • Subsequent recolonization can lead to multiple species per island and further divergence

33. Reproductive Isolating Mechanisms • There are ground finches (6) differing in bill morphology, tree finches (4) forage in trees and are insectivores (1 vegetarian) • There is also a ‘woodpecker’ finch which uses cactus spines, a ‘warbler’ finch is a small insectivore • Combinations ranged from 3 to 10 species with good evidence of character displacement

34. Galapagos Finches

35. Galapagos Finches • Hawaiian honeycreepers

36. Reproductive Isolating Mechanisms • Combinations ranged from 3 to 10 species with good evidence of character displacement • Beak lengths and depths are highly variable from island to island (fig. 7.5)

37. Reproductive Isolating Mechanisms • Small islands only support a single species. Where they are sympatric (co-occur), we see character displacement

38. Reproductive Isolating Mechanisms • Bill depth has changed due to recent environmental events

39. Reproductive Isolating Mechanisms

40. Types of NS