Mutationism, Neutralism, Selectionism

1. Mutationism, Neutralism, Selectionism

2. Neutral mutations Deleterious mutations Overdominant mutations Advantageous mutations

3. New mutations may be: 1. Advantageous 2. Overdominant 3. Deleterious 4. Neutral

4. Probability of fixation: 1. Advantageous - low 2. Overdominant – close to zero (genetic load) 3. Deleterious - very very low 4. Neutral - very low

5. Conditional time to fixation: 1. Advantageous - fast 2. Overdominant - extremely slow 3. Deleterious - fast 4. Neutral - slow

6. Neutral mutations Deleterious mutations Overdominant mutations Advantageous mutations

7. Amount of variability created: 1. Advantageous - none 2. Overdominant - a lot 3. Deleterious - none 4. Neutral - some

8. THE DRIVING FORCES IN EVOLUTION

9. 3 types of evolutionary explanations: • Mutationism = evolutionary phenomena are explained by the joint effects of mutational input and random genetic drift.

10. 3 types of evolutionary explanations: • Neutralism = evolutionary phenomena are explained by the joints effects of mutation, random genetic drift, and purifying selection.

11. 3 types of evolutionary explanations: • Selectionism = evolutionary phenomena are explained by the joints effects of advantageous selection and balancing selection.

12. Selectionism • Gene substitutions occur as a consequence of selection for advantageous mutations. Polymorphism is maintained by balancing selection. • Gene substitution and polymorphism are two separate phenomena driven by different evolutionary forces.

13. Gene substitution is the end result of an adaptive process whereby a new allele takes over future generations of the population if and only if it improves the fitness of the organism.Polymorphism is maintained when the coexistence of two or more alleles at a locus is advantageous for the organism or the population. Selectionism

14. Two separate evolutionary forces! • Genetic polymorphism is permanent - the same alleles are maintained at constant frequencies for long periods of evolutionary time.

15. Adaptation An adaptation is a heritable character state that enhances the fitness of the organisms that bear it relative to alternative character states, especially ancestral character states in the population in which the adaptation evolved.

16. Honey creepers

17. Adaptation Translation into Molecular Lingo: An adaptation is a mutation that enhances the fitness of the organisms that carry it relative to the other alleles in the population.

18. Extreme selectionism leads to The “Panglossian paradigm” “It is proved… that things cannot be other than they are, for since everything was made for a purpose, it follows that everything is made for the best purpose.” Dr. Pangloss in Candide by Voltaire

19. The Panglossian paradigm: “Observe, for instance, the nose is formed for spectacles, therefore we wear spectacles.”

20. The Panglossian paradigm: “Legs were clearly intended for breeches , and we wear them.” Dr. Pangloss in Candide by Voltaire

21. Abbott Handerson Thayer 1849-1921

26. 1911 1901–1909: President of the United States. 1911: A 116-page article on animal coloration.

27. Kimura M. 1968. Evolutionary rate at the molecular level". Nature 217: 624–626. King JL, Jukes TH. 1969. Non-Darwinian Evolution. Science 164: 788–798. The Neutral Theory of Molecular Evolution Motoo Kimura

28. The majority of evolutionary changes and much of the variability within species are caused by random genetic drift of mutant alleles that are selectively neutral or nearly so.

29. Neutrality, in the sense of the neutral theory, does not imply strict equality in fitness for all alleles. It only means that the fate of alleles is determined largely by random genetic drift.

30. Selection may operate, but its intensity is too weak to offset the influences of chance effects. s< 1/(2Ne) Ne = effective population size.

31. That is, either the selection coefficient is small, or the effective population size is small. s< 1/(2Ne) Ne = effective population size.

32. According to the neutral theory, the frequency of alleles is determined by purely stochastic rules, and the picture that we obtain at any given time is merely a transient state representing a temporary frame from an ongoing dynamic process.

33. The neutral theory regards substitution and polymorphism as two facets of the same phenomenon. Gene substitution is a long and gradual process whereby the frequencies of mutant alleles increase or decrease randomly, until the alleles are ultimately fixed or lost by chance.

34. Polymorphic loci consist of alleles that are either on their way to fixation or are about to become extinct. Thus, at any given time, some loci will possess alleles at frequencies that are neither 0% nor 100%. These are the polymorphic loci.

35. All molecular manifestations that are relevant to the evolutionary process should be regarded as the result of a continuous process of mutational input and a concomitant random extinction or fixation of alleles.

36. A single evolutionary force. • Genetic polymorphism is transient - allele frequencies fluctuate with time and the polymorphic alleles themselves are continuously replaced.

37. A population that is free from selection can accumulate many polymorphic neutral alleles. Then, if a change in ecological circumstances occurs, some of the neutral alleles will no longer be neutral but deleterious, against which purifying selection may operate. After these alleles are removed, the population will become more adapted to its new circumstances than before. Kimura (1983)

40. The neutral theory does not preclude adaptation. Adaptive evolution may occur without adaptive selection.

41. The SlightlyDeleterious Model of Molecular Evolution + The SlightlyAdvantageous Model of Molecular Evolution = The Nearly Neutral Model of Evolution

42. selectionism Mean fitness neutralism nearly neutral model time

43. Leigh Van Valen’s “Red Queen Hypothesis” (based “Through the Looking-Glass, and What Alice Found There” by Lewis Carroll (1871). Alice complains to the Red Queen that she is exhausted from running, only to find she is still under the tree where she started. The Red Queen replies: “Now, here, you see, it takes all the running you can do to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that.”

44. Neutralist-selectionist perceptions

45. Neutralist-selectionist perceptions A total resolution of the dispute between neutralists and selectionists would require knowledge of the distribution of fitness effects of mutant alleles.

47. The distribution of fitness effects of random mutations in vesicular stomatitis virus. Random mutations were introduced into the virus, and the fitnesses of the mutants were compared against the unmutated wild type. A fitness of less than one indicates that the mutant was less fit than the wild type, so the mutation was deleterious. A fitness of zero indicates that no mutated progeny were recovered, and that the mutation was therefore lethal. From: Eyre-Walker A, Keightley PD. 2007. The distribution of fitness effects of new mutations. Nature Rev. Genet. 8:610-618.

48. Yeast

49. Fact 1: The cheetah is about to become extinct.

50. Fact 2: Cheetah populations are devoid of genetic variation.