CHAPTER 12 Natural Selection and Adaptation

1. CHAPTER 12 Natural Selection and Adaptation

2. ADAPTATIONS

3. ADAPTATIONS

4. ADAPTATIONS • Most adaptations are complex • The appearance of design

5. ADAPTATIONS • Adaptations have been “designed” by a completely “mindless” process • Evolutionary theory does not admit anticipation of future • Teleological (incorrect) view – processes which invoke goals or end points

6. Adaptation by Evolution by Natural Selection!!! • Evolutionary theory by Natural Selection must be able to account for the origin of complex adaptations that increase ones fitness • But, Natural Selection must also be able to account for traits that do not increase fitness (e.g. bee sting, anaphylactic shock)

7. Adaptation • Hitchhiking • linkage to another allele that increases fitness • Stable Equilibrium • natural selection must be acting in such ways to maintain variation; it does not necessarily cause fixation of a single best genotype

8. Tungara Frog – Conflicting Selection • If males “CHUCK” females will respond favorably YET….. Exposes males to greater risk of predation by bats

9. Tungara FrogConflictingSelection Tuni Tungara

12. Natural Selection • Definition of Natural Selection: • Include a trait must vary among biological entities, and there must be a consistent relationship between the trait and one or more components of reproductive success • Short version of Natural Selection: • Any consistent difference in fitness among phenotypically different biological entities (inherited)

13. Adaptation • Process of becoming adapted Or • To the features of organisms that enhance reproductive success relative to other possible features

14. Recognizing Adaptation? • Includes a Phylogenetic Component • Fleas (wingless adaptation) • Bristle tails (primitively wingless) • Traits evolve from pre-existing ones so its’ phylogenetic position is important • Preadaptation • a feature that fortuitously serves a new function (the kea in New Zealand)

15. What is Adaptation? • Our definition • a feature is an adaptation for some function if it has become prevalent or is maintained in a population because of natural selection for that function

16. Nonadaptive Traits • Trait might be necessary consequence (flying fish) • Evolved by random genetic drift rather than Natural Selection (grouse chick patterns – cryptic but drift in patterns occur among species) • Hitchhiking (linkage of traits) • has not become adaptively altered to a response (big fruits, extinct mammals)

17. How do we recognize adaptations? • Complexity • Design • Experimental Evidence • Comparative Method – uses phylogeny to compare trait evolution among groups of species • Convergent Evolution – trait which is correlated between lineages (2 different groups evolved the same or similar adaptation independently)

18. CHAPTER 13 The Theory of Natural Selection

19. Our View of Evolution up to this point... • Genetic Drift, Inbreeding & Gene Flow act at the same rate on ALL LOCI • NOW.... We will see how selection dictates rates and directions of change

20. The Difference Natural Selection Makes • Natural Selection is profoundly different from previously described mechanisms which lead to evolution (drift, inbreeding, gene flow) • Sexual reproduction – Allele frequency changes proceed INDEPENDENTLY at different loci

21. Natural Selection VS. Evolution • Natural Selection is NOT THE SAME as Evolution! • Evolution is a 2-step process: • Origin of genetic variation by mutation and recombination • Followed by change due to an agent (eg. Natural Selection, drift, etc...)

22. Natural Selection VS. Evolution by Natural Selection • Natural Selection differs from Evolution by Natural Selection • Evolution can occur w/o Natural Selection (e.g. drift) • Natural Selection can occur w/o Evolution (e.g. genotypes differ in each generation by fecundity but their proportions stay the same)

23. So... What is Natural Selection? • Natural Selection can have no Evolutionary effect unless phenotypes differ in genotypes! • Why?  if naturally selected phenotypes are not heritable (genetically based) then Evolution of phenotype cannot occur!

24. So... What is Natural Selection? • Natural Selection = variation in the average reproductive success among phenotypes • Remember: Without variation there cannot be evolution! • EG: Horse tail / Fly

25. Modes of Selection • Directional Selection: if 1 phenotype is the fittest (e.g. size) • Stabilizing Selection: if an intermediate phenotype is fittest • Disruptive Selection: if 2 or more phenotypes are fitter than the intermediates between them

27. Modes of Selectionorange = fittest genotype • Directional Selection A1 A1> A1 A2 = A2 A2 • Stabilizing Selection A1 A1< A1 A2> A2 A2 • Disruptive Selection A1 A1> A1 A2 < A2 A2

28. So, What is Fitness? • The fitness of a genotype is the average per capita lifetime contribution of individuals of that genotype to the population after one or more generations (more detail on p. 366) • Lets look at an example..

29. Calculating Fitness • Genotype A 0.05 (=5%) survive to reproduce Average adult has 60 offspring (fraction surv.) X (ave. fecundity)= R R = per capita replacement rate (0.05) X (60) = 3 = RA • Genotype B 0.10 survive and ave. fecundity = 40 (0.10) X (40) = 4 = RB

30. Calculating Fitness • So, if 20% of population was genotype A and 80% was genotype B • We can calculate the Average Growth Rate per generation of the population R = (0.2)(3) + (0.8)(4) = 3.8

31. Calculating Fitness Ri = absolute fitness (where “i” refers to a particular genotype, e.g. A or B) R = Average fitness

32. Calculating Fitness • What we are REALLY interested in is not R.... • We are interested in RELATIVE FITNESS  W • W = the value of R relative to that of a reference genotype (this tells us how much more fit one genotype is than another)

33. Calculating Relative Fitness (W) • The genotype with the highest Ri is assigned 1 • The other genotypes are calculated in relation to this reference • for our example, we assign WB = 1 (because genotype B has highest R) • Then we can calculate WA RA/RB = ¾ = 0.75 = WA

34. Average Relative Fitness W = (similar to R) = the average fitness of individuals in a population relative to the fittest genotype For our example: (0.2)(0.75) + (0.8)(1.0) = 0.95 = W

35. Relative Fitness and the Rate of Change The rate of change under selection depends on the RELATIVE FITNESS (NOT THE ABSOLUTE FITNESS)

36. Calculating Fitness in Asexual Populations • p = NA / N • q = NB / N • N = population size at beginning of generation • After 1 generation NARA & NBRB • So the new frequency of genotype A = p’

37. Calculating Fitness in Asexual Populations p’ = frequency of genotype A in next generation... p’ = pNRA / (pNRA + qNRB) or p’ = pRA / (pRA + qNRB)

38. Calculating Fitness in Asexual Populations the change in frequency of p: ∆ p = p’ – p or ∆ p = pq(RA-RB) / (pRA+qRB) lets see an example...

39. Calculating Fitness in Asexual Populations Our old example: p = 0.2 and q =0.8 lets calculate ∆ p: (0.2)(0.8) (3-4) [0.2 (3)] + [0.8 (4)] ∆ p = -0.16 / 3.8 = -0.042

40. What did this tell us? RA and RB 6 and 8 or 9 and 12 or 300 and 400 Relative fitness, W, determines the rate of change!!! WA : WB = 0.75 : 1

41. From Relative fitness to Selection! • Now that we can calculate the relative fitness of genotypes we can then quantify selection per genotype! • si = Selection coefficient • si measures the intensity of selection AGAINST a less fit allele and, thus, tells us something about the Selective Advantage of the more fit allele.

42. Example of Si • Remember our recurring example: • WA = 0.75 and WB = 1 • we can also write WA = 1 – sA • Therefore the selection coefficient for genotype A would be sA = 0.25 [WA = 0.75 = 1-sA]

43. Using Si to figure out ∆ p • since WA = 0.75 and SA = 0.25 (remember that we associated p with genotype A) ∆ p = pq([1-sA]-1) p(1-sA+ q1) or -sApq 1-sAp

44. What did this tell us about Selection? -sApq 1-sAp • ∆ p is negative as long as p, q, and s are positive! this means: genotype A will decrease in frequency because its fitness is lower than genotype B

45. What did this tell us about Selection? -sApq 1-sAp = ∆ p 2) ∆ p = directly proportional to the NUMERATOR (-spq) This means that the rate of change is greater when both alleles are common with the maximum being p=0.5 and q=0.5 Numerator Denominator

46. What did this tell us about Selection? -sApq 1-sAp = ∆ p 3) ∆ p = inversely proportional to the denominator which really = W which is the average relative fitness We can express W as: p(1-sA) + q(1) = W Numerator Denominator

47. What did this tell us about Selection? • As p approaches zero, the evolutionary rate SLOWS down! • Meaning: most individuals in the population are carrying the more fit genotype

48. Where in Life Cycles Does Selection Act? • Selection can act on multiple points in sexually reproducing populations • Viability Selection • Sexual Selection • Fecundity Selection • Gamete Selection • Compatibility Selection

49. Where in Life Cycles Does Selection Act?

50. Evolution by Natural Selection • By the way in which changes in allele frequencies are determined by the components of fitness of each zygotic and each gametic genotype • These components of fitness are combined (multiplied) to represent the overall fitness of each genotype • Remember, previously we multiplied the survival X fecundity = overall fitness