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The Genetical Theory of Natural Selection Chapter 6.

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Presentation on theme: "The Genetical Theory of Natural Selection Chapter 6."— Presentation transcript:

1 The Genetical Theory of Natural Selection Chapter 6

2 1.Fitness definition 2.Modes and models of selection General model of selection Directional selection Ex: Warfarin Stabilizing selection Ex: Sickle cell anemia and birth weight Diversifying selection Ex: Seedcracker finch Frequency dependent selection 3.Natural selection outcomes 4.Strength of selection Lecture Outline

3 Points to keep in mind about natural selection: 1.Natural selection is not the same as evolution Origin of Genetic Variation Mutation + RecombinationGenetic Drift + Natural Selection Changes in the Frequency of Alleles and Genotypes 2.Natural selection can have no evolutionary effect unless phenotypes differ in genotypes 3.A feature cannot evolve by natural selection unless it makes a positive contribution to the reproduction or survival of individuals that bear it Natural selection proceeds independently at different loci Evolution

4 Fitness Defining Fitness Arnold Alois Schwarzenegger Homo sapiens

5 Fitness Defining Fitness The fitness of a genotype is the average lifetime contribution of individuals of that genotype to the population after one or more generations, measured at the same stage in the life history Absolute Fitness ( R i )Per capita growth rate of each genotype i Relative Fitness ( W i )Is the absolute fitness of genotype i relative to the absolute fitness of a reference genotype ( R * )

6 Fitness Components of Fitness Individual Viability Probability of survival of the genotype to reproductive age Mating Success Number of mates obtained by an individual (Sexual Selection) Z YGOTIC S ELECTION Fecundity Number of viable offspring per female G AMETE S ELECTION Segregation Distortion Probability of being segregated to the gamete Gamete Viability Probability of survival of the gamete to fertilization Fertilization Success Gamete’s ability to fertilize an ovum

7 Models of Selection Assumptions: large population, random mating, no mutation or migration, viability selection only, discrete generations 1 Locus 2 Alleles A 1 p A 2 1-p=q next generation p’ we are interested in the change of frequency from one generation to the next p’-p=Δp if Δp>0 frequency of A 1 increase Δp<0 frequency of A 2 increase Δp=0 frequencies do not change and we are at an equilibrium

8 Models of Selection A1A1A1A1 A1A2A1A2 A2A2A2A2 Frequency Birth p2p2 2pqq2q2 Fitness w 11 w 12 w 22 Population mean fitness A 1 in next generation A 2 in next generation

9 Models of Selection A 1 in next generation Change in allele frequency “Sticky ends” fixation of A 1 (p=0) or A 2 (q=0) results in no further change Δp=0

10 Fitness Natural selection concerns selection on biological entities within populations Modes of Selection The relationship between phenotype and fitness can be described as one of three modes of selection: Directional SelectionOne extreme phenotype is the fittest Stabilizing SelectionAn intermediate phenotype is the fittest Diversifying SelectionTwo or more phenotypes are fitter than the intermediates between them

11 Models of Selection Directional Selection Intuition Replacement of disadvantageous alleles by more advantageous alleles A 1 replaces A 2 Continuous trait1 Locus 2 Alleles

12 Models of Selection Directional Selection Change in allele frequency Parametrization w 11 =1 w 12 =1- ½s w 22 =1-s Equilibria p=1 (stable) and q=1 (unstable) A 1 always increases as suggested by intuition

13 Models of Selection Directional Selection The number of generations required for an advantageous allele to replace one that is disadvantageous depends on: The initial allele frequencies The selection coefficient The degree of dominance The mean fitness increases as natural selection proceeds

14 Models of Selection Everybody Hates Rats

15 Models of Selection Killing Ratatouille

16 Models of Selection Ex of Directional Selection Warfarin is an anticoagulant that such that poisoned rats often bleed to death from slight wounds Pied Piper of Hamelin

17 Models of Selection Ex of Directional Selection A mutation (Rw) confers resistance by making the rats less sensitive to the poison Rw/Rw Rw/++/+

18 Models of Selection Ex of Directional Selection If a locus has experienced consistent directional selection for some time, the advantageous allele should be near fixation. Thus the dynamics of directional selection are best studied in recently altered environments

19 Models of Selection Stabilizing Selection Intuition Both alleles are maintained in the population A 1 and A 2 at equilibrium Continuous trait1 Locus 2 Alleles Heterozygote advantage

20 Models of Selection Stabilizing Selection Change in allele frequency Parametrization w 11 =1-s w 12 =1 w 22 =1-s Equilibria p=1, q=1 (unstable) and p=½ (stable) A 1 always increases when its frequency is less than ½ but decreases when its frequency is more than ½

21 Models of Selection Ex of Stabilizing Selection The best understood case of heterozygote advantage is the β-hemoglobin locus in some African and Mediterranean human populations SSickle-cell hemoglobin ANormal hemoglobin Allele S results in the formation of S hemoglobin that forms elongated crystals, which carry oxygen less effectively, causing the red blood cells to adopt a sickle shape and to be broken down more rapidly.

22 Sickle-cell DiseaseMalaria AANormalHigher mortality ASSlight anemiaLower mortality SSSevere anemia (Sickle cell) w AA =0.89 w AS =1 w SS =0.2 Models of Selection Ex of Stabilizing Selection AAASSS

23 The heterozygote advantage arises from a balance of opposing selective factors: anemia and malaria AAASSS Models of Selection Ex of Stabilizing Selection

24 In the absence of malaria, balancing selection gives way to directional selection because the AA genotype has the highest fitness. African population q=0.13 African American q=0.05 AA ASSS AAASSS AfricaAmerica Models of Selection Ex of Stabilizing Selection

25 Models of Selection Ex of Stabilizing Selection There is significant stabilizing selection on neonate size. Small infants and large infants die during child birth at a higher rate than average-sized infants. There is also a directional component to selection. Notice that the optimal infant size is one-half of a pound higher than the average infant size in the population. The pattern of low survival of large offspring has different causes than the probability of low survival of small offspring. Small offspring may have had high mortality because of inadequate nutrition during gestation, large offspring may have died because of the large diameter of the cranium relative to the pelvic girdle

26 The previous data was collected back in 1958 before the advent of modern techniques for the care of neonates. It would be interesting to know if the widespread use of cesarean sections and other medical techniques have altered the selection on neonate size Models of Selection Ex of Stabilizing Selection 6-9 Lbs19 Lbs 2010 Indonesia

27 Models of Selection Diversifying Selection Intuition Both alleles are maintained in the population but in their homozygote A 1 and A 2 at equilibrium Continuous trait1 Locus 2 Alleles

28 Models of Selection Diversifying Selection Change in allele frequency Parametrization w 11 =1 w 12 =1-s w 22 =1 Equilibria p=1, q=1 (stable) and p=½ (unstable) A 1 always increases when its frequency is more than ½ but decreases when its frequency is less than ½

29 Seedcracker finch Given the simple Mendelian inheritance for beak size, it is clear that disruptive selection tends to maintain two distinct bill morphs by eliminating birds with intermediate-sized bills Models of Selection Diversifying Selection

30 Natural selection on beak size in seed cracking finches can be traced directly to feeding performance of the two morphs on different sized seeds. Both modes experience disruptive selection which refines the differences between morphs. Seedcracker finch Models of Selection Diversifying Selection

31 Mutation and Migration Deleterious Alleles in Natural Populations Although the most advantageous allele at a locus should be fixed by directional selection, deleterious alleles often persist because they are repeatedly reintroduced either by recurrent mutation or by gene flow from other populations in which they are favored by a different environment

32 Mutation and Migration Deleterious Alleles in Natural Populations Consider an advantageous allele allele at a locus favored by natural selection (directional selection) that tends to mutate to a deleterious form w 11 =1 w 12 =1- ½s w 22 =1-s Allele frequency after selection Allele frequency after selection and mutation Change in allele frequency Equilibria p=0 (unstable) and p=u/s (stable)

33 Mutation and Migration Deleterious Alleles in Natural Populations The frequency of the deleterious allele moves toward a stable equilibrium that is a balance between the rate at which it is eliminated by selection and the rate at which it is introduced by mutation The same result is achieve when there is migration from between a small island in which allele A 1 is favored by directional selection and a large continent in which allele A 2 is fixed ContinentIsland

34 Frequency Dependent Selection Models of Selection In the models considered so far, the fitness of each genotype is assumed to be constant within a given environment Very often, however, the fitness of a genotype depends on the genotype frequencies in the population. The population then undergoes frequency dependent selection

35 I NVERSE FREQUENCY - DEPENDENT SELECTION : The rarer a phenotype is in the population, the greater its fitness Ex. Cichlid fish Frequency Dependent Selection Models of Selection

36 Cichlid fish Perissodus microlepis One of the strangest ways of making a living is found in the behavior of Perissodus microlepis, a cichlid fish that specializes in eating scales. Perissodus microlepis will swoop in on its prey from the blind side and eat some scales. The scale-eater is a classic partial predator that feeds only on part of its prey, but leaves the fish otherwise intact. What is strange about this behavior is that it leads to a curious evolutionary cycle. At any point in time, there are two kinds of scale-eaters. One is always slightly more common than the other. Frequency Dependent Selection Models of Selection

37 In 1982, left-jawed scale-eaters were the most common. The prey are more often attacked on their right flank by a scale-eater with a jaw that curves to the left, so the prey learns to look to the right when being vigilant to attack. While the prey learn to look right, they leave their left flank exposed to the scale-eater with a jaw that curves to the right. This gives the rarer right-jawed morphology an advantage, and they do slightly better that year. The left-jawed morph does slightly worse, because the prey is vigilant to attack from the right flank attack, and the left-jawed morph declines in frequency. Frequency Dependent Selection Models of Selection

38 T HE E VOLUTION OF THE S EX R ATIO Why is sex ratio about even (1:1) in many species of animals? This is quite a puzzle: From a group-selectionist perspective we might expect that a female-biased sex ratio would be advantageous because such a population could grow more rapidly From a individual selection perspective why should a genotype producing an even sex-ratio have an advantage over any other? Frequency Dependent Selection Models of Selection

39 Because every individual has both a mother and a father, females and males must contribute equally to the ancestry of subsequent generations and must therefore have the same average fitness T HE E VOLUTION OF THE S EX R ATIO Let the sex ratio be the proportion of males. Let S be the population sex ratio S. Let s be an individual female sex ratio Suppose that the population sex ratio is 0.25 Female4 offspring Male12 offspring Established genotype individual sex ratio 3:1 x312 grand offspring x112 grand offspring=24 Mutant genotype individual sex ratio 2:2 x28 grand offspring x224 grand offspring=32 Frequency Dependent Selection Models of Selection

40 Multiple Outcomes of Evolutionary Change Initial genetic conditions often determines which of several paths of genetic change a population will follow. The evolution of a population often depends on its previous evolutionary history POSITIVE FREQUENCY DEPENDENT SELECTION The fitness of a genotype is greater the more frequent it is in a population. As a result, whichever allele is initially more frequent will be fixed Ex. Heliconius erato

41 Multiple Outcomes of Evolutionary Change HETEROZYGOTE DISADVANTAGE Monomorphism for either A 1 A 1 or A 2 A 2 is therefore a stable equilibrium, and the initially more frequent allele is fixed by selection. A population is not necessarily driven by natural selection to the most adaptive possible genetic constitution

42 Multiple Outcomes of Evolutionary Change ADAPTIVE LANDSCAPES Metaphor introduced by Sewall Wright INTERACTION OF SELECTION AND GENETIC DRIFT In a finite population, allele frequencies are simultaneously affected by both selection and chance The effect of random genetic drift is negligible if selection on a locus is strong relative to the population size

43 Multiple Outcomes of Evolutionary Change

44 Fitness is the average contribution of an individual of a genotype to the population after one generation There are 3 modes of natural selection when fitness is constant: directional, stabilizing and diversifying The first reduces variability the other two maintain variation There is also frequency dependent selection Frequency dependent selection maintain variability Lecture Ideas

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