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The Genetical Theory of Natural Selection Chapter 6 Lecture Outline 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. 4. Natural selection outcomes Strength of selection Points to keep in mind about natural selection: 1. Natural selection is not the same as evolution Evolution Origin of Genetic Variation Changes in the Frequency of Alleles and Genotypes Mutation + Recombination Genetic Drift + Natural Selection 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 Fitness Defining Fitness Arnold Alois Schwarzenegger Homo sapiens 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 (Ri) Per capita growth rate of each genotype i Relative Fitness (Wi) Is the absolute fitness of genotype i relative to the absolute fitness of a reference genotype (R*) Wi Ri / R Fitness Components of Fitness ZYGOTIC SELECTION Fecundity Number of viable offspring per female Individual Viability Probability of survival of the genotype to reproductive age Fertilization Success Gamete’s ability to fertilize an ovum Gamete Viability Probability of survival of the gamete to fertilization Segregation Distortion Mating Success Probability of being segregated to the gamete Number of mates obtained by an individual (Sexual Selection) GAMETE SELECTION Models of Selection Assumptions: large population, random mating, no mutation or migration, viability selection only, discrete generations 1 Locus 2 Alleles A1 p A2 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 A1 increase Δp<0 frequency of A2 increase Δp=0 frequencies do not change and we are at an equilibrium Models of Selection A1A1 A1A2 A2A2 Frequency Birth p2 2pq q2 Fitness w11 w12 w22 A1 in next generation w11 p2 w12 12 2pq A2 in next generation w22q2 w12 12 2pq Population mean fitness w w11 p2 2w12 pqw22q2 Models of Selection A1 in next generation w11 p2 w12 pq p w p p w11 pw12q w Change in allele frequency p p p p w11 pw12q p w wp pq w11 w12 p w22 w12 q “Sticky ends” fixation of A1 (p=0) or A2 (q=0) results in no further change Δp=0 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 Selection One extreme phenotype is the fittest Stabilizing Selection An intermediate phenotype is the fittest Diversifying Selection Two or more phenotypes are fitter than the intermediates between them Models of Selection Directional Selection Continuous trait 1 Locus 2 Alleles w11 w12 ,w22 Intuition Replacement of disadvantageous alleles by more advantageous alleles A1 replaces A2 Models of Selection Directional Selection w11=1 Change in allele frequency wp pq w11 w12 p w22 w12 q w12=1- ½s w22=1-s Parametrization wp spq Equilibria p=1 (stable) and q=1 (unstable) A1 always increases as suggested by intuition 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 Models of Selection Everybody Hates Rats Models of Selection Killing Ratatouille Models of Selection Ex of Directional Selection Pied Piper of Hamelin Warfarin is an anticoagulant that such that poisoned rats often bleed to death from slight wounds Models of Selection Ex of Directional Selection A mutation (Rw) confers resistance by making the rats less sensitive to the poison Rw/Rw Rw/+ +/+ 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 Models of Selection Stabilizing Selection Continuous trait 1 Locus 2 Alleles w12 w11 ,w22 Heterozygote advantage Intuition Both alleles are maintained in the population A1 and A2 at equilibrium Models of Selection Stabilizing Selection w12=1 Change in allele frequency wp pq w11 w12 p w22 w12 q w11=1-s w22=1-s Parametrization s12ppq wp pq sp sq and p=½ (stable) Equilibria p=1, q=1 (unstable) A1 always increases when its frequency is less than ½ but decreases when its frequency is more than ½ 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 S A Sickle-cell hemoglobin Normal 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. Models of Selection Ex of Stabilizing Selection Sickle-cell Disease Malaria AA Normal Higher mortality AS Slight anemia Lower mortality SS Severe anemia (Sickle cell) wAA=0.89 wAS=1 wSS=0.2 AA AS SS Models of Selection Ex of Stabilizing Selection The heterozygote advantage arises from a balance of opposing selective factors: anemia and malaria AA AS SS Models of Selection Ex of Stabilizing Selection 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 Africa AA AS America SS AA AS SS 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 Models of Selection Ex of Stabilizing Selection 6-9 Lbs 19 Lbs 2010 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 Diversifying Selection Continuous trait 1 Locus 2 Alleles w11 ,w22 w12 Intuition Both alleles are maintained in the population but in their homozygote A1 and A2 at equilibrium Models of Selection Diversifying Selection w11=1 Change in allele frequency w22=1 wp pq w11 w12 p w22 w12 q w12=1-s Parametrization wp pq sp sq s 2p1 pq p=½ (unstable) Equilibria p=1, q=1 (stable) and A1 always increases when its frequency is more than ½ but decreases when its frequency is less than ½ Models of Selection Diversifying Selection 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 Seedcracker finch 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. 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 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 Allele frequency after selection w11=1 p p w12=1- ½s spq w w 1sq Allele frequency after selection and mutation w22=1-s spq p 1u p w Change in allele frequency sq p p 1 u u w wp p 1u sq uw p sq u Equilibria p=0 (unstable) and p=u/s (stable) 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 q u s The same result is achieve when there is migration from between a small island in which allele A1 is favored by directional selection and a large continent in which allele A2 is fixed Island q m s Continent Models of Selection Frequency Dependent 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 Models of Selection Frequency Dependent Selection INVERSE FREQUENCY-DEPENDENT SELECTION: The rarer a phenotype is in the population, the greater its fitness Ex. Cichlid fish Models of Selection Frequency Dependent Selection 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. Models of Selection Frequency Dependent Selection 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. Models of Selection Frequency Dependent Selection THE EVOLUTION OF THE SEX RATIO 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? Models of Selection Frequency Dependent Selection THE EVOLUTION OF THE SEX RATIO 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 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 Female 4 offspring Male 12 offspring Established genotype individual sex ratio 3:1 x3 12 grand offspring x1 12 grand offspring =24 Mutant genotype individual sex ratio 2:2 x2 8 grand offspring x2 24 grand offspring =32 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 Multiple Outcomes of Evolutionary Change HETEROZYGOTE DISADVANTAGE Monomorphism for either A1A1 or A2A2 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 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 Multiple Outcomes of Evolutionary Change Lecture Ideas • 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