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Genotypes, Phenotypes, and Selection

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Genotypes, Phenotypes, and Selection Genotypes,Phenotypes, and Selection Selectionoperates directly on phenotypesbecause phenotypic In manycases, the allelicvariation at a particularlocus does not 'h variationamong organisms rnf luences the relativeprobability of sur- Intluencethe pnenotype. In such cases, the alleres are dden"frorn vivaland reproduction.Those phenotypes, tn turn, are influenced by the actionof seleciionbecause they are selectrvely neutral. Even if an alleles.Although the relationshipbetween alleles and phenotypesis alleledoes result in a phenotypicchange, it stillcould be selectively rarelyknown and often complex, it isstill possible for allelesat genetic neutralif the changein phenotypehas no effecton reproductive locito experrenceselectron. Population geneticists can sample indi- SUCCCSS, vrdualsfor theirgenotype at a locusand comparethe fitnessof indi- vidualswith onegenotype (r.e., the averagefrtness of the genotype) Key Concept with the fitnessesof individualswith othergenotypes. When geno- Allelesare selectively neutral if they haveno effecton the fitnessof typesdiffer consistently in theirf itness,the genetic locus can be said theirbearers. This phenomenon often occurs when genetic variation to beunder selection. The selection coefficient (s) isused to describe at a locusdoes not affect the phenotypeof an individual howmuch the genotypesdiffer in theirfitness. 6.6 Selection:Winning and Losing In Chapter 2, we introduced the concept of selectionas first developedby Charles Darwin and Alfred RusselWallace. Both naturalistsrecognized the profound impor- tance of selectionas a mechanism of evolution. Natural selection arises whenever (1)individuals vary in the expressionoftheir phenotypes,and (2)this variationcauses some individuals to perform better than others.Over many generations,Darwin and Wallace argued, selection can drive large-scaleevolutionary change,allowing new adaptationsto arise.In Chapter10, we will considerthe origin of adaptationsin more detail. For now, let's focus on the question of how selectionchanges the frequencies of allelesin a population. The reproductivesuccess of an individual with a particular phenotype is known Fitness:The success of anorganism as fitness, and selectionoccurs when individuals vary in their fitness. While this at survivingand reproducing, and thus may seemstraightforward enough,studying the actual fitness of real organismsis a contributingoffspring to future surprisingly complicatedmatter. The best way to measurefitness would begin with generations. tallying the lifetime reproductivecontribution of an individual and then noting how many of the offspring manageto survive to reproductiveage themselves. In practice, however,it's hardly ever possibleto make such a detailedmeasurement. Scientistssettle instead for reliableproxies for fitness.They sometimesmeasure the probability that an individual survivesto the ageof reproduction,for example,or they measurethe number of offspring that organismsproduce in a specificseason. Whatever the actualmetric, measuringselection entails comparing thesefitness mea- suresfor many different individuals and relating variation in fitnesswith variation in the expressionof a phenotype. Another difficulty when it comes to measuring fitness is the complicated rela- tionship betweengenotype and phenotype.The fitnessof an organism is the product of its entire phenotype.We'll seein Chapters7 and 8 how scientistscan make mea- surementsof phenotypic selectionto study how complex morphological and behav- ioral traits evolve. But first let's consider how population geneticistsstudy fitness. Instead of studying an entire phenoqpe, they focus on the evolution of allelesat a geneticlocus. Populationgeneticists often distill all of the different fitness components,such Relativefitness (of a genotype): as survival, mating success,and fecundity, into a single value, called w. This value Thesuccess ofthe genotypeat describesthe relative contribution of individuals with one genotype,compared with producingnew individuals (its fitness) the averagecontribution of all individuals in the population. If individuals with a standardizedby the successof other particular genotype,for example,4747, consistentlycontribute more offspring than genotypesin the population(for individuals with other genotypes(e.g., A1A2, A2A2), then their relative fitness will be example,divided by the average greaterthan one. Conversely,if the net contributions of individuals with a genotype fitnessof the population). are lower than those of other individuals, the relative fitness will be less than one. 166 cHAprERsrx rHE wAys oF cHANGE:DRrFT AND sELEcrroN Sometimespopulation geneticistscalculate relative fitness by comparing the fitness of all individuals to the fitness of the most successfulgenotype in the population, rather than by the mean fitness of the population. In such cases,the genotypewith the highest fitness has a relative fitness of w : 1, and all other genotypeshave rela- tive fitnessesthat are between0 and 1.Regardless of which way it is measured,selec tion will always occur if two or more genotypesdiffer consistentlyin their relative fitness.The strength of selectionwill reflect how different the genotypesare in their respectivefitnesses. To understand how selectionleads to changesin the frequenciesof alleles,we can consider the contributions of an allele, rather than a genotype,to fitness. But calculatingthe relative fitness of an allele is more complicatedthan calculatingthat of a genotype,for two reasons.First, alleles in diploid organismsdon't act alone.They are alwayspaired with another alleleto form a genotype.If there is, say,a dominance interaction between them, that interaction will influence the phenotype. Second, selectiondoes not act directly on alleles.It actson individuals and their phenotypes. Nevertheless,it is still possibleto calculatethe net contributions of an alleleto fit- ness.To do so,we must considerthe fitnesscontributions of individuals heterozygous for the allele as well as that of homozygotes,and weigh how many individuals with each genotype are actually present in the population and contributing offspring to Averageexcess of fitness (of an the next generation.Box 6.5 showshow the net fitnesscontribution of an allele,called allele):The difference between the the averageexcess of fitness, is calculated. averagefitness of individualsbearing The averageexcess of fitness for an allele can be used to predict how the fre- the alleleand the averagefitness of the quency of the allelewill changefrom one generationto the next: oooulationas a whole. Lp:px(ao, w) where Ap is the change in allele frequency due to selection,p is the frequency of the A1 allele,w is the averagefitness of the population, and a,a,is the averageexcess of fitness for the A7 allele.This equation can tell us a lot about the nature of natural selection. The sign of the averageexcess of fitness(a o ) , for example,determines whether selectionincreases an allele'sfrequency or decreasesit. Whenever an allele is pres- ent in a population, its frequency is greaterthan zero; and as long as the population exists,its averagefitness, w, is alsogreater than zero (becausew is the sum of all indi- viduals with each genotype times their respectivecontributions of offspring to the next generation).Since both p and w are by definition positive,the sign of Ap must be determined by the averageexcess of fitness of the allele. Whenever the fitness effectsof an allele are positive,selection should increasethe frequency of the allele over time; the converseis true when the fitnesseffects are negative. This equation also tells us that the speedof increase(or decrease)in the fre- quency of an allelewill depend on the strength of selectionthat it experiences-the magnitude of aa,.When the averageexcess of fitness is very large (positiveor nega- tive), the resulting changein allele frequencywill be greaterthan when the average excessof fitness is smaller. Finally,this equation shows us that the effectivenessof selectionat changing an allele'sfrequency dependson how common it is in the population.When an alleleis very rare (p : 0), the power of selectionto act will be low even if the fitness effects of the allele are pronounced. SmallDifferences, Big Results Alleles can differ enormously in fitness. A single mutation can disable an essential protein, leading to a lethal genetic disorder. These alleles experience strong negative selection because children who die of such a disorder cannot pass on the mutation to their offspring. As a result, a typical severe genetic disorder affects only a tiny fraction of the population. But even when alleles are separated by only a small difference in their average excess of fitness, selection can have big long-term effects. That's because populations grow like investments earning interest. '167 6.6 sELEcroN:wrNNtNG AND LosrNG E SelectionChanges Allele Frequencies ll Let'sconsider how naturalselection changes allele frequencies by Genotype: A,A, AtAz A,A, startingwith a populationin Hardy-Weinbergequilibrium at a genetic (p'zxwrr)fw (zpqxwt)fw \q'xwrr)/w locus.We willthen calculate how selection pulls ihe populationout of f,rtt equilibriumand, in so doing,shifts the frequenciesof the alleles. Andfrom theseresults, we cancalculate each a//e/e frequency in this We'lluse the samelocus and alleles that we did in Box6.2, A, and newgeneration as the frequencyof homozygoteindividuals plus half Ar, and starting frequenciesof p and 4 respectively.We've already the frequencyof heterozygotes: seenthat for a
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