Limits to what selection n Accomplish 130 Although selection is perhaps the most owerful of the five principal agents of netic change. there are limits to what it can ccomplish. These limits arise because al ternative alleles may interact in different 2 ways with other genes and because alleles 5 type(the phenomena of epistasis and /e120 often affect multiple aspects of the pheno- pleiotropy discussed in chapter 13). These /3 interactions tend to set limits on how much a phenotype can be altered. For example,115 fo r large clutch size in barnyard chickens eventually leads to eggs with thin ner shells that break more easily. For this 110 reason, we do not have gigantic cattle that 1920 1960 2000 yield twice as much meat as our leading strains, chickens that lay twice as many eggs as the best layers do now, or corn with an ear at the base of every leaf, instead of just at the base of a few leaves FIGURE 20.16 Selection for increased speed in racehorses is no longer effective. Kentucky Evolution Requires Genetic Derby winning speeds have not improved significantly since 1950 Variation Over 80% of the gene pool of the thor oughbred horses racing today goes back to 31 known an cestors from the late eighteenth century. Despite intense directional selection on thoroughbreds, their perfor- mance times have not improved for the last 50 years(fig ure 20.16). Years of intense selection presumably have re- moved variation from the population at a rate greater than it could be replenished by mutation such that now no genetic variation remains and evolutionary change is not possible In some cases, phenotypic variation for a trait may never have had a genetic basis. The compound eyes of in sects are made up of hundreds of visual units, termed om- matidia. In some individuals, the left eye contains more ommatidia than the right eye. However, despite intense selection in the laboratory, scientists have never been able to produce a line of fruit flies that consistently have / more ommatidia in the left eye than in the right. The eason is that separate genes do not exist for the left and right eyes. Rather, the same genes affect both eyes, and differences in the number of ommatidia result from dif- FIGURE 20.17 ferences that occur as the eyes are formed in the develop- Phenotypic variation in insect ommatidia. In some indivic ment process(figure 20.17). Thus, despite the existence the number of ommatidia in the left eye is greater than the uals phenotypic variation, no genetic variation is available number in the right eye. However, this difference is not election to favor genetically based; developmental processes cause the differene 436 Part vI EvolutionLimits to What Selection Can Accomplish Although selection is perhaps the most powerful of the five principal agents of ge￾netic change, there are limits to what it can accomplish. These limits arise because al￾ternative alleles may interact in different ways with other genes and because alleles often affect multiple aspects of the pheno￾type (the phenomena of epistasis and pleiotropy discussed in chapter 13). These interactions tend to set limits on how much a phenotype can be altered. For example, selecting for large clutch size in barnyard chickens eventually leads to eggs with thin￾ner shells that break more easily. For this reason, we do not have gigantic cattle that yield twice as much meat as our leading strains, chickens that lay twice as many eggs as the best layers do now, or corn with an ear at the base of every leaf, instead of just at the base of a few leaves. Evolution Requires Genetic Variation Over 80% of the gene pool of the thor￾oughbred horses racing today goes back to 31 known an￾cestors from the late eighteenth century. Despite intense directional selection on thoroughbreds, their perfor￾mance times have not improved for the last 50 years (fig￾ure 20.16). Years of intense selection presumably have re￾moved variation from the population at a rate greater than it could be replenished by mutation such that now no genetic variation remains and evolutionary change is not possible. In some cases, phenotypic variation for a trait may never have had a genetic basis. The compound eyes of in￾sects are made up of hundreds of visual units, termed om￾matidia. In some individuals, the left eye contains more ommatidia than the right eye. However, despite intense selection in the laboratory, scientists have never been able to produce a line of fruit flies that consistently have more ommatidia in the left eye than in the right. The reason is that separate genes do not exist for the left and right eyes. Rather, the same genes affect both eyes, and differences in the number of ommatidia result from dif￾ferences that occur as the eyes are formed in the develop￾ment process (figure 20.17). Thus, despite the existence of phenotypic variation, no genetic variation is available for selection to favor. 436 Part VI Evolution 1900 110 115 120 125 130 1920 1940 1960 Year Kentucky Derby winning speed (seconds) 1980 2000 FIGURE 20.16 Selection for increased speed in racehorses is no longer effective. Kentucky Derby winning speeds have not improved significantly since 1950. Right Left FIGURE 20.17 Phenotypic variation in insect ommatidia. In some individuals, the number of ommatidia in the left eye is greater than the number in the right eye. However, this difference is not genetically based; developmental processes cause the difference