that tends to fly 30 卫629399 Selected population 3 that tends not to fly toward light Number of generations Birth weight in pounds FIGURE 20.14 FIGURE 20.15 Directional selection for phototropism in Drosophila. In Stabilizing selection for birth weight in human beings. The eneration after generation, individuals of the fly Drosophila were death rate among babies(red curve; right y-axis)is lowest at ar selectively bred to obtain two populations. When flies with a intermediate birth weight; both smaller and larger babies have a trong tendency to fly toward light(positive phototropism) were greater tendency to die than those around the optimum weight used as parents for the next generation, their offspring had a (blue area; left y-axis)of between 7 and 8 pounds greater tendency to fly toward light(top curve). When flies that tended not to fly toward light were used as parents for the next generation,their offspring had an even greater tendency not to my Many examples are known. In humans, infants with inter- toward light(bottom curve) mediate weight at birth have the highest survival rate(fig ure 20.15). In ducks and chickens, eggs of intermediat weight have the highest hatching success. This form of se Directional selection ection is called stabilizing selection. When selection acts to eliminate one extreme from an array of phenotypes(figure 20 13b), the genes promoting Components of Fitness this extreme become less frequent in the population. Thus in the Drosophila population illustrated in figure 20 14, the Natural selection occurs when individuals with one phe elimination of flies that move toward light causes the popu- type leave more surviving offspring in the next generation han individuals with an alternative phenotype. Evolution lation to contain fewer individuals with alleles promos ary biologists quantify reproductive success as fitness, the such behavior. If you were to pick an individual at random number of surviving offspring left in the next generation from the new fly population, there is a smaller chance it Although selection is often characterized as"survival of the would spontaneously move toward light than if you had se lected a fly from the old population Selection has changed fitness. Even if no differences in survival occur. selection the population in the direction of lower light attraction. may operate if some individuals are more successful than This form of selection is called directional selection others in attracting mates. In many territorial animal species, large males mate with many females and small Stabilizing selection mates rarely get to mate. In addition, the number of off- When selection acts to eliminate botb extremes from an spring produced per mating is also important. Large female array of phenotypes(figure 2013c), the result is to increase frogs and fish lay more eggs than smaller females and thus the frequency of the already common intermediate type. In may leave more offspring in the next generation effect, selection is operating to prevent change away from this middle range of values Selection does not change the Selection on traits affected by many genes can favor most common phenotype of the population, but rather both extremes of the trait, or intermediate values, or makes it even more common by eliminating extremes only one extreme Chapter 20 Go ene Populations 435Directional Selection When selection acts to eliminate one extreme from an array of phenotypes (figure 20.13b), the genes promoting this extreme become less frequent in the population. Thus, in the Drosophila population illustrated in figure 20.14, the elimination of flies that move toward light causes the popu￾lation to contain fewer individuals with alleles promoting such behavior. If you were to pick an individual at random from the new fly population, there is a smaller chance it would spontaneously move toward light than if you had se￾lected a fly from the old population. Selection has changed the population in the direction of lower light attraction. This form of selection is called directional selection. Stabilizing Selection When selection acts to eliminate both extremes from an array of phenotypes (figure 20.13c), the result is to increase the frequency of the already common intermediate type. In effect, selection is operating to prevent change away from this middle range of values. Selection does not change the most common phenotype of the population, but rather makes it even more common by eliminating extremes. Many examples are known. In humans, infants with inter￾mediate weight at birth have the highest survival rate (fig￾ure 20.15). In ducks and chickens, eggs of intermediate weight have the highest hatching success. This form of se￾lection is called stabilizing selection. Components of Fitness Natural selection occurs when individuals with one pheno￾type leave more surviving offspring in the next generation than individuals with an alternative phenotype. Evolution￾ary biologists quantify reproductive success as fitness, the number of surviving offspring left in the next generation. Although selection is often characterized as “survival of the fittest,” differences in survival are only one component of fitness. Even if no differences in survival occur, selection may operate if some individuals are more successful than others in attracting mates. In many territorial animal species, large males mate with many females and small mates rarely get to mate. In addition, the number of off￾spring produced per mating is also important. Large female frogs and fish lay more eggs than smaller females and thus may leave more offspring in the next generation. Selection on traits affected by many genes can favor both extremes of the trait, or intermediate values, or only one extreme. Chapter 20 Genes within Populations 435 0 2 4 6 8 10 Number of generations Average tendency to fly toward light 2 1 3 4 5 6 7 8 9 10 11 12 13 14 15 12 18 20 14 16 Selected population that tends not to fly toward light Selected population that tends to fly toward light FIGURE 20.14 Directional selection for phototropism in Drosophila. In generation after generation, individuals of the fly Drosophila were selectively bred to obtain two populations. When flies with a strong tendency to fly toward light (positive phototropism) were used as parents for the next generation, their offspring had a greater tendency to fly toward light (top curve). When flies that tended not to fly toward light were used as parents for the next generation, their offspring had an even greater tendency not to fly toward light (bottom curve). 20 15 10 5 10 20 30 50 70 100 5 7 3 2 23 456 Birth weight in pounds Percent of births in population Percent of infant mortality 7 8 9 10 FIGURE 20.15 Stabilizing selection for birth weight in human beings. The death rate among babies (red curve; right y-axis) is lowest at an intermediate birth weight; both smaller and larger babies have a greater tendency to die than those around the optimum weight (blue area; left y-axis) of between 7 and 8 pounds