5. Selection Selection to Avoid Predators. Many of the most dra- As Darwin pointed out, some individuals leave behind matic documented instances of a daptauon involve gene more progeny than others, and the rate at which they do so hanges which decrease the probability of capture by a is affected by phenotype and behavior. We describe the re- predator. The caterpillar larvae of the common sulphur butterfly Colias eurytheme usually exhibit a dull Kelly green Its of this process as selection and speak of both artifi- color, providing excellent camouflage on the alfalfa plants the breeder selects for the desired characteristics. In natural selection. environmental conditions determine which indi- is kept at very low frequency because this color renders the larvae highly visible on the food plant, making it easier for viduals in a population produce the most offspring. For bird predators to see them. In a similar fashion, the way the natural selection to occur and result in evolutionary change three conditions must be met shell markings in the land snail Cepaea nemoralis match its background habitat reflects the same pattern of avoiding 1. Variation must exist among individuals in a popu- predation by camouflage lation. Natural selection works by favoring individ- of the most dramatic examples of background uals with some traits over individuals with alternative matching involves ancient lava flows in the middle of traits. If no variation exists, natural selection cannot deserts in the American southwest. In these areas, the black oPerate rock formations produced when the lava cooled contrasts 2. Variation among individuals results in differences starkly to the surrounding bright glare of the desert sand in number of offspring surviving in the next gen- Populations of many species of animals--including lizards, eration. This is the essence of natural selection Be- rodents, and a variety of insects--occurring on these rocks cause of their phenotype or behavior, some individu- are dark in color, whereas sand-dwelling populations in als are more successful than others in producing surrounding areas are much lighter(figure 20.7). Predation offspring and thus passing their genes on to the next is the likely cause selecting for these differences in color Laboratory studies have confirmed that predatory birds are 3. Variation must be geneticall lly inherited. For adept at picking out individuals occurring on backgrounds natural selection to result in evolutionary change, ch the the selected differences must have a genetic basis However, not all variation has a genetic basis--even genetically identical individuals may be phenotype cally quite distinctive if they grow up in different environments. Such environmental effects are com mon in nature. In many turtles, for example, indi viduals that hatch from eggs laid in moist soil are heavier, with longer and wider shells, than individu Is from nests in drier areas. As a result of these en vironmental effects, variation within a population does not always indicate the existence of underlyin genetic variation. When phenotypically different individuals do not differ genetically, then differ- ences in the number of their offspring will not alter the genetic composition of the population in the next generation and, thus, no evolutionary change It is important to remember that natural selection and evolution are not the same-the two concepts often are incorrectly equated. Natural selection is a process, whereas evolution is the historical record of change through time. Evolution is an outcome, not a process Natural selection(the process) can lead to evolution(the outcome), but natural selection is only one of several processes that can produce evolutionary change. More- FIGURE 20.7 over, natural selection can occur without producing evo- Pocket mice from the Tularosa Basin of New Mexico whose tionary change; only if variation is genetically based will color matches their background. (a) The rock pocket mouse natural selection lead to evolution lives on lava, (b)while the Apache pocket mouse lives on white 428 Part vI Evolution5. Selection As Darwin pointed out, some individuals leave behind more progeny than others, and the rate at which they do so is affected by phenotype and behavior. We describe the re￾sults of this process as selection and speak of both artifi￾cial selection and natural selection. In artificial selection, the breeder selects for the desired characteristics. In natural selection, environmental conditions determine which indi￾viduals in a population produce the most offspring. For natural selection to occur and result in evolutionary change, three conditions must be met: 1. Variation must exist among individuals in a popu￾lation. Natural selection works by favoring individ￾uals with some traits over individuals with alternative traits. If no variation exists, natural selection cannot operate. 2. Variation among individuals results in differences in number of offspring surviving in the next gen￾eration. This is the essence of natural selection. Be￾cause of their phenotype or behavior, some individu￾als are more successful than others in producing offspring and thus passing their genes on to the next generation. 3. Variation must be genetically inherited. For natural selection to result in evolutionary change, the selected differences must have a genetic basis. However, not all variation has a genetic basis—even genetically identical individuals may be phenotypi￾cally quite distinctive if they grow up in different environments. Such environmental effects are com￾mon in nature. In many turtles, for example, indi￾viduals that hatch from eggs laid in moist soil are heavier, with longer and wider shells, than individu￾als from nests in drier areas. As a result of these en￾vironmental effects, variation within a population does not always indicate the existence of underlying genetic variation. When phenotypically different individuals do not differ genetically, then differ￾ences in the number of their offspring will not alter the genetic composition of the population in the next generation and, thus, no evolutionary change will have occurred. It is important to remember that natural selection and evolution are not the same—the two concepts often are incorrectly equated. Natural selection is a process, whereas evolution is the historical record of change through time. Evolution is an outcome, not a process. Natural selection (the process) can lead to evolution (the outcome), but natural selection is only one of several processes that can produce evolutionary change. More￾over, natural selection can occur without producing evo￾lutionary change; only if variation is genetically based will natural selection lead to evolution. Selection to Avoid Predators. Many of the most dra￾matic documented instances of adaptation involve genetic changes which decrease the probability of capture by a predator. The caterpillar larvae of the common sulphur butterfly Colias eurytheme usually exhibit a dull Kelly green color, providing excellent camouflage on the alfalfa plants on which they feed. An alternative bright blue color morph is kept at very low frequency because this color renders the larvae highly visible on the food plant, making it easier for bird predators to see them. In a similar fashion, the way the shell markings in the land snail Cepaea nemoralis match its background habitat reflects the same pattern of avoiding predation by camouflage. One of the most dramatic examples of background matching involves ancient lava flows in the middle of deserts in the American southwest. In these areas, the black rock formations produced when the lava cooled contrasts starkly to the surrounding bright glare of the desert sand. Populations of many species of animals—including lizards, rodents, and a variety of insects—occurring on these rocks are dark in color, whereas sand-dwelling populations in surrounding areas are much lighter (figure 20.7). Predation is the likely cause selecting for these differences in color. Laboratory studies have confirmed that predatory birds are adept at picking out individuals occurring on backgrounds to which they are not adapted. 428 Part VI Evolution (b) (a) FIGURE 20.7 Pocket mice from the Tularosa Basin of New Mexico whose color matches their background. (a) The rock pocket mouse lives on lava, (b) while the Apache pocket mouse lives on white sand