Selection to Match Climatic Conditions. Many studies of selection have focused on genes encoding en zymes because in such cases the investigator can directly the consequences to the organism of changes in th frequency of alternative enzyme alleles. Often investiga tors find that enzyme allele frequencies vary latitudinally, 9 with one allele more common in northern populations but progressively less common at more southern locations. A superb example is seen in studies of a fish, the mumm hog, Fundulus heteroclitus, which ranges along the eastern 3 0.4 coast of North America. In this fish, allele frequencies of the gene that produces the enzyme lactase dehydrogenase,9 0.2H which catalyzes the conversion of pyruvate to lactate, vary geographically(figure 20.8). Biochemical studies show that the enzymes formed by these alleles function differ ently at different temperatures, thus explaining their geo- 4442403836343230 graphic distributions. For example, the form of the en zyme that is more frequent in the north is a better catalyst at low temperatures than the enzyme from the south Latitude(Degrees North Moreover, functional studies indicate that at low tempera tures. individuals with the northern allele swim faster and FIGURE 20.8 presumably survive better, than individuals with the alter- Selection to match climatic conditions. Frequency of the cole native allele adapted allele for lactase dehydrogenase in the mummichog (Fundulus heteroclitus)decreases at lower latitudes, which are Selection for Pesticide Resistance. A particularly clearwarmer example of selection in action in natural populations is pro- vided by studies of pesticide resistance in insects. The widespread use of insecticides has led to the rapid evolution of resistance in more than 400 pest species. For example, Pesticide the resistance allele at the pen gene decreases the uptake of o arget site O insecticide, whereas alleles at the kdr and dld-r genes de crease the number of target sites, thus decreasing the bind ing ability of the insecticide(figure 20.9). Other alleles en- 48 hance the ability of the insects'enzymes to identify and detoxify insecticide molecules Single genes are also responsible for resistance in other Resistant nsect cell organisms. The pigweed, Amaranthus bybridus, is one of about 28 agricultural weeds that have evolved resistance (a)Insect cells with resistance allele at pen gene to the herbicide Triazine. Triazine inhibits photosynthe decreased uptake of the pesticide sis by binding to a protein in the chloroplast membrane. Single amino acid substitutions in the gene encoding the protein diminish the ability of Triazine to decrease the plant's photosynthetic capabilities. Similarly, Norway rats are normally susceptible to the pesticide Warfarin, which diminishes the clotting ability of the rat 's blood and leads to fatal hemorrhaging. However, a resistance allele at a ngle gene alters a metabolic pathway and renders War- farin ineffective Five factors can bring about a deviation from the (b )Insect cells with resistance allele at kdr gene proportions of homozygotes and heterozygotes decreased number of target sites for the pesticide predicted by the Hardy-Weinberg principle. Only selection regularly produces adaptive evolutionary FIGURE 20.9 change, but the genetic constitution of individual Selection for pesticide resistance. Resistance alleles at ger populations, and thus the course of evolution, can also like pen and kdr allow insects to be more resistant to pesticides be affected by mutation, gene flow, nonrandom nsects that possess these resistance alleles have become more nating, and genetic drift. through selection Chapter 20 Go ene atonsSelection to Match Climatic Conditions. Many studies of selection have focused on genes encoding en￾zymes because in such cases the investigator can directly assess the consequences to the organism of changes in the frequency of alternative enzyme alleles. Often investiga￾tors find that enzyme allele frequencies vary latitudinally, with one allele more common in northern populations but progressively less common at more southern locations. A superb example is seen in studies of a fish, the mummi￾chog, Fundulus heteroclitus, which ranges along the eastern coast of North America. In this fish, allele frequencies of the gene that produces the enzyme lactase dehydrogenase, which catalyzes the conversion of pyruvate to lactate, vary geographically (figure 20.8). Biochemical studies show that the enzymes formed by these alleles function differ￾ently at different temperatures, thus explaining their geo￾graphic distributions. For example, the form of the en￾zyme that is more frequent in the north is a better catalyst at low temperatures than the enzyme from the south. Moreover, functional studies indicate that at low tempera￾tures, individuals with the northern allele swim faster, and presumably survive better, than individuals with the alter￾native allele. Selection for Pesticide Resistance. A particularly clear example of selection in action in natural populations is pro￾vided by studies of pesticide resistance in insects. The widespread use of insecticides has led to the rapid evolution of resistance in more than 400 pest species. For example, the resistance allele at the pen gene decreases the uptake of insecticide, whereas alleles at the kdr and dld-r genes de￾crease the number of target sites, thus decreasing the bind￾ing ability of the insecticide (figure 20.9). Other alleles en￾hance the ability of the insects’ enzymes to identify and detoxify insecticide molecules. Single genes are also responsible for resistance in other organisms. The pigweed, Amaranthus hybridus, is one of about 28 agricultural weeds that have evolved resistance to the herbicide Triazine. Triazine inhibits photosynthe￾sis by binding to a protein in the chloroplast membrane. Single amino acid substitutions in the gene encoding the protein diminish the ability of Triazine to decrease the plant’s photosynthetic capabilities. Similarly, Norway rats are normally susceptible to the pesticide Warfarin, which diminishes the clotting ability of the rat’s blood and leads to fatal hemorrhaging. However, a resistance allele at a single gene alters a metabolic pathway and renders War￾farin ineffective. Five factors can bring about a deviation from the proportions of homozygotes and heterozygotes predicted by the Hardy-Weinberg principle. Only selection regularly produces adaptive evolutionary change, but the genetic constitution of individual populations, and thus the course of evolution, can also be affected by mutation, gene flow, nonrandom mating, and genetic drift. Chapter 20 Genes within Populations 429 1.0 0.8 0.6 0.4 0.2 44 42 40 38 36 34 32 30 Latitude (Degrees North) Frequency of cold-adapted allele FIGURE 20.8 Selection to match climatic conditions. Frequency of the cold￾adapted allele for lactase dehydrogenase in the mummichog (Fundulus heteroclitus) decreases at lower latitudes, which are warmer. Pesticide molecule Resistant target site Insect cell membrane Target site Target site (a) Insect cells with resistance allele at pen gene: decreased uptake of the pesticide (b) Insect cells with resistance allele at kdr gene: decreased number of target sites for the pesticide FIGURE 20.9 Selection for pesticide resistance. Resistance alleles at genes like pen and kdr allow insects to be more resistant to pesticides. Insects that possess these resistance alleles have become more common through selection