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Fitness and Natural Selection Different phenotypes interact with the environment in slightly different ways, leading to variation in fecundity and survival among individuals in populations. The rate of reproduction of a phenotype – the intrinsic rate of increase of the life table for the phenotype – is a measure of its fitness. When differences in fitness among individuals have a genetic basis, those alleles that have the higher fitness reproduce faster and take over the population. The phenotypes with the highest finesses are said to be selected, and the change in genotype frequencies resulting from natural selection is referred to as evolution. We can now point to many cases of selection producing evolution in natural populations, often in response to environmental changes brought about by human activities. Perhaps the most striking case is that of industrial melanism in the peppered moth in England. The English have always been avid butterfly and moth collectors, and such enthusiasts look carefully for rare variant forms. Early in the nineteenth century, occasional dark (or melanistic) specimens of the common peppered moth (Biston betularia) were collected. Over the next 100 years, the dark from, referred to as carbonaria, became increasingly common in industrial areas (today it makes up nearly 100 percent of some populations.) The phenomenon aroused considerable interest among geneticists, who showed by cross-mating light and dark forms that melanism is an inherited trait determined by a single gene. In the early 1950s H.B.D. Kettlewell, and English physician who had been practicing medicine for 15 years and was an amateur butterfly and moth collector, changed the course of his life to pursue the study of industrial melanism. Several facts about melanism were known before Kettlewell began his studies. The melanistic trait is an inherited characteristic, so its spread reflected genetic changes (evolution) in the population. The earliest records of the carbonaria from were from forests near heavily industrialized regions of England. In the absence of factories and other heavy industry, the light form of the moth still prevailed. It was also known that melanism is not unique to the peppered moth; dark forms have appeared in many other moths and in other insects. The peppered moth inhabits dense woods and rests on tree trunks during the day. Kettlewell reasoned that where melanistic individuals had become common, the environment must somehow have been altered in such a way as to give the dark from a survival advantage over the light from. Could natural selection have led to the replacement of the typical light form by the carbonaria form? To test this hypothesis, Kettlewell had to find some measure of fitness other than the relative evolutionary success of the two forms. Population Genetics and Evolution Fitness and Natural Selection To determine whether the carbonaria form had greater fitness than typical peppered moths in areas where melanism occurred, Kettlewell chose the markrecapture method. He marked adult moths of both forms with a dot of cellulose paint and released them. The mark was placed on the underside of the wing so that it would not call the attention of predators to a moth resting on a tree trunk. Kettlewell recaptured moths by attracting them to a mercury vapor lamp in the center the woods and to caged virgin females at the edge of the woods. (Only males could be used in the study because females are attracted to neither lights nor to virgin females.) In one experiment, Kettlewell released 201 typical and 601 melanics in a wooded area near industrial Birmingham, where the tree bark was darkened by pollution. The results were as follows: Number of moths released Number of moths recaptured Percent recaptured Typicals 201 34 16 Melanics 601 205 34 These figures indicated that more of the dark form survived over the course of the experiment. Although consistent with Kettlewell’s original hypothesis, the results could be interpreted otherwise: as differential attraction of the two forms to the traps or as differential dispersion of the two forms away from the point of release. Variables besides differential mortality had to be accounted for. To test the hypothesis of natural selection unequivocally, Kettlewell ran a similar experiment in an unpolluted forest near Dorset, where the tree bark was lighter. Of 496 marked typicals released, 62 (12.5 percent) were recapture; of 473 marked maleness, only 30 (6.3 percent) were recaptured. Thus in the unpolluted forest, light adults had a higher recapture rate than dark adults. If typicals and melanics were differently attracted to light traps or dispersed from a release point at different rates, the level of pollution would not have influenced the results. In fact, only differential survival could account for a reversal in the relative rates of recapture. This confirmed Kettlewell’s hypothesis and established natural selection as being responsible for the high frequency of the carbonaria from in industrial areas. The specific agent of selection was easily identified. Kettlewell reasoned that in industrial area, pollution had darkened the trunks of trees so much that “typical” moths stood out against them and were readily found by predators. Any aberrant dark forms were better camouflaged against the darkened tree trunks, and their coloration conferred survival value (Figure 17.2). Eventually, differential survival of dark and light forms would lead to changes in their relative frequency in a population. To test this idea, Kettlewell placed equal numbers of the light Population Genetics and Evolution Fitness and Natural Selection and dark forms on tree trunks in polluted and unpolluted woods and watched them carefully at some distance from behind a blind. (A blind is a tent-like structure intended to conceal observers from their subjects; it is more often called a hide in England.) He quickly discovered that several species of birds regularly searched the tree trunks for moths and other insects and that these birds more readily found a moth that contrasted with its background than one that resembled the bark it clung to. Kettlewell tabulated the following instances of predation: Number of moths released Number of moths recaptured Percent recaptured Typicals 201 34 16 Melanics 601 205 34 These figures indicated that more of the dark form survived over the course of the experiment. These data were fully consistent with the results of the mark-capture experiments. Together they clearly demonstrate the operation of natural selection, which over a long period results in genetic changes in populations of the peppered moth in polluted areas. Many decades were required for the replacement of one form by the other. The agents of selection were insectivorous birds whose ability to find the moths depended on the coloration of the moth with respect to its background. Kettlewell’s study shows clearly how the interaction between the organism and its environment determines its fitness. Light Colored Moth Typical Dark Colored Moth Carbonaria Population Genetics and Evolution Fitness and Natural Selection Fitness and Natural Selection Questions Directions: Answer each question in a complete paragraph with as much detail as possible 1. How is a phenotype selected? 2. Define evolution according to this article. 3. What phenomena among peppered moths became a special interest in England around the beginning of the nineteenth century? 4. How did Kettlewell determine whether carbonaria peppered moths had a greater fitness than typical peppered moths? 5. What was Kettlewell’s hypothesis and was it confirmed? 6. What agent(s) of selection did Kettlewell identify? 7. Explain this statement: The interaction between an organism and its environment determine its fitness. Population Genetics and Evolution Fitness and Natural Selection