Assessing the Costs of Adaptations
Adaptations typically impose costs as well as benefits, and the evolution of adaptations depends on the trade-off between those costs and benefits. Garter snakes in some populations, for example, can eat rough-skinned newts without being poisoned, but they pay for this ability by sacrificing crawling speed. Determining the costs and benefits of a particular adaptation is difficult because individuals differ not only in the degree to which they possess the adaptation, but also in many other ways. How can investigators study individuals that differ only in the genetically based adaptation of interest? Such individuals can be created by recombinant DNA techniques using cloned or highly inbred populations. In plants, for example, plasmids can be used to transfer specific alleles to experimental individuals. Control individuals also receive plasmids, but those plasmids lack the allele of interest. Plasmid transfer techniques made it possible to measure the cost associated with resistance to the herbicide chlorosulfuron conferred by a single allele in the shale cress, Arabidopsis thaliana. The allele, Csr1-1, results in the production of an enzyme that is insensitive to chlorosulfuron. However, plants with the Csr1-1 allele produce 34 percent fewer seeds than nonresistant plants grown under identical conditions in the absence of the herbicide. The reason for the high cost of resistance is not fully understood, but evidence suggests that the resistance allele results in an accumulation of branched-chain amino acids that interfere with metabolism. Agriculturalists wish to alter the genotypes of plants to give them resistance to herbicides so that the herbicides applied to agricultural fields will kill the weeds, but not the crops. This experiment shows that such benefits may impose a trade-off in terms of crop yield. We saw in the previous section that the possession of certain conspicuous features by males confers reproductive benefits. What kinds of trade-offs do these benefits impose? The cost of long tails was not measured in the experiments with widowbirds but related studies have been done on males of other species. In some mammalian species, including deer, lions and baboons, one male controls reproductive access to many females. These polygynous species tend to be sexually dimorphic— the males appear quite different from the females. Males of these species are significantly larger than females and often bear large weapons (such as horns, antlers, and large canine teeth); size and weaponry are needed to defend a male’s multiple mates against other males of the species. The costs of sexual dimorphism for males of polygynous species were assessed using the comparative method. Such males have higher parasite loads and higher mortality rates than females of their own species because maintaining a large size and bearing large weapons makes them more susceptible to parasites. In addition, when compared to parasite loads in males of closely related monogamous species (in which males and females are essentially monomorphic, appearing quite similar), the dimorphic males carried higher parasite loads in almost every case.
Maintaining Genetic Variation
Genetic drift, stabilizing selection, and directional selection all tend to reduce genetic variation within populations. Nevertheless, as we have seen, most populations have considerable genetic variation. What maintains so much genetic variation within populations? To answer this question, we will show how sexual recombination, neutral mutations, and frequency-dependent selection can maintain variation within populations, and how variation may be maintained over geographic space.
Sexual recombination amplifies the number of possible genotypes
In asexually reproducing organisms, the cells resulting from a mitotic division normally contain identical genotypes. Each new individual is genetically identical to its parent, unless there has been a mutation. When organisms exchange genetic material during sexual reproduction, however, offspring differ from their parents because chromosomes assort randomly during meiosis, crossing-over occurs and fertilization brings together material from two different cells.
Sexual recombination generates an endless variety of genotypic combinations that increases the evolutionary potential of populations. Because it increases the variation among the offspring produced by an individual, sexual recombination may improve the chance that at least some of those offspring will be successful in the varying and often unpredictable environments they will encounter. Sexual recombination does not influence the frequencies of alleles; rather, sexual recombination generates new combinations of alleles on which natural selection can act. It expands variation in a character influenced by alleles at many loci by creating new genotypes. That is why selection for bristle number in Drosophila resulted in flies with more bristles than any flies in the initial population had.
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