As we mentioned at the beginning of this chapter, 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 dif
23.17 Bright Bills Signal Good Health (a) This experiment demonstrated that bright bill color in the male zebra finch does indeed indicate a healthy individual. (b) Female zebra finches (the bird below) preferentially choose mates with the brightest bill color—thus choosing the healthiest males.
fer 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 (see Figure 16.5). 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 chloro-sulfuron conferred by a single allele in the shale cress, Ara-bidopsis 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 (Figure 23.18).
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
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