Affects The Distribution Of Phenotypes

Affects The Distribution Of Phenotypes

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which of the following is NOT a way in which natural selection affects the distribution of phenotypes? a. directional selection b. stabilizing selection c. distruptive selection d. chance events

The phenotypic  of a trait, as we have seen, is a function of the average differences between genotypes and of the  between genotypically identical individuals. But both are in turn functions of the sequence of environments in which the organisms develop and live. For a given , each  will result in a given  (for the moment, ignoring developmental noise). Then a distribution of environments will be reflected biologically as a distribution of phenotypes. The way in which the environmental distribution is transformed into the phenotypic distribution is determined by the norm of reaction, as shown in Figure 25-5. The horizontal axis is environment (say, temperature) and the vertical axis is phenotype (say, plant height). The  curve for the genotype shows how each particular temperature results in a particular plant height. So, the dashed lines from the 18° point on the temperature axis is reflected off the norm of reaction curve to a corresponding plant height on the vertical phenotype axis, and so forth for each temperature. If a large number of individuals develop at, say, 20°C, then a large number of individuals will have the phenotype that corresponds to 20°C, as shown by the dashed ; and, if only small numbers develop at 18°C, few plants will have the corresponding plant height.

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