Dli

(100) where x is the trait mean and var is the trait variance) is used to compare the variances of distributions after correcting for differences due only to the value of the mean. The CV expresses the magnitude of the standard deviation as a percentage of the mean. As an illustration, look at the quantitative trait distributions in Fig. 9.1. Body length and weight for fish have much greater mean values than pupal weights for mosquitoes. But, using the CV, we can compare the variation in these traits to see that the standard deviation is about 5% of the mean for body length in striped bass and almost 21% of the mean for pupal mass in wild-reared mosquitoes. The CV also allows us to properly compare the distributions of pupal mass for laboratory-reared mosquitoes and mosquitoes grown in the wild to see that the spread of pupal mass values around the mean is about two times wider in the wild mosquitoes (CV = 20.68) than in laboratory-reared mosquitoes (CV = 11.23).

Biologically, equation 9.3 helps us to recognize and quantify the determinants of phenotypic variation. Equation 9.3 divides the causes of quantitative trait variation into those due to heredity and those due to the environment, or into the phenotypic variation caused by nature and that caused by nurture. Look again at the bottom panels of Fig. 9.1 that show pupal mass distributions for mosquitoes. Imagine that the laboratory and wild populations of mosquitoes have roughly the same genotype frequencies or VG. The greater pupal mass variance or VP in the wild population could then be explained by greater environmental variance or VE, a common observation since laboratory conditions tend to be more uniform and benign. As an alternative, imagine that the laboratory population has less VG since it was founded from a small sample of individuals and it also has less VE since the conditions individuals experience are more uniform. Then the greater pupal mass variance (VP) in the wild could be caused by both more genetic variation (VG) and more environmental variation (VE) than experienced by individuals in the laboratory. In this fashion it is possible to quantify and compare the relative causes of phenotypic variation.

The V notation compactly expresses the multiple causes of total phenotypic variation, VP, in quantitative traits. Genotypic variation (VG) and environmental variation (VE) are not the only causes of total phenotypic variation. Table 9.1 summarizes additional causes of total phenotypic variation. Notice that VG is actually broken down into three distinct components due to the effects of alleles, the effects of dominance, and the effects of gene interaction or epistasis (epistasis is symbolized VT since VE is

Table 9.1

Symbols commonly used to refer to categories or causes of variation in quantitative traits. Variation

is indicated by V while the specific cause of that variation is indicated by a subscript capital letter (with one

exception). Total genetic variation (Vc) in phenotype can be divided into three subcategories.

Symbol

Definition

VP

Total variance in a quantitative trait or phenotype

Vc

Variance in phenotype due to all genetic causes

VA

Variance in phenotype caused by additive genetic variance or the effects of alleles

Vd

Variance in phenotype caused by dominance genetic variance or deviations from additive

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