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Attention to the problems of estimating broad heritability distracts from the deeper questions about the meaning of the ratio even when it can be estimated. Despite its widespread use as a measure of how “important” genes are in influencing a character, H2 actually has a special and limited meaning. Two alternative conclusions can be drawn from the results of a properly designed heritability study. First, if there is a nonzero heritability, we can conclude that, in the population measured and in the environments in which the organisms have developed, genetic differences have influenced the phenotypic variation among individuals, and so genetic differences do matter to the trait. This finding is not trivial, and it is the first step in a more detailed investigation of the role of genes. 

It is important to notice that the reverse is not true. If zero heritability for the trait is found, this finding is not a demonstration that genes are irrelevant to the trait; rather, it demonstrates only that, in the particular population and environment studied, either there is no genetic variation at the relevant loci or different genotypes have the same phenotype. In other populations or other environments, the character might be heritable. Moreover, we must distinguish between genes contributing to a trait and genetic differences contributing to differences in a trait. The natural experiment of immigration to North America has proved that the ability to pronounce the sounds of North American English, rather than French, Swedish, or Russian, is not a consequence of genetic differences between our immigrant ancestors. 

But, without the appropriate genes, we could not speak any language at all. Second, the value of H2 provides a limited prediction of how much a character can be modified by changing the environment. If all the relevant environmental variation is eliminated and the new constant environment is the same as the mean environment in the original population, then H2 estimates how much phenotypic variation will still be present. So, if the heritability of performance on an IQ test were found to be, say, 0.4, then we could predict that if all children had the same developmental and social environment as the “average child,” about 60 per cent of the variation in IQ test performance would disappear and 40 per cent would remain. 

The requirement that the new constant environment be at the mean of the old environmental distribution is absolutely essential to this prediction. If the environment is shifted toward one end or the other of the environmental distribution present in the population used to determine H2 or if a new environment is introduced, nothing at all can be predicted. In the example of IQ test performance, the heritability gives us no information at all about how variable performance would be if the developmental and social environments of all children were enriched. To understand why this is so, we must return to the concept of the norm of reaction. The separation of phenotypic variance into genetic and environmental components, and , does not really separate the genetic and environmental causes of variation. 

Consider the results presented in Figure 20-9b. When the environment is poor (an environmental quality of 50), corn variety 2 has a much higher yield than variety 1, and so a population made up of a mixture of the two varieties would have a lot of genetic variance for yield in that environment. But, in a richer environment (scoring 75), there is no difference in yield between varieties 1 and 2, and so a mixed population would have no genetic variance at all for yield in that environment. Thus, genetic variance has been changed by changing the environment. On the other hand, variety 2 is less sensitive to the environment than variety 1, as shown by the slopes of the two lines. So a population made up mostly of variety 2 would have a lower environmental variance then one made up mostly of variety 1. 

So, environmental variance in the population is changed by changing the proportion of genotypes. As a consequence of the argument that was just given, we cannot predict just from knowing the heritability of a character difference how the distribution of variation in the character will change if either genotypic frequencies or environmental factors change markedly. So, for example, in regard to IQ test performance, knowing that the heritability is 0.4 in one environment does not allow us to predict how IQ test performance will vary among children in a different environment. All that high heritability means is that, for the particular population developing in the particular distribution of environments in which the heritability was measured, average differences between genotypes are large compared with environmental variation within genotypes. 

If the environment is changed, there may be large differences in phenotype. Perhaps the best-known example of the erroneous use of heritability arguments to make claims about the changeability of a trait is that of human IQ performance and social success. Many studies have been made of the heritability of IQ performance in the belief that, if heritability is high, then various programs of education designed to increase intellectual performance are a waste of time. The argument is that, if a trait is highly heritable, then it cannot be changed much by environmental changes. But, irrespective of the value of H2 for IQ test performance, the real error of the argument lies in equating high heritability with unchangeability. In fact, the heritability of IQ is irrelevant to the question of how changeable it is.