As stated earlier in this chapter, quantitative traits are traits that result from complex interactions between multiple genes and that may be influenced by environmental factors. To understand how these traits evolve, evolutionary biologists analyze the heritability of quantitative traits.
As you can imagine, the first task is to determine what proportion of the trait (or phenotype) is due to genetic factors (the heritable bits) and what portion is due to the environment (the non-heritable bits). As tough as that job is, the analysis is made even more complicated by the fact that any given gene, or allele, may impact the resulting phenotype in an additive or non-additive way.
Additive or non-additive?
The concept of additivity can be a little slippery. To understand it better, think about a single gene with two alleles: for example, wrinkled or smooth peas. In this case, the smooth form (A) is dominant over the wrinkled form (a). An individual with a single allele for the smooth form (Aa) would express the smooth phenotype. Having two copies of the smooth form (AA) produces the same smooth phenotype. In the first case (Aa), the single recessive allele is non-additive, as is the second dominant allele in the second case (AA). Neither allele affects the final expression of the trait. (For details on dominant and recessive genes, see Chapter 3.)
This situation, wherein the combinations of different alleles can result in some alleles not having an effect on the phenotype, can happen across different loci. A gene may have the potential to influence a particular phenotype, but it may not influence the phenotype in an additive way. In other words, when the conditions are right (a certain combination of alleles across loci or a particular interaction with other alleles), the gene may influence the pheno-type. At other times, this same gene may not affect the phenotype, because the necessary combination or interaction didn't occur.
A hot subtopic of quantitative genetics is sorting through the non-additive nature of multiple genes to figure out whether some of them are especially important (or especially important some of the time). Given that some ailments certainly have a genetic component that's controlled by multiple genes, it would be nice if researchers could identify genes that have large effects and then figure out what they do. This is crucially important for understanding the genetics of some human diseases.
Determining phenotypic Variation
Evolution requires that variation exist and that this variation be heritable. The upshot of environmental effects and of the non-additive genetic variation is to decrease heritability and, as a result, decrease the power of selection to transform populations. To determine the strength of selection, scientists separate the variation in a population into the differences due to genetics (both additive and non-additive) and the differences due to environment.
Variation is a property of groups of individuals or populations, not individuals alone, no matter how fickle, unpredictable, or changeable those individuals are. All the analysis performed to determine variation relates to groups of individuals.
To understand all the components of an analysis of the heritability of quantitative traits, consider a simple hypothetical example: height. (Why height? No particular reason. You could use any quantitative trait.)
The phenotype — in this example, whether you're tall or short — is a function of the genotype and the environment. Height has a heritable component: Tall parents tend to have taller offspring. It also has an environmental component: Absent a proper diet, you won't get very tall.
You can further partition the genetic component of the phenotype into additive and non-additive parts. How strong selection is for this trait is determined by the additive component of genetic variation.
Here's the math: The phenotypic variation within a population is the sum of additive genetic variation plus the non-additive genetic variation plus the environmental variation. If you like formulas, here's what this one looks like:
phenotypic variation = additive genetic variation + non-additive genetic variation + environmental variation
Environmental variation isn't heritable. Imagine two people who have a similar genetic makeup, one of whom is taller due only to a better diet. Because the variation between these two individuals isn't due to genetic factors, the taller person won't have taller offspring. But variation that is a function of genetics and not of the environment is heritable.
For the purposes of understanding natural selection, it's helpful to think of heritability as being either the broad-sense or the narrow-sense type:
^ Broad-sense heritability: The total of all of the genetic factors, be they additive or non-additive
^ Narrow-sense heritability: The subset of the genetic component that is additive.
Heritability is measured as a number from 0 to 1, indicating the degree of correlation between the parental phenotype and the offspring phenotype:
^ If the offspring phenotype is predicted by the phenotype of the parents, heritability is 1.
^ If the offspring phenotype is not predicted by the phenotype of the parents, heritability is 0.
In the height example, in which the difference between the height of two people was due simply to diet, the phenotype of the offspring of the tall parent would not be any different from the phenotype of the offspring of the short parent, and heritability would be close to zero.
Think back to the example of the smooth and wrinkly peas. Imagine two pea plants, both of which are heterozygous for the smooth character — that is, each plant has both a dominant smooth allele and a recessive wrinkled allele. Because of the dominant interaction between these two genes, all the peas are smooth. Now suppose that you cross these two pea plants. On average, one quarter of the offspring will have wrinkly seeds. In this very simple case, you can see that although the phenotype of the offspring was a direct result of the genes they inherited from their parents (broad-sense heritability), the phenotype of the parents was an inexact predictor of the phenotype of the offspring. Heritability in the narrow sense was less than 1.
In evolution, narrow-sense heritability is the important form of heritability. Imagine that for some reason, being a pea plant with smooth seeds is advantageous. Natural selection will result in the pea plants with smooth seeds being the ones to leave more descendents, and as a result, the next generation will have fewer plants with wrinkly seeds — but not as many fewer as you would expect based just on the relative selective advantage of having smooth seeds. Why? Some of the plants with smooth seeds will have wrinkly offspring. That phenomenon is the non-additive part of the genetic variation.
Was this article helpful?