Cultural Phylogenetics by Larissa Mendoza Straffon

Author:Larissa Mendoza Straffon
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

The use of the squared symbol here is a reminder that the parameter of heritability is based on the descriptive statistic of variance (i.e., the standard deviation squared) in terms of the two variables used to compute it. Because the denominator (V P) of this fraction is always inevitably larger than the numerator (V G), computed heritability values always range between 0 and 1. (Please note that due to complications that do not concern us here, the parameter described above is strictly referred to as “broad sense” heritability, which is often contrasted with “narrow sense” heritability, or h 2 in biological usage.)

In quantitative genetic terms, therefore, the heritability of a metric trait is the degree to which variation in that trait (measured across a group of individuals) is determined by genetic factors, expressed as a ratio of the degree to which variation in that trait is also determined by additional (i.e., nonheritable) factors. Because total phenotypic variance (V P) is the sum of both genetic and environmental components (i.e., V G + V E), one of the main factors affecting the value of H 2 is the extent to which variation in the trait is determined by nonheritable (i.e., “environmental”) factors. In cases where environment is having a relatively large effect, the computed value of H 2 will decrease. If, hypothetically, H 2 = 0, then there is no heritable source of variation in that trait, and so evolutionary forces (e.g., selection or drift) cannot influence changes in the mean value and/or variance. In cases where H 2 has a value approaching 1, selection or drift has the greatest potential to produce change in the mean and/or variance values of that trait in subsequent generations. Understanding this ratio is, therefore, fundamental to understanding the relationship between how things look in one generation, compared to how they might look in the next generation once drift and/or selection has done its work. Mathematically, this is expressed by what is referred to as “breeder’s equation” (see, e.g., Conner and Hartl 2004).

Breeder’s equation calculates what is termed the “response to selection” (R), using the “selection differential” (S), which is simply the difference between the mean value of the entire population and the mean of those individuals who are not subject to negative selection. It is then computed as:

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