A closer look at noise.
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at all. If heritability is “0” for a trait - that trait is not inherited at all, it is entirely environmental in nature (i.e. a tattoo). A simple trait such as height is usually quite highly heritable (h2 = 0.3). This specifically means that for such a trait, 30% of the phenotypic variation is heritable (selectable) variation. Unfortunately, for very complex traits such as “fitness”, heritability values are low (i.e. .004) – often approaching zero (Kimura, 1983, pp.30-31). This is because total fitness combines all the different types of noise from all the different aspects of the individual. When Kimura says that fitness heritability is generally very low (i.e. .004), he means that almost all variation for individual fitness in due to non-heritable (non-selectable) effects. This means that almost all selection for fitness will be ineffective and wasted. Low heritability means that selecting away bad phenotypes does very little to actually eliminate bad genotypes. To illustrate this, consider seeds from a cottonwood tree. Some seeds will land on fertile ground ideal for growth, with abundant moisture, few weeds, etc. But most seeds will land on places that are too dry, too wet, too many weeds, too much shade, too many people, etc. The result will be great diversity in the health and vigor of the resulting trees, and huge differences in their survival and reproduction. But almost all of this “natural selection for the fittest”, will really only be selection for the luckiest - not the genetically superior. Hence in most natural situations, most phenotypic variation in fitness will only be due to non-genetic noise, and will have very little to do with heritable differences. This is what we mean by low heritability. Low heritability largely neutralizes the effectiveness of selection. Like an automobile with a racing motor but a broken transmission, there can be lots of selection happening, yet almost no genetic progress.