One of paramount interests of quantitative geneticists is the knowledge about how much visible (phenotypic) variability is due to genetic variability. To describe this we usually compute narrow sense heritability h2 = Var(a) / Var(y), where y is phenotypic and a is additive genetic value (see book by Falconer and MacKay). A usual result from analyses is that fitness related traits have much lower heritability (say 0.1 or similar) than morphological traits (say 0.5 or even more). This eventually leads to the conclusion that selection will not be effective for fitness related traits, since there is not much additive genetic variability. However, this can only be partialy true. The heritability can have a small value due to small additive genetic variance and/or large phenotypic variance. It is clear that if there is a lot of environmental variability that the selection can not be the prime force to change the population. However, this still does not mean that we should neglect the genetic variability. David Houle has dealt with this issue and published a paper in Genetics (1992). I have read that paper several times and I always forget the exact conclusion. Well, there is no exact conclusion. However, Houle suggests that heritability is not a good measure of evolvability (ability of population to respond to selection) or variability (strength of forces that maintain and deplete genetic variation). He suggests that standardizing the additive genetic variance with a mean is better, which in turn leads to the suggestion that additive genetic coefficient of variation (CVa = Var(a) / Mean(y)) is a more informative measure than heritability. There is also some more recent work by Houle (his pub. webpage, this looks very interesting), that I need to digest - sometime.
An example: we have two populations A and B (this could also be two different traits!), with means 5 and 10, additive genetic variance 5 and 5, heritabilities 0.2 and 0.2. Based on heritabilities we could conclude that selection will have the same effect in both populations, but this is not true in relative meaning. In population A the additive genetic variance is euqal to the mean, whereas it is only half of the mean in population B. The additive genetic coefficient of variations would be 0.45 and 0.22 for population A and B, respectively. This means that we can achive relatively greater response to selection in population A than in population B.