Biology view on trait variability

Question

In reading the annotated Origin... I have come to the following note by Costa on p. 168:

Again, modern biologists would disagree with Darwin's idea that especially well-developed traits vary to a greater degree than expected simply because they have been varying and undergoing selection in the past (as evidence by their considerable development)...

I am not sure I understand what the problem with Darwin's view is. From reading his work I understood that he, Darwin, is simply saying that traits that have been more recently acted on by natural selection would vary more than those that have been acted in the past and over a longer period of time (given that conditions remain relatively the same - I assume).

What is the modern view of biologists on this?

P.S. The excerpt to which this side note refers, follows:

In these remarks we have referred to special parts or organs being still variable, because they have recently varied and thus come to differ; but we have also seen in the second Chapter that the same principle applies to the whole individual; for in a district where many species of any genus are found - that is, where there has been much former variation and differentiation, or where the manufactory of new specific forms has been actively at work - there, on an average, we now find most varieties or incipient species.

Answer 1

The variance of a trait's allele frequency can follow different trends over time depending on several factors such as whether one or several of the alleles are consistently selected for or the population dynamics. For example Disruptive Selection will tend to increase the allele variance. On the other hand Directional selection, or Stabilizing selection will tend to decrease variance by driving the allele frequency to one side of the distribution (in the first case) or by "cutting down" its sides in the second case. It is true that stabilizing and directional selection processes are more common especially under relatively stable environments. Therefore for the majority of cases, during the first phase of selection for a trait variance will most commonly decrease . This, however, is not a rule. There are additional, rare, but from an evolutionary point of view extremely important cases where allele frequency variance can see a sudden "ballooning". Consider for example the case of selection processes after mass extinction events. Then, because of the sudden availability of previously occupied niches, genetic drift due to small population sizes as well as changed conditions the previously stabilized traits will start...well...drifting. The same is true for almost any circumstances that will destabilize equilibrium and lead to speciation.