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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.

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Could you please clarify? Do you mean well-developed (whatever that means really) or recent? The two are not synonymous. Could you maybe include Darwin's original sentence that we modern types are supposed to disagree with? –  terdon Oct 4 '12 at 12:21
    
I cannot really clarify it as I don't really understand what the author means. I can, and will, include the original passage to which this side-note refers. –  drozzy Oct 7 '12 at 3:56
    
In fact, I can't find what Costa is saying in Darwin's words, as it seems Darwin is saying complete opposite. –  drozzy Oct 7 '12 at 4:10

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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.

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But that has little to do with the actual theory, which is trying to explain the "general principles" of things. The events such as catastrophe will in a sense "revert" things back to an earlier time, after which the quickening of variability will commence. BUT it will still tend to slow down, eventually, and vary to a lesser degree, again satisfying the theory. N.B.: I haven't read about the Disruptive Selection or Directional Selection yet, so I am only talking about your latest point here. Thank you and excuse me! –  drozzy Oct 7 '12 at 3:44
    
Now that I've read about Disruptive Selection, I don't see how it impacts the argument. For once the selection is stabilized, it will tend to vary less. E.g. in the example of white/black rabbits - once the black color is chosen, it will vary very little compared to when the rabbits were initially specialized from, say, previous-rabbit-species, which were, for example of green color. Nor is there anything in Drawin's theory to suggest that simultaneous "genes" cannot exists in species at the same time, if selection deems them useless. –  drozzy Oct 7 '12 at 3:50
    
Directional selection sounds just like a regular natural selection to me. Perhaps I am missing some fine detail? Excuse my ignorance - I am just starting this subject! –  drozzy Oct 7 '12 at 3:55

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