Evolution Requires Reduction in Genetic Diversity

The key to pre-adaptation is that it occurs before the ecological change that makes it beneficial -- black mice, black pepper moths, etc -- so that selection just shifts the frequencies of alleles in the populations towards the beneficial traits and away from the least beneficial traits.Post-adaptation would be the additional changes that continue the adaptation process beyond what was available in the initial population.Do the largest beaks in the population survive and breed more than other sizes, and then does the following generations have even larger beaks that improve survival and breeding -- does the trait distribution move outside the original distribution.Again we can look at Pelycodus:

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Evolution of Pelycodus
an early Eocene Primate from Big Horn Basin, Wyoming

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The chart documents the evolution of the Eocene lemur-like primate Pelycodus into Notharctus. The horizontal scale is an index of molar tooth size. Each horizontal bar gives the mean (vertical bar), two standard deviations (thick horizontal bar) and range (thin horizontal bar) for the indicated number of skulls from a series of fossil deposits. The index increase from 1.0 to 1.4 is an approximate doubling in size. The inset photograph shows a reconstruction of Notharctus venticolis, the species in the upper right of the series. [diagram after Gingerich]

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Each layer shows the population distribution of sizes and the trend is for larger and larger individuals until with P.Jarrovii ALL the individuals are larger than the original P.ralstoni individuals ... this adaptation is the rule imho, although you could argue that each layer had members that were pre-adapted to be selected for their larger size, that is really just the natural selection of the available variations, and it is the long term trend that is the overall evolution.In my opinion.Enjoy.