Not all beneficial alleles come with deleterious side effects, in case I gave that impression. Of course, not all beneficial alleles come from mutations either. Hybridization between closely related species can lead to advantageous alleles introgressing into a population from another species. Loren Rieseberg, who has been studying hybridization between sunflower species for some time, has an article in the upcoming issue of the American Naturalist (press release here) on the introgression of an advantageous allele from one species to another. This is probably quite common and may have influenced the evolution of the human brain.
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ppl should get familiar with rieseberg's work, it might come in hand in other areas that are less obscure than sunflowers :) (also, people should check out some of h. allen orr's work on mutations of large effect spreading through populations, it seems complementary to rieseberg's stuff)
Would this be an example of hybrid vigor - heterosis?
Amit
Hybrid vigor refers to the increased fitness of F1 hybrids. This example is looking at long term hybridization between two populations (in this case, separate species). You can think of it as migrants bringing beneficial alleles into a population. I don't believe the authors are arguing that the alleles increase in fitness because the F1 hybrids are more fit.