Comment by textor

Another factor is initial variance. Russian foxes experiment uses some hundreds of animals, perhaps. Suppose we operate on the scale of millions of specimens, a significant portion of the entire species. There could be a many-sigma difference between the average and the smartest living crow in the world (not sure how to estimate this), but the former one would probably struggle to survive in the city while the latter would thrive. Urbanization, then, can be thought of as a bottleneck that promptly eradicates the left part of the extant distribution, changing the population average even in a single generation.