Another interesting finding was that when they deleted the Prx1 enhancer in the mouse, they didn't find any immediate effects on forelimb length, "revealing regulatory redundancy", according to the authors.
Via this bit in Science.
Regulatory divergence modifies limb length between mammals
Chris J. Cretekos, Ying Wang, Eric D. Green, James F. Martin, John J. Rasweiler IV, and Richard R. Behringer
Genes and Development 22:141-151, 2008
Abstract: Natural selection acts on variation within populations, resulting in modified organ morphology, physiology, and ultimately the formation of new species. Although variation in orthologous proteins can contribute to these modifications, differences in DNA sequences regulating gene expression may be a primary source of variation. We replaced a limb-specific transcriptional enhancer of the mouse Prx1 locus with the orthologous sequence from a bat. Prx1 expression directed by the bat enhancer results in elevated transcript levels in developing forelimb bones and forelimbs that are significantly longer than controls because of endochondral bone formation alterations. Surprisingly, deletion of the mouse Prx1 limb enhancer results in normal forelimb length and Prx1 expression, revealing regulatory redundancy. These findings suggest that mutations accumulating in pre-existing noncoding regulatory sequences within a population are a source of variation for the evolution of morphological differences between species and that cis-regulatory redundancy may facilitate accumulation of such mutations.