Pleiotropy is the effect of one gene affecting multiple traits, as when Drosophila genes are expressed in more than one place during embryogenesis. For a bunch of examples of that, see
Repression and loss of gene expression outpaces activation and gain in recently duplicated fly genes, Oakley, Østman, and Wilson, 2006, PNAS, 103, 11637.
In my own work on computer simulations of epistatic interactions, it is clear that pleiotropy has the effect of changing the phenotype more per mutation than withoput pleiotropy. With pleiotropy, more than one trait gets affected by each mutation. And each trait can potentially be affected by the same amount (i.e. numerical value), so if a mutation affects five instead of one trait, it can change the fitness five times as much. Admittedly, this is a result from simulating a very simple model, but that it has biological relevance is suggested (among other considerations) in a recent Nature paper by Günter Wagner et al. where they measure pleiotropy in mice. They show that a substitution at a QTL has an effect on each trait that increases with the total number of traits affected. While mutations in many genes produces only a small effect on a few traits, those that affect many traits does so with higher effect.
Pleiotropic scaling of gene effects and the ‘cost of complexity’, Wagner, Kenney-Hunt, Pavlicev, Peck, Waxman, Cheverud, 2008, Nature, 452, 27.
The significance of this observation is that once the environment changes, and the population is forced to adapt, those organisms that exhibit much pleiotropy in the genomes can adapt really fast. And that organisms can adapt way faster than we normally imagine became increasingly clear when a paper came out this year about a lizard, Podarcis sicula. This lizard evolved differences in head morphology, bite strength, and digestive tract structure in a very, very short period of time. How short? Ten thousand years? That would be short by evolutionary standards. But no, it took just 36 years! In about 30 generations the lizards evolved larger heads, stronger bites, and cecal valves - a structure in the gut that can constrict, slowing down the passage of food, giving more time for digestion. Behaviorally the lizards changed their diet to include much more plant material, and the morphological changes were adaptations to this new lifestyle. Clearly the lizards became a new species, as they moved into a new niche. You can read more about that all over the web - it deservedly got a ton of coverage - but I recommend Science Daily for this one.
Herrel, Huyghe, Vanhooydonck, Backeljau, Breugelmans, Grbac, Van Damme, and Irschick (2008). Rapid large-scale evolutionary divergence in morphology and performance associated with exploitation of a different dietary resource PNAS, 105 (12)
31 minutes ago in Catalogue of Organisms