Thanks to rod-the-farmer for the link.
http://www.sciencedaily.com/releases/2009/11/091102171726.htm

ScienceDaily (Nov. 3, 2009) — Researchers at the University of Pennsylvania have developed a theoretical model that informs the understanding of evolution and determines how quickly an organism will evolve using a catalogue of "evolutionary speed limits." The model provides quantitative predictions for the speed of evolution on various "fitness landscapes," the dynamic and varied conditions under which bacteria, viruses and even humans adapt.

A major conclusion of the work is that for some organisms, possibly including humans, continued evolution will not translate into ever-increasing fitness. Moreover, a population may accrue mutations at a constant rate -- a pattern long considered the hallmark of "neutral" or non-Darwinian evolution -- even when the mutations experience Darwinian selection.

While much is known about the qualitative aspects of evolutionary theory -- that organisms mutate and these mutations are selected by the environment and are gradually absorbed by the entire population, very little is known about how, or how quickly, this is accomplished. Information on evolution between consecutive generations is hard to come by, and the lack of understanding has real-world implications. Public-health officials would have an easier time preparing targeted vaccinations, or combating drug resistance, if they understood the evolutionary speed limits on viruses and bacteria such as influenza and M. tuberculosis.
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http://www.sciencedaily.com/releases/2009/11/091102171726.htm