A new research paper appears to demonstrate that adaptative evolution happens along the lines of neutral stochastic evolution and is not accelerated.
Kryazhimskiy S, Tkacik G, Plotkin JB, The dynamics of adaptation on correlated fitness landscapes. PNAS 2009 (right now the PDF is freely available)
From the Press release:
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.
According to the study, a population’s fitness and substitution trajectories —t he mutations acquired to achieve higher fitness — depend not on the full distribution of fitness effects of available mutations but rather on the expected fixation probability and the expected fitness increment of mutations. (...)
Researchers demonstrated that linear substitution trajectories that signify a constant rate of accruing mutations, long considered the hallmark of neutral evolution, can arise even when mutations are strongly beneficial.
If confirmed, this research should lay to rest the hypothesis that dismiss the informative value of genetic phylogenies based on presumpt adaptative selection: adaptative or not, mutations accumulate on similar rates, depending mostly on other factors, like population size (greater or weaker drift, possibly leading to fixation).