I think it's more like when the environment is perfectly stable for a very long time, so that even alleles which are only microscopically less fit get filtered out. If the environment keeps varying a little, there can be competing alleles, one of them more fit during one decade, another during another decade. But there could be an equilibrium such that if one of the alleles gets less frequent it works as an advantage, and then the equilibrium would remain even after long time. This could happen e.g. with sexual selection -- when some visible trait becomes sufficiently rare, it may become a status symbol to get a mate with that rare trait.

Large connected landmasses have a lot more evolutionary pressure than Australia or small islands. This is demonstrated by the fact that they are inhabited by less advanced animals, which in turn is demonstrated by the fact that they are very vulnerable to invasive species from the large continents, while animals from the small landmasses mostly can't survive on the large continents. This is expected, because larger areas means there is room for more species which can compete with each other, and evolution generally faster in larger populations because it increases the probability that at least one organism has a beneficial mutation. So according to your theory, we would expect species from large continents to be a lot more vulnerable to environmental changes than island species. This might be true, but it seems highly unlikely to me.