In “On Retrospective Analysis and Coalescent Theory” (Biologic Institute Perspectives August 5, 2012), Ann Gauger notes,
The theory is that in small populations (smaller than a trillion, say) drift can overwhelm the power of selection. In such a case, organisms do not have sufficient numbers for beneficial mutations to arise and be fixed with any frequency. Most mutations are lost to drift before becoming established, even when they are beneficial. The significance of natural selection is thus greatly reduced in shaping evolutionary history.
The idea that evolution is driven by drift has led to a way of retrospectively estimating past genetic lineages. Called coalescent theory, it is based on one very simple assumption — that the vast majority of mutations are neutral and have no effect on an organism’s survival. (For a review go here.) According to this theory, actual genetic history is presumed not to matter. Our genomes are full of randomly accumulating neutral changes. When generating a genealogy for those changes, their order of appearance doesn’t matter. Trees can be drawn and mutations assigned to them without regard to an evolutionary sequence of genotypes, since genotypes don’t matter.