Effect of Small Populations
nBut for transitions across the upper levels of the biological classification system, many mutations would be needed, probably 100s or 1000s.
nThe chance of getting 5 (or 10) mutations of the right sort in a population varies with the 5th (or 10th) power of the population size, so large populations are heavily favored.
nThis more than cancels out the benefit of a small population.