In a 1937 paper , J. B. S. Haldane covered some aspects of the evolutionary process in a particularly clear way. Not everything in the following paragraphs is correct or uncontroversial today, but Haldane gives a clear exposition of the power of small differences on an geological timescale:
On the other hand, the evolutionary process is exceedingly slow. Forms usually change little in 100,000 years. Now Haldane (1924) showed that a dominant character causing an increase of 0.1 per cent. in the fitness of its carriers would increase from a frequency of .001 per cent. to one of 99 per cent. in a random mating population in 23,400 generations, and somewhat more rapidly in an inbred population; in fact, on a geological time scale, almost explosively. But a difference in fitness of this magnitude could not be detected. In order that an observed viability difference of 0.1 per cent. should exceed twice its standard error, we should have to observe at least sixteen million individuals. To detect so small a difference in fertility we should have to count their progeny.
It may be possible to observe evolution by natural selection in a species which is adapting itself to a new environment. In other cases we can very rarely hope to notice evolutionary changes within a human lifetime. From the standpoint of an individual human observer species may be regarded as almost in equilibrium. Our only reason to hope for observable evolution is that owing to glaciation, agriculture, fishing and industry, the balance of nature has recently been upset in a manner probably without precedent in our planet's history; and hence on the Darwinian theory we should expect that evolution was proceeding with extreme and abnormal speed.
Every so often as I'm reading classic pieces in population genetics, I look for statements that anticipate our recent observation that human evolution has accelerated during the last few thousand years. Haldane's comments on rapidly changing environments are among the first I've noticed along these lines, of course they pertain not to humans but to other possible examples of evolution in the natural environment.