You are factuall incorrect. Natural selection does not propose 'small changes in a particular direction". Environmental stresses creates evolutionary winners and losers. The winners do not "adapt" to a change in environment, they merely survive. The genetic advantage they have for survival existed as a result of variations in the gene pool due to genetic drift. The genetic differences existed before the particular selection event took place. It only became an "advantage", after the event. If a different selection event had occured, the same genetic difference may have been fatal.
Well, I differ and here is my evidence.
"The Origin of Species" by Charles Darwin Chapter 14 - Recapitulation and Conclusion
As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modification;
Successive, favourable --- that is direction.