Variability is ubiquitous, but explanations difficult

The fantastic development of genetic techniques has disclosed a previously unimagined amount of variability in humans, as well as in all the organic world generally. Scientific attempts to understand this variation began with the seminal work of Darwin (1859), but the harmonious fusion of genetic and evolutionary concepts occurred in the first half of the twentieth century only, with the empirical contributions of Dobzhansky (1937), Mayr (1942), Simpson (1944, 1949) and Stebbins (1950), while the mathematical foundations of what had been called the synthetic theory were established by Fisher (1930), Wright (1930, 1931), and Haldane (1932). According to these scholars, whom Mayr (1980) called ‘bridge builders’ due to their ability to cross disciplinary boundaries, the basic facts of evolution are now known. Mutations (including any type of change in the genetic material) provide the variability that will be tested by natural selection. The latter establishes which part of this variation should be maintained or favored, and which part eliminated.

Besides these primary factors, several others should be considered, the most important being related to the constraints developed by population structure (size, subdivisions, mobility). In humans cultural influences cannot be overemphasized, since key technologic improvements may be of paramount importance for the fate of a given community or society.

Having said that, it is important to emphasize that in humans the specific roles of each of these factors can seldom be established in specific situations.