In 1888 Gulick published his results in a paper entitled “Divergent Evolution Through Cumulative Segregation”, and a year later Alfred Wallace himself recognized that Gulik’s mechanism was indeed different from natural selection. Wright gave to genetic drift the name of “Gulik’s effect”, but since it was he who proved its general nature and its mathematical basis, it has become known as “Sewall Wright’s effect”. To our purposes, however, what matters is that genetic drift is not only predicted by theory but does actually exist in life, and truly represents a second mechanism of evolution.

The Modern Synthesis

The rediscovery of Mendel’s laws, the demonstration of crossing over, the chromosome theory of heredity, the link between sex and XX or XY chromosomes, the discovery of the first Mendelian disorders in man (alcaptonuria and brachydactily), were all obtained in the first ten years of the 20th century. In that brief period of time, light was thrown on the millennial mystery of heredity, and genetics became a science.
A change in a gene was called “mutation”, a sober word for “sudden creation of hereditary novelty”, and the first geneticists (Hugo de Vries, Carl Correns, William Bateson and Wilhelm Johannsen) regarded mutations as the only real moving power of evolution. They almost instinctively rejected natural selection, but this is understandable because they were studying the genetics of individual organisms, and in this field mutation is everything.
Later on, however, as attention shifted from individuals to populations and geneticists learned the lesson of the Hardy-Weinberg law, it became clear that mutations alone are not enough. People realized that an evolutionary mechanism must necessarily be a two-step process: a first stage where mutations occur, and a second one where mutations spread in a population and their destiny is actually decided (many are lost and only a few become “fixed”). The deciding force, as we have seen, can be either Darwin’s natural selection or Wright’s genetic drift, and this of course raises the problem of understanding which of the two is more important in nature.