The evolutionary mechanisms proposed by Darwin contained a gap: the explanation for the origin of diversity; this being a reason for some people to question his work. Although both Mendel and Darwin lived at the same time, he had no contact with the ideas of the precursor of Genetics. In fact, some sources claim that Darwin had some of Mendel’s publications in his library without, however, having read them.
Mendel, unlike Darwin, was not someone of recognized prestige. In this way, his work remained almost anonymous for a long time, until it was rediscovered in the early 19th century.
In the mid-1930s, some scientists began to recognize the link between natural selection and genetics; with Dobhansky (geneticist), Mayr (zoologist), Simpson (paleontologist) and Stebbins (botanist) as the main authors of these ideas. Thus, the Synthetic Theory of Evolution, also known as Neo-Darwinism, or Neo-Darwinist Theory was formulated, relating these two fronts of Biology.
This theory considers that mutations, gene combinations, and natural selection are the main factors that culminate in evolution; being the gene combinations consequences of the independent segregation of the chromosomes, and permutations that occur during meiosis. Both it and the mutations are related to the genetic variability of the population.
The latter occur at random, and, by natural selection, they can be maintained, as adaptive traits; or cause the demise of certain individuals. Thus, when positive (first case), this type of selection will allow some representatives of a population to have more chances of survival, reproducing and giving rise to individuals that are also better adapted, if this factor is hereditary.
Natural selection, in this way, eliminates individuals who have aspects that are disadvantageous for a given situation, which are, therefore, less adapted to it. Thus, considering that environments are not stable and constant systems, different selective pressures can occur within a population, avoiding the elimination of certain alleles that would not be maintained if the environment were homogeneous.