Selective breeding began in Mexico, around 10,000 years ago. Selective breeding is the process of mating of two animals or plants in an attempt to produce offspring with desirable phenotypes or for the elimination of certain traits.
By selective breeding the maize was domesticated and easier for farmers to grow. However, selective breeding, at least intentionally was not a common practice at this time. The farmers would choose the best kernels (displaying the best phenotype) and replant those in hopes they will grow as nice, or better than the plant they took them from. Assuming the genotype would also be desirable. Thus, using artificial selection. Many things could make the corn more acceptable to the farmer, such as the size of the kernels, the colour to look more appealing, however, they were not able to be bread to be resistant to pests.
This happened once the biologists became interested in genetic manipulation, and the alleles on the chromosomes that could be used to track genes.. The farmers did this with no awareness of the impact they could have on the species, once biologists became more interested in artificial breeding and expanded it to genetic manipulation it was no longer a natural process and it would begin to have wider implications.