The evolution of species almost always occurs through natural selection. Natural selection is a process whereby some individuals with certain traits have a higher reproduction rate than individuals with other traits leading to a change of relative trait frequency within a population. Differences in reproduction rates often occur as a result of interactions between individuals and their environment. In some settings humans can act as a selecting force encouraging desirable traits to be fixated within a crop population while limiting undesirable traits.
Artificial Selection of Recessive Phenotypes
The common field pea, Pisum sativum, has been an important crop in human civilizations since 10,000 BCE. Very early on in their domestication, farmers selected sweet wrinkled peas over round starchy peas for planting in their fields. Because wrinkled peas are recessive a farmer could conceivably plant and harvest an entire field of sweet peas in just one generation. Wrinkled pea seeds contain only the r allele; when a field of wrinkled peas are planted, 100% of the F1 generation will be wrinkled because no R alleles are present in the P1 generation. Conversely, if a farmer were trying to select for starchy peas it would be considerably harder. Since the round seed phenotype results from both the RR and the Rr genotypes, when a field of round peas is planted, both the R and r alleles are present (75% R and 25% r). The resulting F1 generation will be a mixture of RR, Rr and rr genotypes (ratio of 9:6:1). Through continual selective breeding over many generations it would be possible to fix the dominant trait within a population, but since the dominant allele masks the recessive allele fixation of the former takes longer.
Darwin used the example of artificial crop selection as an example to support his theory of natural selection.