Final answer:
The concept described is frequency-dependent selection, specifically negative frequency-dependent selection when referring to rare alleles increasing in reproductive success. The Hardy-Weinberg equation is used to predict expected allele frequencies in a non-evolving population, while deviations from these frequencies suggest evolutionary forces, such as selection pressure, are at play.
Step-by-step explanation:
The process described in the question, where the reproductive success of purple flowers decreased and the reproductive success of yellow-flowered individuals increased when the yellow allele started to become rare, is known as frequency-dependent selection. This evolutionary mechanism favors phenotypes that are either common or rare, depending on the context. In this particular case, it is likely negative frequency-dependent selection because the rarer yellow allele increases in reproductive value as it becomes less common.
Regarding the study of a population of flowers and the selection pressure on the color of the flower, where blue flower color is dominant to red and there are 600 blue and 200 red flowers, the Hardy-Weinberg equation would be the tool used to calculate expected allele frequencies for flower color based on the phenotype frequencies observed in the field. However, if bees prefer red flowers and thus potentially increase their reproductive success, selective pressures might be acting on the population, which could lead to deviations from Hardy-Weinberg equilibrium over time.