In 2002, Peter and B. Rosemary Grant studied a population of Darwin's finches on one of the Galapagos Islands that feeds on seeds of various sizes. After a drought that caused only large seeds to be available to the birds, they found that natural selection favored those birds that had larger beaks and bodies. Once the rains returned and smaller seeds became much more abundant, however, natural selection favored those birds that had smaller beaks and bodies. Which process is the best interpretation of this scenario?
a) genetic driftb) founder effectc) microevolutiond) macroevolutione) bottleneck effect

Microevolution is defined as the changes in allele expression and frequency within a specific population and which take place within a relatively short period of time regarding evolution. There are different reasons why microevolution might take place, instead of genetic evolution at a much larger scale, and throughout a much larger period of time, which would be typical of macroevolution. These reasons are: mutations, natural and artificial selection, gene flow and genetic drift. In the case of the bird found in Galapagos by Peter and B. Rosemary Grant in 2002, the differences in the finches arose due to natural selection when confronted with a change in their climatic conditions, and therefore, the availability of food. These changes in the environment drove the birds with larger beaks to be first privileged in food acquisition and also in reproduction, making their genetic make-up more prevalent, while those with smaller beaks, due to shortage of food availability, became incapable of passing on their genes to future generations. Natural selection played its role and one set of genes expressed themselves more frequently due to the circumstances and the reproductive process. But when things changed, the situation reversed. This is all characteristic, given the small changes and the reversion, of microevolution, or evolution at a small scale.