Answer and Explanation:
Natural selection is the result of the phenotype-environment interaction which determines gene destiny in space and time, selecting beneficial alleles and increasing their frequency in the population. It is a consequence of the individual differential capability to reproduce and leave offspring. There must be also a genetic base called heritability. Natural selection involves interactions between individuals, the physical environment, and the biological environment.
Natural selection results in adaptation, an increase of the aptitude phenotype. In many cases, adaptations, resulting from natural selection, can be correlated to environmental factors or selective pressures applied by other organisms.
The selective agent is the environmental characteristic that determines the differential survival of the phenotypic classes.
Some intraspecific phenotype variations follow a geographical distribution and can be correlated to changes in environmental conditions, such as temperature or humidity.
Adaptive evolution refers to changes in the organism that lead to the evolution of the species and that are adaptive to certain conditions of the environment. Birds adapt to diverse ecological niches showing specialized beak forms to improve functions such as feeding, mating, defending, competing, and etcetera. These factors play an important role in beak shaping. Natural selection produces adaptive evolution.
During 1977 Daphne Major island went through an important drought, and many plant species produced few or no seeds during and after the event. The medium ground finch population, which is seed-feeder species, was hardly affected by this phenomenon. As seeds´ production dropped and the sizes of the extisting seeds were considerably smaller, the finch population declined drastically from about 1400 individuals to a few hundred in a period of two years, approximately. The fact is that the island weather controls plant species development, and thus, seeds availability as food for the birds. Years later after the drought, the finches population managed to recover but showing differences in the size of their beaks. Before the drought finches had a stubby beak and many years after recovering the average size of the beaks was larger than the beak size of the original population. This difference in phenotype was due to the fact that during the drought, small seeds were very scarce, but large seeds with thick husks were still available. Small birds with small beaks starved because they were not able to open the husk and eat the interior of the seed. But those birds with bigger and stronger beaks were able to do so, and hence, managed to survive. The bird population adapted to the new environmental conditions and resource availability. Natural selection acted on the beak size, changing it.
There are different kinds of natural selection, but in the present example, directional selection is operating.
<u>Directional selection</u> increases the proportion of individuals with an extreme phenotypic trait, in this case, large beaks. This selection presents more frequently in those cases in which interactions between living organisms and the environment modify in the same direction.
Furthermore, beak size is not only related to feeding strategies, but also to reproduction. Female finches only mate with males that have the same beak size.