Environmental factors model organisms' phenotypes. Natural selection benefits certain traits over others to increase organisms' fitness and adaptation. Oceanographic barriers and streams model the dolphins' skulls.
Natural selection selects beneficial alleles and increases their frequency in the population.
Natural selection results in adaptation, which means the increase of the aptitude phenotype. Aptitude is the contribution of each genotype to the next generation.
In many cases adaptations, resulting from natural selection can be correlated to environmental factors or selective pressures applied by other organisms or habitats.
Let us remember that a mutation is a change or alteration in DNI sequences that introduce new variants.
Many of these are eliminated, but some of them might succeed and be incorporated into each individual. These mutations are the ones that have been selected by natural selection.
So, when thinking about examples of environmental factors that could influence natural selection and decrease genetic diversity, we might mention oceanographic barriers and streams.
Let us analyze an example.
Risso's dolphin (Grampus griseus), is a cosmopolitan species that inhabit template waters all around the globe. The species feeds mainly on squids. Squids distribution depends on streams and is limited by oceanographic barriers. Hence, the distribution of Risso´s dolphins is also associated with oceanographic barriers. Even when they have the physical capability of displacing big distances, they usually inhabit the same areas or regions following their principal prey. So around the world, there are many different populations of the same species that may or may not meet.Streams among different regions vary in temperature, direction, and strength. This environmental condition seems to model Risso´s dolphin's morphology. Differences in the dolphin's skulls might be associated with their habitat. According to the geographical regions, some populations exhibit longer and wider skulls, while other populations exhibit taller and shorter skulls. These measures are related to the development of different bones composing the skull and their associated musculature. According to the type of streams the populations are exposed to, they develop characteristics that make them more hydrodynamic and provide support to the skull, especially while deep diving and foraging.  On each population, natural selection benefits a trait over the other one, increasing the fitness of the animals in the region. Dolphins get to adapt to their enviroment.Intrapopulation Genetic variability decreases because individuals of the same population need to adapt in the same way to the same environmental condition. Their skulls are either long and wide or short and tall. Interpopulation Genetic variability increases. The more varying are streams in different regions, the more different will be the individuals' skulls.
Summing up,
•The selective pressure or modeling environmental factor is the type of streams and oceanographic barrier.
•The dolphin's response to streams is the differential development of the skull in different regions
•Natural selection benefits long and wide skulls in some areas, and tall and short skulls in other areas.
•Genetic diversity depends on the study scale. At a global scale and among populations, genetic diversity increases. On a local scale and studying one population, genetic diversity decreases.
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