A pressure on diversity that preserves favorable variations and rejects injurious variations within a species. This works through the passing of inheritable traits through generations, with the individuals exhibiting favorable variations maintaining a high level of fitness while individuals exhibiting injurious traits leave fewer offspring. These traits are eventually dropped from the population.
The Power of Man's selection:
The selection for extremes in pigeons by man due to man's tendency to find extremes more interesting, shows how over a short period of time artificial selection can shape the anatomy and behavior of a species. However, Darwin argues that because man selects for arbitrary traits due to personal preference, he could never actually help to create more fit animals like the geological time scale of natural selection.
The influence of the opposite sex upon preferred traits allows for a higher fitness of animals with generally desirable traits. It does not kill the disadvantageous traits, but rather just prevents them from reproducing.
Competition of individuals that have either been isolated or have physical barriers which occupy the same niche, for the same resources will increase the effect of injurious and advantageous traits allowing one type to succeed while the other will die off. This is how extinction occurs in populations.
Divergence of Character:
In an effort to prevent intense competition for limited resources animals adapt to use different habitat and food. These variations continue as long there are alternative resources and the environment remains stable. If no new competitive species are introduced many variations may occur.
H1N1 developed a drug-resistant strain of the virus after less than two weeks within two hosts with compromised immunity. How did these little buggers do it? NATURAL SELECTION. Virus reproduction rates are ridiculous so you can witness the miracle of natural selection in real time. At first the drugs began to kill the virus but then "laboratory tests of virus strains isolated from patients showed that some strains contained a genetic mutation (the H275Y mutation) that [made] the virus less susceptible to some neuraminidase inhibitors." These viruses survived to reproduce while the rest were killed off by the enzyme inhibitors. Now there is a more adapted strain if H1N1 virus and I'm sure it mutated again and again producing strains with no selective advantage that were also killed off.