Selection pressure is an abstract force that shapes organisms as they evolve due to mutation, natural selection, and genetic drift. For instance, a selection pressure on the ancestors of giraffes for longer necks led to modern giraffes. Selection pressures on small mammals living in open plains led to muscular legs and adeptness at running. Selection pressure operates over durations as short as a few generations, and as long as millions of generations.

A selection pressure can derive from practically anything, as long as it acts in a relatively consistent fashion over long timeframes, and actually impacts the reproductive or survival rates of a species. Potential sources of selection pressure may include availability of prey, presence of predators, environmental stresses, competition with other species (including humans), and intra-species competition. In the eyes of evolution, differential reproductive capacity is all that matters -- if a certain predator only consumes old animals that are already incapable of reproducing, the predator will have no impact on the evolution of the prey species whatsoever.

It is important to know that selection pressure has no intelligence, foresight, rhyme, or reason. The locus of selection is the individual, not the species. Thus, new adaptations do not appear "for the good of the species" -- new adaptations only become fixed in a population if they are good for each individual that has it, even if it collectively makes life worse for the species. New adaptations can be partially self-destructive, as long as their net effect promotes the inclusive fitness of the organism. Accordingly, animals like the Komodo Dragon bite down into their own gums with their sharp teeth when they feed, increasing their likelihood of lethal infection, but providing an advantage because their contaminated, blood-filled mouths poison prey.

Selection pressure can operate more quickly than one might think, and this is especially true under conditions of selective breeding, when the selection pressure is intelligently applied by humans. One of the most striking examples are seen in a series of experiments in the Soviet Union by scientist Dmitri Belyaev. The object of the experiments was to domesticate the silver morph of the Red Fox, which was achieved in just 10 generations of selective breeding. These foxes lost their distinct musky smell, wagged their tails like domestic dogs, and showed no fear of humans, even licking their hands to show affection. Less frequently mentioned is that these experiments also produced a group of highly aggressive foxes which would leap at their cage walls ferociously when humans walked by. In short, selection pressure can operate in significant ways over short time periods.