Evolution of species has a lot to do with natural selection. Organisms with different phenotypes have different chances for survival in a particular environment. Some species are better suited to survive in hot conditions, other species are better suited to the cold environment. Naturally, they cannot exist side by side, because the natural selection has smartly chosen which genes are preferable on a particular territory.
Selective pressure is an inevitable part of natural selection. To be precise, there are dozens of pressures that make species die out, mutate, develop new traits essential to survive. Native African people, for example, never have a fair skin, and desert plants are always meant to store a lot of water in their cells. Obviously, only dark-skinned people are well-equipped to survive in the equatorial area. In course of evolution, water-rich succulents have outlived many plants that were not meant to survive with little water under the blazing sun.
Interestingly, the immune system is a strong selective pressure to pathogenic organisms. Some of them have phenotypes that make them highly resistant to the host’s immune reaction, which makes bacteria and viruses mutate. This way some pathogens can disguise themselves within the host, which makes them difficult to find or fight.
Climate, food, predators, diseases, and human influence are typical selective pressures. They urge organisms to move, transform, or die out – the fierce law of wild nature. When humans are better prepared to resist absolutely any trouble from within, animals are defenseless, and that makes natural selection work.