1. change
The natural selection is a process that occurs as a direct consequence of the living conditions in the environment. When the environment starts to change though, the organisms that live in it face a big challenge. They either have to change, adapt to the new conditions, and thus survive, or if they are not able to do so, to die out. This is the biggest evolutionary pressure for the organisms, and while some come out as winners from it, many others die out .
2. variation
In order for the process of natural selection to occur, the population needs to have some genetic variation. The environment is the ultimate push factor, but if the population doesn't have genetic variation in it, than the species will die out. The genetic variation is very important as in that way the individuals choose who will they mate with, in accordance with their traits, so that only the advantageous traits can be passed on to the next generation, so that it can be more competitive in the environment.
3. traits
The natural selection is a very simple process. The individuals of a population, when choosing a mate, look for the individuals that exhibit traits that are advantageous for survival. Mating with individuals with traits that are advantageous guarantees that the offspring will inherit those traits, thus it will have higher chances to survive. The individuals that do not have advantageous traits though will be avoided as mating partners, and that would rapidly lead to their decline and elimination from the gene pool, as they will not have any offspring to continue their line.
