Environment IS natural selection, so a change in environment is a change in selection pressure.

When nothing happens to exert strong population pressure on that population, natural selection favors the allele frequency already present. When mutations cause new traits, natural selection weeds these traits out because they're not as efficient as the others.

This is the same as evolution of caveman to modern man.

These plants and animals were subjected to artificial selection so that the traits humans wanted in the organisms were selected for and the organisms not having these traits were culled. This, with some modification, is a good analogue for natural selection and artificial selection shows how organisms can be shaped over time with the proper selective pressures. The selective pressure of humans in artificial selection and the selective pressure of the environment in natural selection.

Adaptions that lead to greater survivability and reproductive success in the immediate environment of the individual organisms under selection pressure.