How do environmental, selective pressures, shape the evolution of organisms?

If there is an environmental or selective pressure it will contribute to the evolution of an organism by giving an adnantage to the organisms of a population who have a trait which allows them to survive that change. The advantageous genes will be passed on and the number of organisms with the desirable feature will increase over time, however the number of organisms that lack the characteristic will decrease over time. This is evolution by natural selection.