However, some phenotypes are either selected for or against by the conditions in which an organism lives. For example, people that live in places with strong sunlight, like near the equator, are likelier to survive and reproduce if they have dark skin to protect them from UV damage.
A selective pressure is any reason for organisms with certain phenotypes to have either a survival benefit or disadvantage. In the example above, strong sunlight is a selective pressure that favors darker-skinned people: lighter skin would be a disadvantage in these regions.