Climate Factors
Environmental stresses such as droughts and floods can change the organisms that live there. The conditions are different, and therefore the organisms may have to change over generations to adapt to living on less water, higher temperatures, develop webbed feet for swimming or become more interested in burrowing to find water. For example, most frog species lay eggs that are suitable either for land or water laying. Due to an unpredictable environment, a frog species in Panama is able to lay eggs in both places to best adapt to the changing environment.
Physical Factors
Physical environmental changes such as moving over time into higher altitudes, areas of swamp or drier areas like desert can have an effect on the organisms that live there. For example, people who live in Tibet, where most of the land is at high altitude with lower oxygen content in the air, have developed physiological differences to lowlanders. They have more red blood cells, which carry oxygen around the body. Their ancestors survived and had more children, who also had more children, and so on, breeding out the less well adapted people with lower red blood cell counts.
Predation and Food Factors
Predator populations change coloration and develop different predatory skills depending on the environment and on the reactions of their prey. For example, a cheetah's coat allows it to meld with the savannah grasses, and its speed allows it to keep up with its fast-moving antelope prey. Conversely, the antelope population develops signalling systems, camouflage coats and rapid responses to increase the chances of surviving the cheetah attack.
Genetic Variation
In order for any selection pressure to alter the characteristics of a population over time, the population has to have the genetic variation necessary. The variation in the genes, and the accidental development of a mutation in the genes, gives some members of a population an adaptive advantage over the other members. This difference makes it more likely that the organism with the new, improved characteristic will survive and breed, compared to the organism with the less suitable characteristic. For example, the human population has variation which allows some members to resist certain diseases better than other people. An example of this is the prevalence of sickle cell anemia in people who live in some areas of Africa with lots of malaria. The anemia helps protect against the disease, and so more people in this area have it than in areas less affected by malaria.