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Slow changes in organisms are made through natural selection. In any population of plants or animals, genetic mutations will occur. Badly (or not so well) adapted mutations are less likely to survive and breed but well adapted mutations will do both. The new characteristics that make them better adapted than their competitors are therefore passed on to their offspring.
Natural selection and adaptation through inherited characteristics occurs slowly but behavioral adaptations can arise over much shorter time periods. Environmental and other changes may affect the behavior and distribution of living organisms causing significant changes in populations - plants not suited to the new conditions will die and animals will either die or move elswhere. Population size depends on factors including competition, predation and human influences.
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