There are a seemingly endless variety of fish that live in the oceans and in fresh water. Although most fish have common features (such as gills and fins), you can find fish with many differences. For example, fish vary in size, shape, color, and behavior. The great diversity of fish that exist is a result of evolution.

Natural selection describes the evolutionary process by which species develop biological adaptations—particular traits that allow them to better survive in their environment. In this process, organisms with traits that are not well suited to their environment tend to die out while those with traits better suited to their environment are more likely to survive and reproduce. As a result, the favorable traits become more common with each generation.

A fish that is well suited to its environment has adaptations that allow it to survive in the conditions of the water (such as temperature and whether it is salt water or fresh water) and physical traits (such as body shape and coloring) that protect it from predators and help it get food. For example, a streamlined body helps some fish swim fast (which makes them good predators) and a spotted pattern helps fish blend into the background (which hides them from predators or prey).

While there are many physical characteristics that make a fish fit for its environment, there are also behavioral adaptations. Behavior is defined as an organism's response to its environment. Some behaviors are learned, but an organism also has instinctive or innate behaviors that are inherited from its parents, such as hibernation, migration, and mating habits. Behavioral traits that function to improve the organism's chances for survival are determined by natural selection in the same way that physical traits are selected. Behavioral adaptations in fish include how they evade predators or find food, where they live (such as on a reef or in open water), and whether or not they swim in schools.

Physical and behavioral traits often evolve in tandem. For example, fish with striped body patterns are easily seen in open water or against a reef background. Consequently, striped fish better protect themselves from predators if they swim in schools, where their stripes blend together and confuse predators.

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