Any heritable characteristic that increases an organism's ability to survive and reproduce in its environment is called an adaptation. Adaptations can involve body parts or structures, like a tiger's claws; colors, like those that make camouflage or mimicry possible; or physiological functions, like the way a plant carries out photosynthesis. Many adaptations also involve behaviors, such as the complex avoidance strategies prey species use. Examples of adaptations are shown in Figure 16–9.

Survival of the Fittest Darwin, like Lamarck, recognized that there must be a connection between the way an organism “makes a living” and the environment in which it lives. According to Darwin, differences in adaptations affect an individual's fitness. Fitness describes how well an organism can survive and reproduce in its environment.

Individuals with adaptations that are well suited to their environment can survive and reproduce and are said to have high fitness. Individuals with characteristics that are not well suited to their environment either die without reproducing or leave few offspring and are said to have low fitness. This difference in rates of survival and reproduction is called survival of the fittest. Note that survival here means more than just staying alive. In evolutionary terms, survival means reproducing and passing adaptations on to the next generation.

In Your Notebook If an organism produces many offspring, but none of them reach maturity, do you think the organism has high or low fitness? Explain your answer.

VISUAL SUMMARY

ADAPTATIONS

FIGURE 16–9 Adaptations take many forms.

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