Why do individuals with low levels of fitness die?

How do adaptations help an organism as they struggle for existence?

Not getting enough physical activity can lead to heart disease—even for people who have no other risk factors. It can also increase the likelihood of developing other heart disease risk factors, including obesity, high blood pressure, high blood cholesterol, and type 2 diabetes.

Darwin suggests that an organism's struggle for existence is part of what determines why some species' characteristics survive and others become extinct. These adaptations illustrate how unique characteristics of particular organisms have developed, allowing them to thrive in their specific environments.