We say than a behavior is adaptive if it leads to improved survival and reproductive success. All organisms that have behavior (i.e. animals) are under evolutionary pressure to choose adaptive behaviors over maladaptive ones. In simple animals such as insects there is little dispute that behavior is primarily instinctual (innate or genetically determined). In humans, much behavior is clearly partially or entirely learned. The Nature Versus Nurture dispute is largely about whether humans have any important behavioral instincts. Yet whether behavior is learned or innate, human behavior must be reasonably adaptive or we would die off.

We also say that the organism adapts to its environment, evolving toward greater fitness. As a noun, an Adaptation is an evolved anatomic feature or other trait of an organism that increases its fitness. Although we could speak of a behavioral adaptation, it is often more convenient to say that “Behavior X is adaptive.” For example, it is adaptive for squirrels to store nuts by burying them.

Insofar as Truth is akin to usefulness, we can say that in evolutionary reasoning, showing that a trait is adaptive is the primary way of establishing truth (see Evolutionarily Stable Strategy.)

See also Just-So Stories, Adaptation and Adaptive behavior.