Organisms are machines designed by their evolution to play a certain role. The role, and the stage – the environment – where it is played, is called the organism's niche. For example, most cats – tigers, leopards, mountain lions – play the role of solitary hunters. Wolves and wild dogs are social hunters; antelope are social grazers; and so on. The basis for the modern idea of niche is Charles Darwin's discussion of an organism's “place in the economy of nature.”

A niche defines the pattern of behavior – the adaptive repertoire – compatible with an organism's survival and reproduction. A niche doesn't tell an organism how to behave. It just punishes it – by death or reproductive failure – for doing the wrong thing. A niche is a filter not a creator. Niches are best defined by example. It is pretty obvious that the talents required of a good leopard are quite different from those needed by an effective antelope. For leopards, powerful means of attack, a digestive system attuned to meat, and a visual system adapted to attend to one thing at a time work well. But a prey animal like an antelope needs a good way to evade attack, a lengthy gut able to cope with poor herbivore diet, and a visual system able to detect threat from any quarter. Hence, the claws and teeth of the leopard, its forward-facing eyes and short digestive tract, as well as the rapid running and maneuvering of the antelope, its lengthy gut, and sideways-facing eyes – all have an obvious functional explanation.

The behavioral adaptations required by different niches are usually less obvious than morphological (form) differences, especially if they involve the ways that past experience affects present potential, that is, differences in learning. The match between adaptation and niche is no less close because it is hard to see, however.

For simple niches, such as those filled by most nonsocial invertebrates, a set of built-in responses to commonly encountered environments suffices to get the organism to its evolutionary goal, which, for bugs as much as billionaires, is survival and reproduction (Darwinian fitness). The animal need only avoid bad things and approach good ones, all signified by signals innately coded. Stimulus–response mechanisms, plus some sensitivity to rates of change, are sufficient for a wide range of surprisingly intelligent behavior.