Natural selection was Darwin’s brilliant hypothesis, independently conceived by Wallace, that accounts for adaptations, features that appear “designed” to fit organisms to their environment.

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Taxis - movement toward or away stimuli

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Fixed action pattern - A predictable series of actions triggered by a cue, sometimes called the key stimulus.

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Stimuli
Anything that can trigger a physical or Behavioral change

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Sign stimulus (releaser)

The essential feature of a stimulus, which is necessary to elicit a response.

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Male stickleback fish develop red colour on their ventral side in the breeding season and maintain territories. 

When a conspecific male intruder enters their territory, resident males perform an aggressive display. 

The ethologist Niko Tinbergen presented models of different shapes to territorial male stickleback fish. He found that models of any shape elicited aggressive displays, provided the ventral part of the models was coloured red.