There is a consistent suite of neural and chemical features that influence the behaviour of many animals, however, including mammals such as humans. For instance, the brain region known as the amygdala, and the chemicals epinephrine (i.e. adrenalin) and norepinephrine are the key biological underpinnings of the experiences of fear and anger – part of what is termed the fight-or-flight response. Similarly, there are brain regions (the orbito-frontal cortex and the nucleus acumbens) and a chemical (dopamine) involved in learning, pleasure and addiction. An animal's nature – including human nature - depends to a great degree on these physiological elements, which have a genetic basis.
Yet what an animal actually does in any situation, while influenced by these internal forces (nature), is also greatly affected by its prior experience and circumstances (nurture). A dog has powerful natural instincts to chase after something receding from them, but they do not use this solely to hunt prey. In fact, a domestic dog often feels this instinct most powerfully in the urge to chase a tennis ball. The language used to discuss biology today frequently makes general claims of the form "evolution designed dogs to hunt prey", but this overstates the matter. Millions of years of natural selection refined the hunting instincts of ancestral dogs, but those same instincts can be used in many different ways – including chasing tennis balls and chewing up shoes.
Evolutionary history can help explain how certain instincts might have come to be, but it cannot tell us much about actual instances. Natural selection is the ultimate statistical abstraction, so far removed from the everyday life of individual animals that it can tell us very little (but not nothing) about how they actually live. We still learn about animal behaviour by watching how creatures live in their natural environment. Evolutionary theory, at best, helps put those observations into a historical context, allowing for informed guesses as to how certain instincts may have come about. It's an important part of the story of life, but it cannot substitute for individual stories.
Part 6 of 23 in the Pentenary series.