You often hear “that can’t happen ­ it would be bad for the species”. But this is not true. Individuals, not species, are born, live, eat, and die. And only those individuals that reproduce will make any contribution to the next generation. Behaviours that benefit individuals will persist, even if they lead to a decline in numbers, or even extinction.
In a narrow sense, then, individuals are a fundamental unit of selection. In any generation, some characteristics are more likely to be favoured than others. For instance, in time of famine, organisms with lower metabolisms will need less food, and will be more likely to survive and leave offspring. In times of plague, organisms with better resistance to disease will be the ones most likely to survive, and so on. But of course, what is beneficial in one generation may be a hindrance in the next.
