The theory name always gets shortened. More completely it is, “Evolution by means of Natural Selection”. The theory also applies to brains and emotions, not just bodies.
Genes try out different combinations pretty much randomly, since sexual reproduction, apart from being fun is a good way to mix up genes to try out new combinations. Once you have a new combination (one that is based on the genes the parents have, which were effective enough to allow the parents to breed) that gets “tried out” in real life.
The genes might make somebody slightly smarter, or more stupid, than others, be more selfish or more altruistic. They might make somebody analytical and not emotional, or the reverse. And those genes, however they are manifested in us, are potentially passed on to our children, if we have any.
The environment then performs a sort of “Goldilocks” experiment on the results of the genes. This person is too selfish, nobody likes them, nobody mates with them. This person is too altruistic, everybody likes them they have lots of mates, but they give away all their food to the poor, and their children die of starvation. But this person is just right – they are pleasant and helpful enough to be well liked, but they look after themselves and their offspring.
Therefore they have lots of children who survive and the genes that made them like they were get passed on to lots of people in the next generation. Those children inherit the genes that made their parent so successful in reproducing. So they tend to have lots of children too.
This applies to all sorts of thoughts, emotions, decision processes and so on, as well as physical characteristics like strength or eyesight. Genes that result in the person they produce being more successful in getting food, mates and the other requirements of life (e.g. stronger bodies to hunt, nicer personalities to attract mates, larger brains to solve problems) or producing more and better offspring (e.g. emotions that lead to finding and keeping a better mate or protecting and feeding offspring so they grow up successfully) will mean that those people have more offspring that will grow up to have offspring of their own. The less reproductively successful people will have less offspring who breed, so over time, the proportion of the genes that produce successful people will increase in the population, while the genes the produce less successful people decline.
The population ends up dominated by people who have the most living children!
The “selection” is done by the environment. It isn’t about being “nice”, “selfish”, “strong” or “rich” or anything else, it is only about what works to drive increased numbers of offspring in the environment – that is, what is reproductively successful in the real world.
Stephen Pinker has written a couple of books on this that you might like:
[amazon-product]014027605X[/amazon-product][amazon-product]0140244913[/amazon-product]
