Natural selection describes how evolution works.

More organisms are produced then can possibly survive. This is best illustrated by looking at organisms with a high breeding speed. For instance common brown rats have an average litter of eight young and pregnancy takes only 21 days average. You go figure how long it takes to get one million rats if you start with only two.

(No I can’t do maths for crap, that’s what I am becoming a biologist for)

There’ll be a struggle for existence. At one point the environment will fail in supplying enough food water and space for all organisms produced. Here’s were selection comes in. Those individuals best adapted to the environment will logically manage to get the best resources, like food, water, territory, mates, you name it.

Only those best adapted to their environment will survive bad times so they can reproduce for themselves. This how ‘nature selects’ the ‘fittest’, best equipped for surviving.

So the most succesfull indivuals pass on their genes to the next generation. Hereby they spread their genes further trough the population. This happens generation after generation. After a number of generations considerable changes in physiology as a response to the selective environment can be noted.

An extreme case are our common dogs, all dogs belong to the same species, be they a poodle or a golden retriever. In this case man has selected out the dogs with traits they liked, and interbred dogs until they had the desired physical appearance.

Natural selection only works on genetically inheritable characteristics though. Simply because an non genetic adaptation cannot be inherited. A non genetic adaptation can be something an organism learned.

There are for instance chimpansees that have learned to use sticks to catch termites. When a small stick or twig is stuch into a termite hill the termites will start biting the ‘intruder’. The chimpansee can then just retract the twig and eat the bugs. They can of course teach their young this trick, but the young don’t know it by instinct or anything.

Same thing is that human babies don’t know language by instinct. The ability to speak has to do with brain, tongue and troath etc. Many genetically inheritable traits have an influence on our ability to speak. But language itself is a learning process.