How does the theory of evolution works? [DT02]
If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection.
As describe by Charles Darwin in Chapter 4 of his book the origin of the species, selection of a specific trait from a certain organism will be favored by its usefulness and will have a higher likelyhood of being carried out through reproduction. Fitness of an organism pertains to its ability to reproduce despite the struggles of competition and subjected to environmental forces.
The choices available to any organism: adapt or die. We are included in this dynamic motion of survival with other animals. Our ancestors were forced to choosing to adapt and use wooden spears to take down mamoths and smash stone tools to break its bones when they are traced to scavenge even marrow. This is how polar bears hibernate for some months to cope up with the deep freezing months, memorizing the time when to hunt and stock up on body fats.