Can anyone please help me by explaining (simply) what Charles Darwin theory of Natural Selection is, please?
No, that is not correct! Natural selection acts on populations, not on individuals. Consider this: in the example above, if you got calloused hands, and then you became a parent, would your baby be born with callouses? Of course not. This idea, known as Lamarckism, has been disproven again and again.
What Darwin's theory really said was this:
1) More individuals are born than the environment can support, so there is competition for resources
2) Not all individuals have the same genes
3) Some genes create an advantage when competing for resources
4) The individuals with the advantageous genes do better in the competition, so they get the opportunity to make more babies than the less successful competitors
5) The babies of the successful competitors will tend to carry the successful genes, which become more numerous over time
6) Sometimes a mutation will occur which creates a new gene; some of these new genes will be advantageous to the individual that has then, so will be passed on as above
7) If this goes on long enough, we will see changes in an entire population of individuals.
Natural Selection is based on two facts of life; sex and death. In a nutshell, if you are dead you can't have sex. In order to have sex, you need to stay alive.
Life in the wild is dangerous and difficult. Many many things can kill you (hunger, cold, accidents, disease etc etc) and death is always ready to pounce. So, death removes many many members of each generation of every species before they get a chance to reproduce. Only the good survivors survive long enough to pass their DNA to the next generation. Over 1000s of generations this filtering out of the less prepared individuals selects more and more suitable DNA for each environment. A basic example would be a speceis that gradually migrates into a colder region. The cold will kill off most of them, but the survivors will be those more suited to cold. Over time, features which protect against cold will become more and more developed.
Natural selection favors those animals/plant from each generation which are better adapted to their environment.