What is the central force in evolution?

There is no central force, except that anything which does not hinder a gene's chance of reproducing will allow whatever species that gene belongs to live on.

If a trait helps a species to survive, that trait will be "selected".

Look at it this way, if a particular trait means that some members of a species is less likely to breed than other members, that trait will die out eventually. This could mean that lions which have shorter claws than their fellows, or cheetahs which can't run as fast as their fellows will go hungry, and be less successful in having and rearing cubs which carry their genes.

That is not so much a force driving evolution as a force culling those members of a species which are "less fit" for survival than their fellows