Why does natural selection continue to work over time?

Answer 1

Natural selection creates a stronger species that is able to live longer and produce more. It continues to work because after a few generations, the traits will become common in the population.

Answer 2

Natural selection is the process whereby a species changes slowly because genetic variations are "selected" by merit of the environment killing off members of the species which do not have them.

For instance take horses. In any group of horses there will be countless variations, most of which don't impact survival or reproduction, but some of those variations might make the horse faster, slower, have stronger/weaker muscles, better or worse endurance etc etc etc, so if say a fast predator migrates into those hoses' environment it will pick off the slower, less strong ones with less endurance etc, and the genes responsible for those traits will become more dominant in the gene pool because those members of the species which have them will survive longer and reproduce more over many generations.

Natural selection might favor sharp teeth, claws, skin which retains moisture better, a stronger immune system or countless other things.

Complex or new mechanisms occur via natural selection as well, usually through a series of intermediaries. Countless mechanisms said to be "unevolvable" because of their complexity have been broken down through a series of useful slight improvements, including such notable examples as the bacteria flagellum and the eye.

Answer 3

As long as the prerequisites for natural selection are met, it will continue to happen. So as long as there are lifeforms that reproduce with variation, and those variations compete against one another for resources and mating opportunities, natural selection is happening.

Answer 4

It doesn't. What is selected for, or how severe the selection pressure is, depends entirely on the physiology of the variant or population, and on circumstances.