"If a species is evolving because of environmental changes, the would natural selection just be a weeding out of the weaker part of the gene pool?
Yes, sort of. There is a tendency for most with little knowledge of evolution to assume that the word 'fittest' means strongest. In the sense of the least sickly it is true, but it does not necessarily mean the physically strongest. What it does mean is that any organisms that have an advantage, whether it be physical strength as in many carnivores, more speed as in many prey animals, physical attractiveness as with birds, better hunters as with humans or even more resistant to bacterial or parasite infections, will have a better chance of having a high number of offspring that survive til they reach the age of reproduction. Its all about how many kids you have that have their own kids, not just about who is the strongest.
I think I have a better understanding about how natural selection works now from others that have explained it as well.
But I do appreciate your replies and help.
Each of you has a unique way of explaining and from that it has helped me better understand it's workings.
Thanks.