How natural selection is related to evolution?
Excellent questions, and as well, the very heart of evolution.
Firstly, every living thing produces offspring that might have a mutation...a gene that the parent does not have. If this gene is lethal, the plant or animals will not survive to adulthood to pass it one. (and at this point, as always, most mutations are indeed lethal. A bird with only one wing is hatched, a plant that has no gene for leaves, etc.) But on occasion, an individual is hatched, or born, or arrives on the scene that has a somewhat better, say eye, or wing, or claw or skin---better than his parents had. Since that gene is better, it will help this individual to survive to adulthood, and in so doing, pass this gene onto the next generation. No one selected this gene.... "Natural selection" did.. That is, that in nature, the better off some individual is, the more likely it will survive, as opposed to the individual bird born with only one wing. In 40 or so generations, in that immediate area, an animal with that superior gene, will be the only type of that species...again, in that area. And if that area is isolated long enough, from others in that species, that species will become somewhat different that those on, say, the other side of a high mountain range.
The bottom line here is that mutations occur all the time in all living things....all the time. Good mutations improve the species. Bad ones kill the individual, and it does not pass them on. And "Nature" selects simply because of the rule of "survival of the fittest"..... (and actually it is better to say, "survival of the most adaptable". And all sorts of gene mutations can provide an individual better and more adaptable.
This process began as soon as there were single cells that reproduced thru simple cell division. And it continues to this day, tho most people don't know what to look for.
Evolution in action example:
1. Speckled Moths in England were predominately grey and white before 1740. With the beginning of the Industrial Revolution, and the burning of huge amounts of coal, the tree bark that was predominately grey and white, began to turn mostly dark, with the soot of the burning coal. A morph individual, that was dark, was easily seen by birds, and eaten. But as time went on, and the tree bark became darker and darker, it was then, the usual white and grey ones that were most readily seen, and thus eaten, and the morph dark one was the survivor. Nature was selecting the survivors. However after the 1954 killer smog in London, legislation was passed to clean up the coal burning manufacturing plants, and over the next 20 years or so, those trees that were once grey, then black, then became a white grey again. I think you are seeing what was going to happen.... the white grey moths now were again the survivors, and the morph dark were easily spotted by birds and eaten. There are still morph dark ones hatches, and they are now still eaten.
That gene will always show up in some individuals. This is the genius of natural selection, and evolution.... always trying to make a plant or animal a bit better able to cope with its environment.