Natural Selection is one of the key concepts that one must understand in order to understand evolution. Natural Selection is established by the following facts:
1) There is variation amongst living things.
2) Some variations help living things survive longer and leave behind more offspring, who share these successful traits.
3) Variations that help organisms survive and reproduce become common in populations because of this.
Living things which reproduce the most become the most common. For Example, Sean Carroll explains in his book "The Making of the Fittest" how a certain species of mouse comes in both dark and light coated forms. Populations of mice that live on dark, volcanic rock tend to be dark, simply because they blend in with their environment. Lightly colored mice are easily spotted against the dark backround by birds and other predators.
PBS Evolution explains the case of the Darwin Finch:
"The major factor influencing survival of the medium ground finch is the weather, and thus the availability of food. The medium ground finch has a stubby beak and eats mostly seeds. Medium ground finches are variable in size and shape, which makes them a good subject for a study of evolution.
The first event that the Grants saw affect the food supply was a drought that occurred in 1977. For 551 days the islands received no rain. Plants withered and finches grew hungry. The tiny seeds the medium ground finches were accustomed to eating grew scarce. Medium ground finches with larger beaks could take advantage of alternate food sources because they could crack open larger seeds. The smaller-beaked birds couldn't do this, so they died of starvation."
Another case to ponder is that of Lactose tolerance in humans: Only populations which have herded cattle for centuries seem to have the ability to digest milk. The people of Japan and China, for instance, did not herd cattle in the past, and today they are usually unable to consume dairy products. Why is that? In earlier times food was not as abundant as it is now. Having the ability to digest dairy products would have been a major advantage: It meant an extra source of food. During times of famine, being able to consume something as nutricious as milk or cheese meant the difference between life and death for some. Through the centuries, the population would certainly go through many periods of famine, and each one would reduce the number of lactose intolerant people, while preserving the lactose tolerant. Eventually the population is overtaken by those with lactose tolerance.
We now know that the variations Darwin described are due to changes in DNA called mutations. These genetic changes affect everything about living creatures. Mutations can add letters to the genetic code, delete them, substitute them, and copy genetic material. An insertion mutation, which adds a letter to the genome, has been observed to give bacteria the ability to digest nylon. Gene Duplications produced an "antifreeze" in the blood of arctic fish. Many other examples exist of mutations creating new and beneficial genes.
In addition, Darwin made the scientific prediction that intermediate forms would be found in the fossil record. For instance, if fish evolved into amphibians, we will find fossils that show characteristics of both. Darwin was proven correct.
