Evolution and Natural Selection
(S7L5.a and b)

Evolution is the theory that all organisms share a common ancestor and that we developed new features over time to help us survive. For evolution to occur, natural selection must occur, as well. A man by the name of Charles Darwin was one of the first to comprehend this theory. He studied the organisms on the Galapagos Islands, off the coast of Chile in South America. One thing he observed was how the finches on each of the different islands were adapted to different things, depending on the environment of the island.

Here's Darwin's own summary of natural selection (taken from his book "Origin of Species"):

If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But, if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterized. This principle of preservation, I have called, for the sake of brevity, Natural Selection.

Going back to evolution, evolution may be change over time, but it does not mean something like the leaves falling off trees, or mountains being eroded. Evolution is to change genetically. Over the millions of years life has existed on this planet, new species have emerged through evolution by becoming better adapted for their environment.

Take whales for example. Studies have shown that whales probably started off as an animal similar to a wolf. As they figured out it would be easier to hunt in water, their legs and tails became more suited to swimming over the generations. As they continued to evolve to the water, the prehistoric whales saw no need to be on the land, and thus, their legs became smaller over time, as they fully lived in the ocean.