|How Does Evolution Occur?
Darwin's Theory of Natural Selection has been divided into five parts to explain clearly how evolution happens in nature.
1. Organisms produce more offspring than actually survive.
|Bear and fish both struggle to survive. This bear, by catching the fish, is succeeding in the struggle and surviving. The fish did not survive, however, but left more fit fish behind.|
The fish eggs at left are only a small fraction of those laid by a single fish. If all of the fish hatched and each of them laid as many eggs, which all hatched as well, there would be too many fish for the fishes' environment to support. 
|2. Every organism must struggle to survive.
One reason that not all organisms survive is that there are not enough resources, things that they need, to go around. Organisms must struggle to get what they need to survive, competing against other organisms that want the same things they do. They also have to struggle to get away from predators and to overcome disease. For example, a fox struggling to catch a rabbit, which struggles to escape. As explained in part one, not all of them make it.
3. There is variation within a species.
4. Some variations allow members of a species to survive and reproduce better than others.
5. Organisms that survive and reproduce pass their traits to their offspring, and the helpful traits gradually appear in more and more of the population.
Here is an example demonstrating each of these points, told as the story of a population of brown hares in a polar region becoming white hares:
1. There are hares living in a cold, snowy, polar region. All of the hares are brown* and many of them are killed and eaten by other animals each year, so more are born than survive.
2. These hares are competing for life and struggling to survive, partly by trying to avoid predators, to "not be the one who gets eaten."
3. Each hare is a little bit different from other hares, but one time a few hares are born albino, white, because of a mutation in the genes.
4. This variation in color helps the albino hares to survive. Against the white snow, predators have a harder time seeing them, so more of the albino hares are able to survive and therefore to reproduce.
5. In the next generation there are more white hares, because the white hair gene is passed on. In this generation, too, more white hares survive and reproduce. They pass the albino trait on to their offspring, who also survive and reproduce more than do the brown hares. Gradually, there are more and more white hares in the population, until the entire population is made of white hares.
* You might ask: What were brown hares doing in a polar region? The brown color in an Arctic region was caused by one of two things, both of which are major causes of change and the arising of new species. One possibility was that a group of hares migrated from a warmer region and became isolated, the other is that the region where the hares lived became colder.
|This wild goat is well equipped with a lot of hair, which its ancestors evolved long ago and which helps it to survive cold mountain weather.|