Consider this: you have two mice, once of which is brown and the other is bright pink. In the field, the brown mouse blends nicely with the ground and the pink mouse sticks out like - well, like a pink mouse. The pink mouse is going to be more easily spotted by predators, giving it less time in its life to make pink mouse babies. Any pink mouse babies that it does have will also have a hard time hiding from predators, thus even fewer pink mouse grandbabies.
In this case, brown fur would be considered a selective advantage, making it easier for the mouse to live longer and have more shots at reproducing. However, if the mice lived in a different environment, such as Barbie's dream house, the pink mouse may actually have the upper hand, being less easily spotted against the pink carpet, pink linoleum, among the pink curtains, and so on. It's all about fitting your world.
Since the advantages are particular to an environment, natural selection can lead to species that are specifically suited only to a specific place, or can even result in brand new species over time.
Natural selection is one of the accepted mechanisms of evolution.
Others:
- Adaptation
- Genetic drift
- Gene flow
- Mutation
- Speciation
