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Defining Natural Selection
Imagine a species of beetles that can be either green or black. Birds cannot see the green beetles well because they blend in with the grass, but the black beetles are easy to spot. Thus, more of the black beetles are eaten by birds. This means that the black beetles have less of a chance to reproduce, while the green beetles reproduce more frequently. As a result, the next generation of offspring will be mostly green. After a few generations, all of the beetles will be green. This is the process of natural selection.
Stabilizing Selection
Stabilizing selection occurs when the extremes are selected against. For example, a bird species may come in a range of sizes. The very small birds may be selected against because they are vulnerable to predators. The very large birds may be selected against because they are not able to fly or move as fast, making them unable to catch enough food. Thus, the middle sized birds are the ones that are best able to survive and reproduce.
Disruptive Selection
Disruptive selection occurs when the extremes are selected for. For example, a frog species may have distinctive markings that makes it identifiable to predators. Frogs with markings that are extremely colorful or very dull may not be recognized by predators as their normal prey, making them less likely to be killed. This translates to a greater potential to reproduce and have offspring with the same unusual markings. Over time, the original markings will become less common.
Directional Selection
Directional selection occurs when one extreme is selected for. For example, let's say that the more intelligent a dolphin is, the better its chance of survival will be. Thus, the intelligent dolphins live longer and produce more offspring than the unintelligent ones. As a result, the dolphin population will become increasingly intelligent over time.