Definition

Pretend the world was really cold outside, so cold that everywhere on earth was 20 degrees less than normal - permanently. Many people, animals, plants, etc would die, but a few would survive. For instance, humans with thicker skin could survive the cold, and plants that don't lose much water, such as evergreens, would also survive. As a result, the human, for instance would end up procreating and make babies that also have this thick-skin trait. This is a hypothetical example showing how "nature selects" traits that are best suited for survival. The thick-skinned humans were more suited for survival, and so had the highest fitness. As a result, they were able to procreate. After 10 generations, most surviving humans would have thick skin.

Natural Selection is basically the survival of the fittest. If only certain species could survive in certain conditions, those species would reproduce to produce offspring with the same traits. Therefore eventually, all of this type of species in that area would all have the same traits: the traits that helped their parents survive in the earlier conditions.

The strong survive, and the weak die. Think about giraffes. Did you know that a long time ago, giraffes had smaller necks? As trees grew taller through evolution, the giraffes with shorter necks could not reach their food and perished, while the longer neck giraffes survived to reproduce. As mentioned above, survival of the fittest.

Natural Selection is when the best type of species survive. Here is an example.

Lets say that there are two people. For some reason, the way that they eat depends on a race. Whoever can get to the food first wins the race and gets to eat the food. Well lets say that one person is faster than the other. The faster person would win more races and get to eat more. The slow person would die of starvation. This way, there are only fast people!

Now this doesn't work in human life, because we take care of our sick and disabled. But in a wild animal population, the slower animals always die first. That way, the big strong animals can mate and have big strong offspring. This way, the population can be strong