<span>The natural selection.
Natural selection is the differences in survival and reproduction as the consequence of differences in phenotypes.
In natural selection, genotype variations that will increase the chance of survival and reproduction of some organism are preserved and will be inherited. So genotypes variations are responsible for phenotypes and, thus, help the survival of species in a particular location.</span>
Answer:
The earth has different layers because as it formed, the lighter parts (like continental crust) floated to the surface, and the really heavy parts (like iron and nickel in the core) sank to the middle. It is just like when you mix oil and water: the oil will float to the surface because it is lighter (or less dense) than the water.
Universe's average temperature and rate of expansion support the idea that it began as one super-dense and hot mass 13.8 billion years ago.
