* No Variation = No Differences: If all individuals in a population were identical, they would all respond to environmental pressures in the same way. There would be no advantage or disadvantage for any particular trait, so no individual would be more likely to survive and reproduce.
* No Advantage = No Selection: Natural selection depends on certain traits being advantageous for survival and reproduction in a particular environment. Without variations, there would be no advantageous traits for natural selection to favor.
* No Selection = No Evolution: Evolution is the gradual change in the genetic makeup of a population over time. If there is no natural selection to drive this change, the species would remain stagnant.
Think of it like this:
Imagine a group of people trying to survive a harsh winter. Some have thick coats, some have thin coats. Those with thick coats are more likely to survive the cold. This means they are more likely to reproduce, passing their genes for thicker coats to their offspring. Over time, the population will evolve to have thicker coats, because the individuals with that advantageous trait were selected by the environment.
Without variation, everyone would have the same thin coats, and nobody would be more or less likely to survive. The cold would kill off a random portion of the population, but there would be no selection pressure for a specific trait.
In short, variations are the raw material for natural selection. Without variation, there is no way for natural selection to act, and therefore no way for evolution to occur.
