The Grand Canyon  is very old. If you look at the Grand Canyon you will see that it is made up of many different layers. The very first layer at the very bottom is the oldest. This make sense because one layer has to be there for the next one to form on top of the other. Sometimes scientists find fossils in the different layers. Since we have the layers we are able to know how old the fossil is
Since we have fossils, we are able to find out how different animals change and develop. This is called evolution. Lets take birds for example. Lets say this bird eats insects from the ground and out of trees. Lets also say that one bird  born with a longer beak and It survives better than the rest because of that longer beak. THis bird with produce offspring with a longer beak. Over about 1000 years the entire species will have that beak. This is called natural selection.    
 
      When you have too many of one species or even too many different kinds it is called over population. This can be good and bad. Mostly bad though. Lets take moths for example, You have way too many living one  area. This can be harmful to the moths. Having so many in can make it harder for them to get food and find a place to live. Witch can lead to the population to die off. 
        After many years of overpopulation, the spices will begin t change so that they have a better chance of surviving. This is called evolution. One example Is the Tortoise  If the Tortoise lives on a wet area the neck will be rounder so that it can eat grass. But a Tortoise hat lives on a drier island its neck will have a slight turn-up so that it can reach up to higher plants and fruits. when an area is over populated the animal will slowly change to make it easier for them to live in the situation that they live in