20 points fast i only have two minutes

What word is used to describe differences of individual organisms

Plz ! ive been stuck on this forever

Need an answer asap 45 points and brainlest to the first person to answer why do scientific theories, such as biological and chemical evolution, represent the strongest explanation of the changes observed in the fossil record. must be in sentence form how can scientific theories on evolution and the fossil record change over time? must be in sentence form