as the ability of a living thing to survive / to reproduce increases , the likelihood of passing on its traits to its decendents will increase & reproduction increases the number of a living thing ->  if living things with one trait outnumber those living things with a different trait in a population then the living things with that one trait probably have a better chance of surviving and reproducing  if living things with one trait outnumber those living things with a different trait in a population then the living things with that one trait probably have a better chance of surviving and reproducing & a sheep is a kind of living thing ->  if sheep with one trait outnumber those sheep with a different trait in a population then the sheep with that one trait probably have a better chance at surviving and reproducing  if sheep with one trait outnumber those sheep with a different trait in a population then the sheep with that one trait probably have a better chance at surviving and reproducing & sheep with light wool outnumber sheep with dark wool in the sheep population on hirta -> sheep with light wool have a better chance of surviving and reproducing than sheep with black wool on hirta