What happens to blood once oxygen hits and why?

it changes colour

Explanation:

When blood passes through the lungs, the hemoglobin absorbs oxygen from the air we breathe and turns bright red. Our heart then pumps this blood all over the body. When it reaches its destination, the oxygen is released from the hemoglobin, and the hemoglobin turns a darker red-brown.

substitution is the answer in this case

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.