At one point, you were just an undifferentiated single cell. You are now made of trillions of cells. Some of these function as liver cells, some as muscle cells, some as red blood cells, while others play different roles. What name is give to the process that is responsible for this?

Explanation:

Cell differentiate into billions of cells from a single cell. At the time of fertilization, sperm and egg fuse to form a zygote. Zygote is a single cell from which all the cells of an organism originate. Whole cells of  the body are generated from a single cell by the process, called cellular differentiation. Cell differentiate into different morphology and function due to differential gene expression, Due to the differential gene expression, different organs are generated.

the correct answer is:

a population changes in response to a change in the environment.

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.