Give an example of an organism's fitness

There are several ways to measure fitness; for example, “absolute fitness” measures the ratio of a given genotype before and after selection while “relative fitness” measures differential reproductive success — that is, the proportion of the next generation's gene pool that is descended from a particular organism

Explanation:

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.