If a single female immigrant brings a trait to a small, sparsely populated island and within many generations it is present in 50% of the population, it would be an example of

b is dominant over the allele for the white fur.

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.