How can we explain the observation that traits, such as skin color, are fairly consistent within a racial group, but alleles for skin color are not consistent

Skin color is a quantitative trait influenced by the combined action of many genes, thereby different gene pools can result in the same skin color.

Explanation:

Quantitative traits (also known as continuous traits, ) are phenotypic traits that depend on the combined action of many genes and the environment. These traits show continuous variation, changing gradually over a range of values in the population. Examples of quantitative traits include height, skin color, weight, etc. A gene pool is the sum of all the individual genes in a population. In consequence, different populations may show different allele combinations (gene pools) that result in the same phenotype for a quantitative trait (skin color).

a unicellular paramecium's contractile vacuole expels excess water

Explanation:

this happens so that the water levels inside the cell are constant, mantaining homeostasis.