What accounts for changes in the characteristics of organisms over time?

The correct choice highlights the foundational mechanisms of evolution that drive changes in the characteristics of organisms over time. Gene mutations introduce new genetic variations into a population, which can lead to new traits. Genetic drift is the process by which allele frequencies in a population change due to random sampling, especially in small populations, leading to the increase or decrease of certain traits purely by chance.

While natural selection is also a vital factor, it specifically refers to the survival and reproduction of individuals based on advantageous traits, shaping populations over time. Migration contributes to genetic diversity by introducing new alleles into a population but does not directly account for internal changes within a population's characteristics. Selective breeding, on the other hand, is a human-directed process that results in specific traits being favored but does not encompass the natural processes that account for changes in characteristics over time in wild populations.

Thus, the combination of gene mutations and genetic drift offers a more comprehensive explanation on a fundamental level of how traits develop and change in organisms over time within varied environments.