What defines an organism's physical appearance or visible traits?

The physical appearance or visible traits of an organism are defined by its phenotype. The phenotype encompasses all observable characteristics, which can include traits such as height, eye color, and the presence of specific features. These traits result from the interaction between the organism's genetic makeup and its environment.

In genetics, the genotype refers to the specific alleles or genetic makeup of an organism, which may not always manifest in observable traits due to environmental influences or other factors. Gemmules are a historical concept related to heredity that is not commonly used in modern genetics. Microevolution refers to small-scale evolutionary changes within a population and does not specifically address physical traits. Therefore, the correct response is that the phenotype is what represents the visible traits of an organism.