What principle is illustrated by the experiment that shows varying the temperature affects the number of facets in fruit fly eyes?

The principle illustrated by the experiment showing that varying the temperature affects the number of facets in fruit fly eyes is the norm of reaction. This concept refers to the range of phenotypic expressions that a single genotype can produce under different environmental conditions. In this case, the number of facets in the eyes of fruit flies is not solely determined by their genetic makeup; rather, it can be influenced by temperature variations in their environment, which leads to observable differences in their traits.

The norm of reaction highlights the interplay between genetics and the environment, suggesting that the same genetic code can yield different physical traits (phenotypes) based on external factors. This principle is crucial in understanding how organisms adapt to changing conditions and how physical traits can vary widely even within species when subjected to different environmental influences.