The results of phenotypic differences will be due to differences in environment rather than to differences in genetics.
Work Step by Step
Consider what could be the cause of the phenotypic differences between two organisms of the same species, such as identical twins: genetics and environment. Then ask how identical twins form: the splitting of one embryo, which means that each twin would have the same alleles for any given gene--no genetic diversity between them. Thus, genetics cannot be responsible for any phenotypic differences, leaving the environment as the only possible cause of differences.