17.1.1—Genetic and environmental variation

Phenotypic variation is the difference in observable characteristics between individuals of the same species. The difference may arise from genetic factors, environmental factors, or an interaction between both.

• Genetic differences can be passed to offspring when they are present in the genetic material of gametes, so they can contribute to inherited differences between individuals.
• Environmental conditions can alter how an organism grows or functions without changing the inherited genetic information; those environmental effects are not passed to offspring as the same phenotype.
• A phenotype can reflect both: genotype sets biological possibilities, while the environment affects how those possibilities are expressed.

A useful cautious framework is: phenotype = genotype expressed in an environment. It is not a literal numerical equation and does not mean that every phenotype can be separated into independent genetic and environmental percentages.

Variation is a population-level comparison, not a label that every individual is “different in every way”. This card defines causes of phenotypic variation; it does not classify discontinuous or continuous distributions, explain detailed genetic mechanisms, or test differences between sample means.