Increased calf muscle in schoolchildren as a result of regular exercise is an example of variability
The increase in calf muscle in schoolchildren as a result of regular exercise is an example of modification (3) variability. Modifications are called phenotype changes caused by environmental influences and not related to genotype changes.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.