In a study of heathland ecosystems, the mature heights of individual common heather plants, Calluna vulgaris, are found to vary between 20 cm and 60 cm.
Which of the following correctly describes this variation in height among these heather plants?
Interspecific variation controlled by environmental factors only
Intraspecific variation controlled by genetic factors only
Intraspecific variation controlled by both genetic and environmental factors
Interspecific variation controlled by both genetic and environmental factors
