Two centuries ago, Tufe Peninsula became separated from the mainland, isolating on the newly formed Tufe Island a population of Turfil sunflowers. This population' s descendants grow to be, on average, 40 centimeters shorter than Turfil sunflowers found on the mainland. Tufe Island is significantly drier than Tufe Peninsula was. So the current average height of Tufe' s Turfil sunflowers is undoubtedly at least partially attributable to changes in Tufe' s environmental conditions.
The mainland's environment has not changed in ways that have resulted in Turfil sunflowers on the mainland growing to be 40 centimeters taller than they did two centuries ago is an assumption on which the argument depends.