The Galapagos Islands formed at least 8 million years ago and a tortoise population from the South American mainland was estimated to have found its way to the island at least 5,000 years ago. A small isolated population of Galapagos Tortoises emerged. Based on a sample, this population's descendants can grow to be on average, 25 kilograms heavier than similar tortoises found on the South American mainland. The Galapagos Islands are drier than the South American mainland. Thus, the current average weight of Galapagos Tortoises is undoubtedly at least partially attributable to changes associated with the Galapagos' environmental conditions.
Which of the following is an assumption on which the argument depends?