May/June 2026 Paper 42

The green-eyed tree-frog, Litoria genimaculata, lives in tropical rainforests in Australia. Around 5 million years ago, the tree-frog population split into northern (N) and southern (S) populations that were separated by a dry area with no rainforest. 6500 years ago, the N and S tree-frog populations came into contact again as a warmer climate allowed rainforest to grow again in the area between the N and S populations.

Figure 3.1 is a map showing an area, labelled the contact zone, where N and S populations of tree-frog currently come into contact.

N tree-frogs and S tree-frogs can be distinguished by genetic techniques. Outline the genetic techniques that could be used to distinguish between N and S tree-frog DNA.

Explain how a period of geographical separation, such as the one that occurred between the N and S tree-frog populations, can eventually lead to speciation.

Scientists were interested in tree-frogs in the contact zone. The scientists made these observations: • In the wild, S female tree-frogs are strongly attracted to the mating call of S male tree-frogs but are not attracted to the mating call of N male tree-frogs. • In the laboratory, when an S female tree-frog is mated with a male N tree-frog, none of the fertilised eggs develop into offspring. Suggest, with reasons, what these observations show about speciation between N tree-frogs and S tree-frogs.

There is a large variation in body length within the population of N tree-frogs and within the population of S tree-frogs. Describe the factors that interact to cause this variation in body length.