Evolution and Speciation · Paper 2 — Free Biology HL Practice Questions

The three-spined stickleback (Gasterosteus aculeatus) is a small fish found in both marine and freshwater environments in the Northern Hemisphere. After the last Ice Age (approximately 10,000 years ago), many marine stickleback populations became trapped in newly formed freshwater lakes and streams. These isolated populations have since diverged into distinct freshwater forms. Data from a study of two stickleback populations in British Columbia, Canada, are shown below: - Population A: Found in a large, deep lake (Loon Lake). Males build nests in open, sandy areas. Breeding occurs in June. Average number of lateral bony plates $= 32$. - Population B: Found in a small, shallow stream (Beaver Creek) that flows into Loon Lake. Males build nests under rocks in fast-flowing water. Breeding occurs in May. Average number of lateral bony plates $= 8$.

The apple maggot fly (Rhagoletis pomonella) originally infested only hawthorn (Crataegus) fruits in North America. Around 1850, a population began to infest introduced apple (Malus domestica) fruits. Today, the two host races are genetically distinct and show partial reproductive isolation. Mating occurs on the host fruit on which the female lays her eggs. In a laboratory experiment, female flies from the apple race were given a free choice of fruits on which to lay eggs. Results: - Apple fruits: $78$ eggs laid - Hawthorn fruits: $22$ eggs laid

The Grant family's long-term study of Darwin's finches (Geospiza spp.) on Daphne Major island in the Galápagos has provided direct evidence for natural selection in the wild. In 1977, a severe drought reduced seed availability. Small, soft seeds were rapidly depleted, leaving primarily large, hard seeds. Researchers measured beak depth in the medium ground finch (Geospiza fortis) population before and after the drought.