Natural Selection · Paper 2 — Free Biology HL Practice Questions

In a population of marine iguanas (Amblyrhynchus cristatus) on the Galápagos island of San Cristóbal, researchers measured the body length of 500 adult iguanas. Iguanas with a medium body length ($60$–$70\,\text{cm}$) were best able to dive and feed on algae in the intertidal zone. Very small iguanas (under $50\,\text{cm}$) were easily swept away by strong currents, while very large iguanas (over $80\,\text{cm}$) had difficulty manoeuvring in rocky crevices where algae grow. Over a 10-year period, researchers recorded lifetime reproductive output: Size class — Body length — Mean offspring per lifetime Small — $<50\,\text{cm}$ — $2$ Medium — $60$–$70\,\text{cm}$ — $8$ Large — $>80\,\text{cm}$ — $3$

A population of Darwin's finches (Geospiza fortis) on the island of Daphne Major in the Galápagos experiences a severe drought in 1977. The drought reduces the availability of small, soft seeds, forcing finches to eat large, hard seeds that require a deep beak to crack open. Researchers measured beak depth (a heritable trait) before and after the drought. Before drought (1976) — $9.2\,\text{mm}$ — $0.8\,\text{mm}$ After drought (1978) — $10.1\,\text{mm}$ — $0.6\,\text{mm}$