adaptive radiation

Meaning: One ancestral lineage gives rise to many species, each adapted to a different niche, over a relatively short evolutionary time. It is usually set off by ecological opportunity: colonizing a new island chain or lake, evolving a key adaptation, or surviving a mass extinction that leaves niches empty. With little competition, populations exploit different resources, selection favors different heritable traits in each, and reproductive isolation (often allopatric at first) makes them separate species. Examples are Darwin's finches and Hawaiian honeycreepers.

On the MCAT: A passage describes colonists reaching an island group, with descendants that differ in beak size, diet or habitat. You identify divergent evolution, niche partitioning and speciation. Predict that more open niches and less competition favor greater diversification, and that competition between related species can drive further specialization. Questions may ask whether the resulting structures are homologous.

Common confusion: Do not mix it up with convergent evolution. Adaptive radiation produces different traits from a common ancestor (homologous structures). Convergent evolution produces similar traits in unrelated lineages facing similar pressures (analogous structures). It is also not individuals changing to meet a need; it is a shift in allele frequencies across generations through natural selection.