Adaptive radiation is the relatively rapid diversification of one ancestral species into several closely related species, each adapted to a different ecological niche. In IB Biology, this is Additional Higher Level content in A4.1 Evolution and speciation.
Adaptive radiation usually occurs where several niches are vacant-for example, after an ancestral species colonizes a new habitat or when extinction removes competing species. It increases biodiversity and can allow related species to coexist because they use different resources and therefore compete less directly.
How Adaptive Radiation Occurs
- An ancestral population contains heritable variation, generated by mutation and sexual reproduction.
- Individuals encounter different habitats, food sources or other selection pressures within the ecosystem.
- Different phenotypes have greater fitness in different niches, meaning that their genotypes have greater survival value and reproductive potential.
- Natural selection increases the frequency of advantageous alleles in each population.
- Reduced gene flow and reproductive isolation allow the populations to diverge genetically.
- Speciation produces several related species, each occupying a different niche.
Darwin’s finches are a classic example. An ancestral finch population diversified into species with different beak forms suited to different food sources. By specializing, the species reduced interspecific competition and could coexist.
A common misconception is that individual organisms deliberately adapt to available niches. This is incorrect: natural selection acts on existing heritable variation, and evolutionary change occurs in populations over generations. Adaptive radiation is also a form of divergent evolution, not convergent evolution, because related populations become increasingly different.
Exam Technique
For an IB “explain” question, present a linked sequence: vacant niches → different selection pressures → differential survival and reproduction → changing allele frequencies → reproductive isolation and speciation. Do not merely state that species “adapt”; identify natural selection, heritable variation, reduced competition and the formation of multiple closely related species.
