Why did mammals diversify so rapidly and extensively after the end-Cretaceous extinction 66 million years ago, filling ecological roles as large as elephants and as specialized as bats?
AThe extinction caused a dramatic increase in mammalian mutation rates, accelerating evolution
BMammals had already evolved most of the necessary adaptations during the Cretaceous and were waiting
CThe extinction of dinosaurs freed enormous ecological opportunity — available niches with no competitors — driving rapid diversification through natural selection
DMammals reproduced faster after the extinction, generating more variation for selection to act on
This is a textbook example of ecological opportunity driving adaptive radiation. When the dinosaurs went extinct, they vacated dozens of ecological roles — large herbivore, apex predator, aerial hunter, aquatic predator. Mammals had existed for over 100 million years but remained small and ecologically restricted while dinosaurs occupied those niches. The removal of competitors created open ecological space, and natural selection rapidly diversified the mammalian lineage to fill it. Mutation rates and reproductive speed (A, D) weren't the drivers — the rate of diversification was governed by available niche space, not genetics.
Question 2 Multiple Choice
Darwin's finches on the Galápagos show a characteristic pattern in their molecular phylogeny: the earliest lineage splits occurred fastest, and diversification slowed over time. What best explains this 'early burst' pattern?
AEarly finch populations had higher genetic diversity, which was depleted over generations
BInitial colonizers found many open niches with little competition; as those niches filled, opportunities for further divergence diminished
CGeographic isolation between islands decreased over time as the islands drifted closer together
DNatural selection becomes less effective once species reach an optimal body size
The early burst pattern reflects the logic of ecological opportunity: at the start, a radiating lineage encounters many unused niches, so divergent selection is strong and speciation is rapid. As those niches fill with specialized species, new arrivals face competitors and resources are scarcer, slowing further diversification. This is not about genetics running out (A) but about ecological space being saturated. The pattern appears in both molecular phylogenies and the fossil record across many adaptive radiations — cichlids, Hawaiian honeycreepers, Caribbean Anolis lizards — making it a signature of radiation driven by ecological opportunity.
Question 3 True / False
Adaptive radiation is primarily driven by increases in genetic mutation rate, which produce more variation for selection to act on during periods of rapid diversification.
TTrue
FFalse
Answer: False
Ecological opportunity — not mutation rate — is the primary driver of adaptive radiation. The ancestral lineage of Darwin's finches had the same mutation rate before and after colonizing the Galápagos; what changed was the availability of unoccupied ecological niches. Radiations require that genetic variation is available (it generally is), but the pace and direction of diversification are governed by ecological factors: what niches are open, what competitors are absent, and what key innovations allow access to new resources. Mutation provides raw material; ecology provides the directional pressure.
Question 4 True / False
Adaptive radiation typically produces a burst of early rapid speciation that decelerates over time as ecological niches become occupied.
TTrue
FFalse
Answer: True
This 'early burst' pattern is one of the defining characteristics of adaptive radiations and appears in both the fossil record and molecular phylogenies. Early in a radiation, many niches are empty and divergent selection is strong — each new variant that exploits a different resource gains a competitive advantage. As niche space fills, opportunities diminish, competition increases, and the rate of new speciation slows. The result is a phylogenetic tree with rapid early branching and increasingly sparse later branching, sometimes called a 'pulled-down' tree shape.
Question 5 Short Answer
Why is ecological opportunity considered the key ingredient of adaptive radiation, rather than genetic factors like high mutation rates or large population sizes?
Think about your answer, then reveal below.
Model answer: Genetic variation is necessary but not sufficient for adaptive radiation. Most lineages have sufficient genetic variation to diversify but don't undergo rapid radiation — they remain ecologically constrained by competitors already occupying available niches. What makes radiation possible is the absence of those constraints: open niches, no competitors, and sometimes a key innovation that unlocks previously inaccessible resources. When ecological opportunity disappears (niches fill), radiation slows regardless of continued genetic variation. The pace and pattern of speciation are fundamentally governed by what ecological space is available, not by the speed at which new variants arise.
A telling example: finches existed for millions of years on continents with diverse ecological niches but didn't radiate because those niches were occupied. The same lineage radiated explosively on the Galápagos because the islands offered open space. Genetics was identical; ecology was different. This is why the concept of ecological opportunity is central to understanding why radiations happen when and where they do.