21.1 Adaptive Radiation
Key Takeaways
Adaptive radiation is one ancestral lineage splitting into many species that use different ecological niches.
The pattern is speciation plus ecological divergence, often after a key innovation or after a lineage reaches open niches.
Darwin's finches on the Galapagos and Hawaiian honeycreepers are island radiations in which beak shape matches different foods.
Mammals diversified into many ecological roles after the end-Cretaceous extinction, which removed the non-avian dinosaurs.
A larger population of one species is only growth, and similar bodies in unrelated animals are convergence, not a radiation.
21.1 Adaptive Radiation
Adaptive radiation is the splitting of one ancestral lineage into many species that use different ecological niches. A niche is the way a species makes a living: the foods it takes and the places it uses. The descendants are relatives, and traits tied to those roles have diverged. The pattern is speciation plus ecological divergence. It is not one mutation, and it is not a larger head count of a single species.
Two settings make a radiation especially likely. A key innovation is a new trait that opens ways of living the ancestor did not use, such as a feeding structure that can later be modified for many foods. The other setting is open niches, resources few competitors already use. Islands favor the pattern because a colonizing lineage can meet foods that no similar resident already uses. Habitats left open after a mass extinction do the same work on a continent. One mutation does not finish the job. Repeated speciation over many generations produces the species.
Darwin's finches on the Galapagos
Darwin's finches are more than a dozen related species on the Galapagos Islands, plus a close relative on Cocos Island. They descend from one ancestral lineage that reached the islands from South America. The islands offered seeds, insects, and cactus, without a full set of similar birds already using those foods. From that start the lineage split into species whose beaks match different diets.
A large ground finch has a deep, strong beak and cracks large hard seeds. A cactus finch has a longer beak and feeds on cactus flowers and fruit and on insects that live on the cactus. A warbler finch has a slender beak and takes small insects. The woodpecker finch uses a twig or a cactus spine to pull insects out of bark. These are separate species with separate feeding roles. They are not one flock that only became more numerous while every bird kept the same diet.
Selection inside one species is real. During a severe drought on a small Galapagos island, hard seeds remained, and medium ground finches with deeper beaks survived better. That episode is selection in one population. It is not the radiation. The radiation is repeated speciation, with descendant species tied to different foods. A bill shape is evidence of radiation only when both the species and the foods have split.
Hawaiian honeycreepers
Hawaiian honeycreepers show the same island pattern in a different ocean. They descend from one finch-like ancestor that colonized the Hawaiian Islands. Many plant and insect foods were available, and few competing songbirds were already there. That lineage diversified into a large collection of species, and the bill again tracks the niche.
The iiwi has a long downcurved bill and drinks nectar from tubular flowers. The akiapolaau has a straight lower mandible, used like a pick, and a curved upper mandible, used as a probe, so it takes insects from wood in a woodpecker-like way even though it is not a woodpecker. The palila has a thick bill and cracks seeds of the mamane tree. One ancestor, many bills, many foods: that is the radiation.
Many honeycreeper species are now extinct or endangered. Habitat loss, introduced predators, and avian malaria carried by introduced mosquitoes removed species the radiation had produced. A later loss is not a new radiation, and a smaller surviving population is not evidence that the original split was only growth.
Mammals after the end-Cretaceous extinction
The third example is continental. Mammals already existed before the end of the Cretaceous, and most of them were small. The end-Cretaceous extinction removed the non-avian dinosaurs and many other animal lineages. Ecological roles those animals had filled were left open. After the extinction, mammals diversified into large herbivores, carnivores, burrowers, climbers, gliding and flying forms, and, later, fully aquatic forms such as whales. That spread of relatives into different roles is an adaptive radiation into open niches.
The extinction was not the origin of mammals, and it was not a plan aimed at mammals or at humans. Marsupials in Australia also diversified into many roles, another radiation of one lineage. When an Australian marsupial resembles a placental mammal elsewhere, that resemblance is convergence.
What a radiation is not
Population growth only changes how many individuals are alive. A flock of seed-eating finches can double and remain one species on the same seeds. No speciation occurred, and the niche did not split.
Convergent evolution produces similar traits in lineages that are not close relatives. Sharks and dolphins both have streamlined bodies, but dolphins are mammals and sharks are fishes. Radiation makes close relatives more different in how they live. Convergence makes distant lineages more alike. If the animals only look alike, do not call the pattern a radiation.
| Question to ask | Darwin's finches | A flock that only grows |
|---|---|---|
| How many species does the story produce? | More than a dozen species across the islands | One species |
| Do the animals use different foods? | Yes. Beak shape tracks seeds, cactus, or insects | No. The same seeds as before |
| What changed? | Speciation plus ecological divergence | The number of individuals |
| Is one mutation the whole pattern? | No. Many generations and many splits | No new species formed |
Warning
Adaptive radiation is not convergent evolution. Radiation is one lineage splitting into many species that use different niches, often after a key innovation or after arrival where niches are open. Convergence is unrelated animals becoming similar because they live in similar ways. A larger population of one species is only growth.
A three-part check
- Name one ancestor, then several descendant species, then different foods or habitats.
- A population that only grows, or that changes during one drought, has not radiated.
- A key innovation or open niches, on islands or after the end-Cretaceous extinction, is the opportunity. One mutation still does not build every species.
One ancestor, many species, and different roles means adaptive radiation. A bigger flock is growth. Similar shapes in distant relatives belong with convergence.
Which statement describes adaptive radiation?
Unrelated animals evolve similar shapes because they live in similar environments.
A single mutation appears, and that mutation by itself is the radiation.
One ancestral lineage splits into many species that use different ecological niches.
One population grows larger while every individual keeps the same niche.
Which example is an adaptive radiation?
An insect wing and a bird wing both produce lift, although insects and birds are not close relatives.
Hawaiian honeycreepers descended from one finch-like ancestor and diversified into species with different bills and foods.
Sharks and dolphins both have streamlined bodies, although dolphins breathe air and nurse their young.
A seed-eating finch population on one island doubles, and every bird still cracks the same seeds.
During a drought, medium ground finches with deeper beaks survive better because the remaining seeds are hard. How should that observation be used?
The observation shows convergent evolution, because the surviving birds belong to unrelated families that only look similar.
The observation shows only that the population grew, so radiation and growth are the same thing.
The observation shows natural selection on a trait. The Galapagos finch radiation is the larger pattern of many species with different beaks and foods.
The drought itself is the adaptive radiation, because one season of selection created every finch species.
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