IB Biology HLEvolution & SpeciationPaper 1 & 2~9 min read
Adaptive Radiation
Drop one species onto an island chain with nothing else living there, and something dramatic happens. Instead of one species slowly changing, you get many species quickly — each one doing a different job.
📚 What you need to know
Natural selection can produce the rapid evolution of multiple species from one common ancestor. This is adaptive radiation.
The new species are likely to share some similar features because of their shared ancestry.
The differences that arise often allow them to live together in one habitat, because they fill different ecological niches.
An organism’s ecological niche is its role within its ecosystem — the food it eats, the conditions it needs, the predators it provides food for.
Examples: Darwin’s finches in the Galapagos islands, and Hawaiian honeycreepers, a group of more than 50 bird species.
Adaptive radiation is the usual explanation for homologous structures.
What adaptive radiation is
Definition
Adaptive radiation is the rapid evolution of multiple species from a single common ancestor
The word “radiation” is doing the work here: think of species radiating outwards from one starting point, like spokes from the centre of a wheel. The word “rapid” matters too. This is not the usual slow accumulation of change — it is many speciation events packed into a short stretch of evolutionary time.
The reason it happens quickly is usually opportunity. A new island, a lake with no fish in it, or a habitat emptied by a mass extinction leaves lots of unused resources. Any variant that can exploit one of them faces very little competition, so selection strongly favours it.
Every one of these birds is still recognisably the same kind of animal. Only the tool on the front of the face has changed.
Niches: why the new species can coexist
Ordinarily, two very similar species living in the same place compete hard, and one usually loses. Adaptive radiation avoids that, because the differences that arise between the new species let them fill different ecological niches.
Definition
An ecological niche is an organism’s role within its ecosystem
A niche is more than an address. It includes the food that the organism eats, the environmental conditions it requires, and the predators it provides food for. Two species with genuinely different niches are not really competing, so they can share one habitat indefinitely.
Four species, one tree, four different jobs. Split the resources finely enough and there is room for everyone.
The two examples to know
Group
Where
What happened
Darwin’s finches
The Galapagos islands
Many species of small birds, observed by Darwin, differing mainly in beak shape and diet
Hawaiian honeycreepers
The Hawaiian archipelago
A group of more than 50 bird species, several of which co-exist on the same island by feeding differently
Both examples are island groups, and that is not a coincidence. Islands supply the two ingredients adaptive radiation needs at once: geographical isolation between islands so populations can diverge, and empty niches so almost any new variant has somewhere to make a living.
The link back to homologous structures
Adaptive radiation is the explanation usually offered for why homologous structures exist. Organisms with homologous structures all evolved from a shared common ancestor, but adapted to different environments in the process — which is exactly what a radiation is.
Be careful with how strongly you word this. Adaptive radiation does not provide proof that these organisms evolved from a common ancestor. It is a good explanation for the existence of homologous structures, and the evidence supports it, but “explains” is not the same as “proves”.
Worked examples
WE 1
Explaining a radiation
A single bird species reaches an island group with no other land birds. Within a few million years there are twelve species. Explain how this happened. (4 marks)
Point 1: isolation
Populations on different islands are geographically isolated, so there is no gene flow between them.
Point 2: different pressures
Each island offers different food and conditions, so different selection pressures act and different alleles become more frequent.
Point 3: reproductive isolation
The populations become reproductively isolated, so speciation occurs repeatedly.
Point 4: why it is fast
There is little competition because many niches are empty, so new variants survive easily. This is adaptive radiation.
Empty niches plus isolation equals many species, quicklythe “empty niche” point is what makes this adaptive radiation rather than ordinary speciation
WE 2
Living together without competing
Explain how several closely related honeycreeper species can co-exist on one island. (3 marks)
Point 1: the differences
Adaptive radiation produced differences between the species, for example in beak shape.
Point 2: different niches
Those differences let each species fill a different ecological niche — a different role in the ecosystem, including different food.
Point 3: the result
Because they use different resources they are not competing directly, so they can live in the same habitat.
Different niches, so no direct competitiondefine “niche” as a role, not a place — that is the mark most often lost here
💡 Exam tips
Learn the definition with the word rapid in it, and with common ancestor.
Define ecological niche as a role, and give two parts of it (food eaten, conditions required).
Name both examples: Darwin’s finches in the Galapagos, Hawaiian honeycreepers with more than 50 species.
Explain co-existence through different niches, not through the species “getting along”.
Link adaptive radiation to homologous structures if the question mentions shared body plans.
Say “explains”, not “proves”.
⚠ Common mistakes
Defining a niche as a habitat. A niche is a role; a habitat is a place.
Forgetting the shared ancestor. Adaptive radiation starts from one species, which is what makes the new species similar.
Confusing it with convergent evolution. Radiation spreads apart from one ancestor; convergence brings unrelated lineages together.
Saying the birds changed their beaks to suit the food. Variation in beak shape came first, then selection.
Claiming it proves common ancestry. It is an explanation supported by evidence.
Up next: Speciation in Plants — the one situation where a brand new species can appear in a single generation.
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