C. Evolution · EvolutionPatterns and Pace: Adaptive Radiation, Convergence, Coevolution and Punctuated Equilibrium
C3.4 C3.3 C2.3 C2.1About 70 minutes
Learning goals- I can describe divergent evolution and adaptive radiation, using Darwin's finches as an example.
- I can explain convergent evolution and coevolution with examples.
- I can compare gradualism with punctuated equilibrium and describe the contribution of Eldredge and Gould.
- I can explain how the fossil record is used to test ideas about the pace of evolution.
Words to know
divergent evolution
When related populations or species become more and more different over time.
adaptive radiation
Rapid divergent evolution of one ancestral species into many species, each adapted to a different niche.
convergent evolution
When unrelated species evolve similar traits because they live in similar environments or have similar ways of life.
coevolution
When two or more species evolve in response to each other (e.g., flowers and their pollinators, predators and prey).
gradualism
The idea that evolution happens through slow, steady change over long periods.
punctuated equilibrium
The idea that species stay much the same for long periods (stasis) and change relatively quickly when new species form.
stasis
A long period with little change in a species' form.
Big patterns from small steps
So far you have seen how populations change: mutation, selection, drift, gene flow, and how new species split off. Zoom out over millions of years and those small steps create big patterns in the history of life. Some groups explode into dozens of species; unrelated animals end up looking alike; species become locked in arms races with each other.
There is also a lively scientific debate about the pace of evolution: slow and steady, or long pauses and sudden bursts? This lesson covers both the patterns and the pace.
Divergent evolution and adaptive radiation
In divergent evolution, related populations become more different over time as they adapt to different environments. Homologous structures (like vertebrate forelimbs) are evidence of it.
When one ancestral species diverges rapidly into many species, each filling a different niche, it is called adaptive radiation. It usually happens when there are many empty niches:
- after a species colonizes new islands or lakes with few competitors;
- after a mass extinction removes many competitors. After the extinction of the non-avian dinosaurs 66 million years ago, mammals radiated into forms as different as bats, whales, horses and primates.
Darwin's finches are the classic example. Biologists recognize more than a dozen species on the Galápagos, all descended from a small group of ancestors that arrived from the mainland a few million years ago. Their beaks are adapted to different foods: large, deep beaks crack hard seeds; slender beaks probe flowers for nectar or catch insects; one species even uses twigs or cactus spines as tools to pry insects out of wood.
Convergent evolution
In convergent evolution, unrelated species evolve similar traits because they face similar challenges. The traits are analogous (similar function, different origin).
- Streamlined swimmers: sharks (fish), dolphins (mammals) and the extinct ichthyosaurs (reptiles) all evolved torpedo-shaped bodies and fins.
- Wings: insects, birds and bats each evolved flight separately.
- Desert plants: cacti in the Americas and some euphorbias (spurges) in Africa are not closely related, yet both evolved thick, water-storing stems, spines and reduced leaves.
Convergence shows that natural selection can find similar solutions to similar problems, starting from different ancestors.
Coevolution
In coevolution, two species are such strong selective forces on each other that a change in one leads to a change in the other, back and forth.
- Flowers and pollinators. Hummingbirds have long bills and are attracted to red, tube-shaped flowers rich in nectar; many such flowers are hard for bees to use. Some orchids have nectar tubes so long that only moths with very long tongues can reach the nectar.
- Predators and prey. Faster prey select for faster predators, and the reverse: an evolutionary "arms race."
- Plants and herbivores. Milkweeds make toxic chemicals that deter most insects. Monarch caterpillars evolved the ability to tolerate these toxins and store them, which makes monarchs distasteful to birds (connecting to the mimicry you learned about in the first lesson).
- Hosts and parasites, including diseases. As a host evolves resistance, the parasite evolves new ways to infect.
Worked exampleExample 1: Let's name the pattern
Name the pattern: (a) The cichlid fish of Lake Victoria in Africa have diversified into hundreds of species with different diets. (b) The marsupial sugar glider of Australia and the placental flying squirrel of North America both glide on flaps of skin. (c) A bat species hunts moths using sound; some moths evolved ears that detect bat calls and dive away, and some bats evolved calls the moths hear less well.
- (a) One ancestral group splitting rapidly into many species filling different niches: adaptive radiation (divergent evolution).
- (b) Unrelated animals (a marsupial and a placental mammal) evolving similar gliding membranes: convergent evolution.
- (c) Each species' adaptation selects for a counter-adaptation in the other: coevolution.
(a) adaptive radiation; (b) convergent evolution; (c) coevolution.
The pace of evolution: gradualism vs punctuated equilibrium
Darwin, inspired by Lyell, pictured evolution as gradual: tiny changes adding up steadily over long periods. If that were always true, the fossil record would show many smooth series of in-between forms.
In 1972, the American palaeontologists Niles Eldredge and Stephen Jay Gould pointed out that, for many species, the fossil record shows something else: species appear, stay much the same for millions of years (stasis), and then new species appear relatively suddenly (in geological terms). They proposed punctuated equilibrium: most change happens in short bursts, often when a small population splits off and forms a new species, followed by long periods of little change.
"Suddenly" in geology can still mean tens of thousands of years, which is quick compared with millions of years of stasis but slow by human standards. Speciation in small, isolated populations (where drift and strong selection act quickly) would also leave few fossils, because small populations leave few remains.
Today most biologists agree that both patterns occur. Some lineages change gradually; others show long stasis and rapid change. The debate led scientists to measure rates of change in the fossil record far more carefully.
Gould also wrote many popular books on evolution, including Wonderful Life (1989), about the remarkable animals of Canada's Burgess Shale.
Worked exampleExample 2: Let's interpret fossil data
In one rock sequence covering 5 million years, a snail species shows the same shell shape in every layer for 4.9 million years. Then, in layers covering about 50 000 years, a new species with a very different shell appears and the old one disappears from that site. (a) Which model does this support? (b) Give one alternative explanation a careful scientist should check.
- (a) Long stasis (4.9 million years) followed by rapid change (about 50 000 years, which is 1% of the time span): this fits punctuated equilibrium.
- (b) Alternative: the new species may have evolved gradually somewhere else and then migrated into this site, replacing the old species. Or some rock layers may be missing (erosion), making change look faster than it was. Scientists would look at other sites and check that the layers are continuous.
The pattern supports punctuated equilibrium, but migration from elsewhere or missing rock layers must be ruled out.
Just for funCoevolution is a never-ending arms race. Biologists call it the Red Queen hypothesis, after the Red Queen in Through the Looking-Glass, who tells Alice: "it takes all the running you can do, to keep in the same place." Species have to keep evolving just to keep up with each other.
Hands-on activityPasta radiation: model adaptive radiation
You need: about 10 kinds of dry pasta, beans or buttons (different shapes and sizes), a sheet of paper, glue or tape, and pens.
- Choose one simple item (for example an elbow macaroni) as your ancestor. Glue it at the bottom centre of the paper.
- Imagine the ancestor reaching a group of islands with five empty niches: hard seeds, soft fruit, insects in bark, nectar, and fish in shallow water.
- Draw a branching tree from the ancestor to five "species" (choose pasta shapes for each) and write the niche and one adaptation next to each tip (for example, shell pasta: deep beak for cracking hard seeds).
- Add one example of convergence: a species in a different part of your tree that evolved a similar shape for a similar niche. Mark it with a star.
- Write two sentences explaining why adaptive radiation happens quickly when niches are empty.
Keep small items away from young children (choking hazard).
What to look for: a branching tree from one ancestor; each species matched to a niche with a sensible adaptation; a correctly explained convergence; the explanation that empty niches mean little competition, so populations using new resources are strongly favoured and diverge quickly.
Your turn
1What is adaptive radiation?
Rapid divergent evolution of one ancestral species into many species, each adapted to a different niche.
2Give two situations that often trigger adaptive radiation.
Colonizing new islands or lakes with empty niches, and the aftermath of a mass extinction.
3Sharks and dolphins have similar body shapes. Which pattern of evolution does this show?
4Define coevolution and give one example.
Two species evolving in response to each other. Examples: flowers and pollinators (hummingbirds and red tubular flowers), milkweed and monarchs, predators and prey, bats and moths.
5Who proposed punctuated equilibrium, and in what year?
Niles Eldredge and Stephen Jay Gould, in 1972.
6How does punctuated equilibrium differ from gradualism?
Gradualism: slow, steady change. Punctuated equilibrium: long periods of little change (stasis) with rapid change when new species form.
7Why do convergent traits not show close relationship?
They evolved independently in unrelated lineages because of similar environments; they are analogous, not inherited from a common ancestor.
More practice: warm-up
A long period with little change in a species' form.
2Which group of birds is the classic example of adaptive radiation?
Darwin's finches on the Galápagos.
3Are the wings of birds and insects homologous or analogous?
4What happened to mammals after the extinction of the non-avian dinosaurs?
They underwent adaptive radiation into many new forms (e.g., bats, whales, horses, primates).
5Which Canadian fossil site did Gould write about in Wonderful Life?
The Burgess Shale (British Columbia).
More practice: on your own
1Explain why Darwin's finches have such a variety of beak shapes.
They descended from a few ancestors that reached islands with many empty niches. Populations on different islands and using different foods faced different selective pressures on beak shape, and with limited gene flow they diverged into separate species, each with a beak suited to its food: adaptive radiation.
2Explain why many flowers pollinated by hummingbirds are red and tube-shaped, while many bee-pollinated flowers are not.
Through coevolution, flowers that were red and tubular attracted hummingbirds (which see red well and have long bills) while excluding many insects; hummingbirds carried their pollen to the same kind of flower efficiently, so these flower traits were favoured. Bees see red poorly and prefer other colours and shapes, so bee-pollinated flowers evolved differently.
3Explain how the milkweed–monarch relationship is an example of coevolution and connects to mimicry.
Milkweed evolved toxins that deter herbivores; monarch caterpillars evolved tolerance and store the toxins. That makes monarchs distasteful to birds, and their bright warning colours advertise it. Other bad-tasting butterflies such as viceroys share the pattern (Müllerian mimicry).
4A fossil sequence shows a gradual, steady increase in body size of an animal over 10 million years. Which model does this support? Does it disprove the other model?
It supports gradualism for that lineage. It does not disprove punctuated equilibrium, because most biologists think both patterns occur in different lineages.
5Why might punctuated equilibrium leave few fossils of in-between forms?
Rapid change often happens in small, isolated populations and over (geologically) short times. Small populations over short periods leave few fossils, so the transition is rarely preserved.
Challenge
1The Hawaiian honeycreepers are a group of birds with very different beaks (some for seeds, some for nectar, some for insects), all thought to descend from one ancestral finch-like species. Use the ideas of empty niches, isolation and selection to explain how this group formed.
A few founders reached the isolated Hawaiian islands, which had many empty niches and few competing bird species. Populations on different islands or in different habitats were isolated (little gene flow) and faced different selective pressures (different foods), so beak shapes diverged. Repeated colonization of islands and divergence produced many species: an adaptive radiation, much like Darwin's finches.
2Eyes have evolved independently many times (for example, in octopuses and in vertebrates), and octopus and human eyes are remarkably similar in design. Explain whether this is homology or convergence and what it tells us about natural selection.
It is convergent evolution: octopuses (molluscs) and vertebrates are only distantly related, and their common ancestor did not have a camera-type eye. The similar design arose independently because the same physical problem (forming a clear image) favoured similar solutions. It shows that natural selection can reach similar solutions from different starting points.
3Explain how the punctuated equilibrium debate is an example of how science works.
Eldredge and Gould proposed a new hypothesis based on patterns in the evidence (stasis in the fossil record). Other scientists tested it by measuring rates of change in many fossil sequences. The result was a refined understanding (both patterns occur), not the overthrow of evolution. Science advances by testing and revising ideas against evidence.
Quiz
1. One ancestral species gives rise to many species filling different niches. This is:
2. Cacti in the Americas and euphorbias in Africa look alike because of:
3. Flowers and their pollinators changing in response to each other is:
4. Punctuated equilibrium was proposed by:
5. Which pattern does long stasis followed by rapid change support?
6. Adaptive radiation is most likely when:
Big idea (C3.4, C3.3, C2.3): over long times, evolution produces patterns (divergence, adaptive radiation, convergence, coevolution). Eldredge and Gould's punctuated equilibrium (C2.3) is presented as a scientific debate about the pace of evolution, now largely resolved as "both patterns occur."
How to help: The pasta radiation activity is fun to do together. On a walk or in a garden, look for coevolution: which insects visit which flowers?
Common difficulties:
- Thinking punctuated equilibrium disproves Darwin. It refines ideas about pace; the mechanism (natural selection and other processes) is the same.
- Mixing up convergent and divergent. Divergent = related species becoming different; convergent = unrelated species becoming similar.
- Taking "sudden" literally. In geology, rapid can still mean tens of thousands of years.
Sources: Darwin's finches, adaptive radiation after the end-Cretaceous extinction, convergent evolution examples, milkweed-monarch coevolution, Eldredge and Gould (1972), Gould's Wonderful Life (1989) and the Red Queen quotation from Lewis Carroll are well documented.