What is coevolution?
The word evolution means slow change in a group of living things over many generations. Coevolution means two (or more) groups change each other. The "co" means together.
It works through natural selection. Each species is part of the other's surroundings. When one species changes, the other faces a new test. Those with a better match survive and breed more. Their young carry the match forward.
Coevolution is not planning. The moth does not "want" a longer tongue. The ones that happen to have one simply leave more young.
Flower and moth: a partnership
Take a flower with nectar at the bottom of a long tube and a moth that drinks it.
- Moths with longer tongues reach more nectar, so they live longer and have more young.
- Flowers with longer tubes make the moth touch the pollen. Their seeds are made more often.
- The new, longer-tubed flowers now favour even longer-tongued moths. The cycle repeats.
Both sides gain, so this kind of coevolution is a mutualism. Many fruit trees and their seed-spreading birds, and flowers and bees, share such partnerships.
Arms races: hunters, prey, parasites and hosts
Not every partnership is friendly. In an evolutionary arms race each side keeps getting a better "weapon" against the other.
- Prey and hunter: faster deer, faster tigers; better camouflage, sharper eyes.
- Plant and plant-eater: a plant makes thorns or bitter chemicals, and the eater slowly learns to cope with them.
- Host and parasite or germ: hosts build better defences, parasites find ways round them. Bacteria that survive a medicine multiply, which is why medicines must be used carefully.
Here one side wins only for a while. The race can go on for millions of years.
How biodiversity forms
Biodiversity is the variety of life: variety of genes within a species, of species, and of ecosystems.
It forms in steps:
- Variation: mutations and the mixing of genes make members of a group slightly different.
- Isolation: a river, mountain, island or different habitat keeps groups apart, so their genes do not mix.
- Natural selection: each habitat has its own climate, food and partners, so each group changes in its own way.
- New species: after many generations the groups cannot breed together. They are now separate species (speciation).
Coevolution adds to this. A flower and a moth that fit each other in one habitat fit differently in another, so each habitat can grow its own matched pairs. Partners also make new homes and food for other species, so the web of life gets richer. Over about 3.5 billion years this has produced millions of species.
Why biodiversity matters and what can go wrong
Because species depend on partners, a loss spreads. If a special moth disappears, its flower may fail to make seeds. If a fig wasp is lost, the fig tree is lost. Cutting forests, pollution and climate change break such partnerships quickly, much faster than coevolution can build new ones. That is why protecting habitats protects whole sets of partners.
Try it: predict, then check
Before step 6, predict: if one habitat becomes two, how many species will you have? Then tap the buttons and count. At home: watch a flower for 10 minutes and note which insect or bird visits, where it touches the flower, and what it eats. Write down one thing the flower does to attract it and one thing the visitor does to help the flower.
Key formulas and definitions
- Coevolution = change in species A causes change in species B, and the change in B causes change in A
- Mutualism: both partners gain (flower + pollinator). Arms race: each side improves against the other (prey + hunter)
- Biodiversity forms: variation → isolation → natural selection in each habitat → new species
- Biodiversity has three levels: genes, species, ecosystems
Worked examples
1. A flower has nectar 4 cm deep. Most moths have tongues 2 cm to 3 cm long, but a few have 4 cm tongues. Explain what happens over many generations.
Only the 4 cm moths reach the nectar, so they live longer and have more young who inherit long tongues. The average tongue gets longer. If long-tubed flowers then get more pollen carried, the flower tube also gets longer, and the cycle continues.
2. Is a cactus growing spines to avoid being eaten coevolution?
Not by itself. It is an adaptation. It becomes coevolution if the animals that eat the cactus also change in response (for example, learn to avoid or get past the spines), and the cactus changes again.
3. Two islands have the same ancestral flower. Island A has short-tongued moths, island B has long-tongued moths. What will you probably see after many generations?
Flowers on island A will have short tubes, flowers on island B long tubes, because each matches its moth. If they stay apart long enough, they may become separate species. This adds to biodiversity.
4. Why does antibiotic misuse speed up an arms race between humans and bacteria?
Medicines kill most bacteria but not the rare ones with resistance. Those survive and multiply, so resistant bacteria become common and a new medicine is needed. Careful use slows this.
Common mistakes
- Saying the species "tries" or "wants" to change. Change happens because some members happen to match better and leave more young.
- Calling every adaptation coevolution. Coevolution needs change in both partners, each caused by the other.
- Thinking coevolution is always friendly. Arms races between hunter and prey, or host and parasite, are coevolution too.
- Mixing up the words biodiversity (the variety of life) and a single big habitat. Biodiversity counts genes, species and ecosystems.