Coevolution explained with six examples

Discover how coevolution works, who discovered this phenomenon and examples of it across the natural world.

What is coevolution?

In nature, species sometimes interact very closely. For example, predators interact closely with their prey and parasites with their hosts. Where these interactions occur, species can place selective pressures on each other, which influences each other’s evolution. This process is known as coevolution.

An evolutionary arms race can be sparked by coevolution. This is an ongoing process where species evolve adaptations and counteradaptations against one another in order to survive.

For example, some bats hunt hawkmoths. To defend themselves, some hawkmoths evolved the ability to make ultrasonic noises that temporarily upset the bats’ sonar, which the bats use to find the hawkmoths in the dark. In return, some bats solve this problem by using echolocation at a pitch the hawkmoths can’t detect to locate them.

Are there different types of coevolution?

Coevolution can take several forms. For example:

• Pairwise or specific coevolution involves two species that strongly influence each other, such as a particular plant and its specialist pollinator.

• Diffuse coevolution is also known as multi-species or guild coevolution. It occurs when several species collectively influence one another. An example is pollination systems in which plants interact with multiple pollinators, such as bees, flies and butterflies.

• Gene-for-gene coevolution occurs when a host evolves a gene to protect itself from a pathogen. In response the pathogen evolves a gene to avoid or overcome that defence.

How is coevolution linked to natural selection?

Natural selection is the mechanism that drives coevolution. In natural selection, individuals that survive and reproduce more effectively in response to evolutionary pressures are more likely to pass their genes on to the next generation. Over time, this leads to genetic changes.

Many different environmental pressures affect evolution by natural selection. For example, some species evolve traits that help them to survive in extreme environments. In coevolution, the environmental pressures come from interactions with another species.

Who discovered coevolution?

The idea of interacting species influencing each other’s evolution can be traced back to Charles Darwin.

In his most famous work On the Origin of Species he discussed evolutionary relationships between flowering plants and insects. He used the term ‘coadaptation’ to describe how they could shape each other’s traits.

The modern concept of coevolution gained prominence in the 1960s, following the work of American scientists Paul Ehrlich and Peter Raven. In their paper Butterflies and Plants: A Study of Coevolution, they proposed that plants and insects evolve together through reciprocal selective pressures.

Six examples of coevolution

Coevolution occurs across nature. Here are some examples, which is your favourite?

Figs and fig wasps

Unlike typical flowers, figs have their floral structures enclosed within a hollow chamber. Inside there are tiny male and female flowers, which are only accessible through a small opening called an ostiole.

So, if the flowers are hard to reach, how does pollination occur? The answer lies in fig wasps. A female fig wasp carrying pollen from another fig enters through the ostiole. They’re adapted for crawling through this narrow opening, having evolved a specialised head shape and wings that can easily tear off.

Once inside, the female fig wasp does two things – lays her eggs and pollinates the female flowers, which allows them to mature into the fig fruits we eat. When her eggs hatch and the larvae develop, the new male and female wasps mate. The males then chew exit tunnels but never leave – they and the original female die inside the fig. However, the new females collect the plant’s pollen and exit the fig. From there they fly off to repeat the cycle.

“It’s a classic example of coevolution,” explains Dr Gavin Broad, our Principal Curator in Charge of Insects. “It’s an obligate relationship where the figs need the wasps and the wasps need the figs.”

The fig offers a relatively safe environment with all the nutrients needed for the wasp larvae to develop. In turn, the fig sacrifices a percentage of its developing seeds in exchange for the pollination service.

Mirror orchids and scoliid wasps

Mirror orchids have evolved flowers that closely mimic the appearance, texture and chemical signals of a single species of scoliid wasp called Dasyscolia ciliata. In response to these cues, male wasps attempt to mate with the flower – a behaviour called pseudocopulation – during which pollen attaches to the wasp’s body. When the deceived male visits another orchid to attempt to mate, pollination occurs.

This interaction has strongly influenced the orchid’s evolution, including traits such as flower shape, colouration, reflective surfaces and species-specific pheromone scents.

In return, the wasps experience selection to better discriminate between real females and deceptive flowers. However, this is an example of asymmetrical coevolution. While the orchids benefit from successful pollination, the wasps incur costs, including reduced mating opportunities and wasted time and energy.

Darwin’s star orchid and hawkmoths

Darwin’s star orchid, which is native to Madagascar, has an exceptionally long nectar spur – often about 25–35 centimetres long. The nectar is located only at the very tip. When naturalists Charles Darwin and Alfred Russell Wallace each examined the flower, they predicted that a moth with an equally long proboscis must exist to reach the nectar and pollinate the orchid. At the time, no such moth was known. Decades later, a hawkmoth – known as Wallace’s sphinx moth – with a proboscis long enough to do so was discovered.

The coevolutionary process involves reciprocal pressure on both species. Orchids with longer spurs are more likely to be pollinated by moths that push their heads deep into the flower, ensuring contact with the pollen. At the same time, moths with longer proboscides gain access to nectar that shorter-tongued competitors can’t reach, giving them a feeding advantage.

This kind of plant-insect relationship occurs in other species too. For example, horseflies in the genus Philoliche have exceptionally long, rigid proboscis to reach inside plants with long floral tubes. However, they’re not always strictly specialised to a single plant species, and the plants may be visited by multiple long-tongued pollinators – this is known as diffuse coevolution.

Ants and aphids

Ants and aphids have experienced coevolution where both parties benefit. Aphids feed on plants and excrete a sugar-rich liquid called honeydew, which is attractive to ants. The ants ‘milk’ this honeydew from the aphids with their antenna. In return, the ants provide the aphids with protection from predators.

Due to the mutualistic benefits, many aphid species have evolved traits that increase their value to ants. For example, they’ll produce larger quantities of honeydew with adjusted sugar composition and release chemical signals to attract the ants.

Pitcher plants and mosquitoes

Pitcher plants have tubular leaves that are filled with fluid that traps and drowns insects. The plant then uses enzymes to break these insects down for their nutrients. However, the pitcher plant mosquito has found a way to live and even reproduce inside this fluid-filled trap.

“Pitcher plants are carnivorous and they destroy lots of other insects, but this mosquito has learned to live in the plant,” explains Dr Erica McAlister, our Principal Curator of Flies and Fleas.

“As the pitcher plant ages, its digestive enzymes weaken, so it needs the little mosquito larvae to go around and break down the dead animals into smaller bits that are small enough for the enzymes of the pitcher plant to get enough nutrients from. This amazing relationship has developed between these two apex predators together in this tiny death zone,” remarks Erica.

Female mosquitoes lay their eggs on the inner walls of the pitcher. When the larvae hatch, they develop in the pitcher fluid, having evolved resistance to the acidic, enzyme-rich conditions that kill a lot of other insects. They feed on bacteria and decomposing prey inside the pitcher rather than on the plant itself.

The pitcher plant tolerates the mosquito because its presence doesn’t significantly reduce prey capture and enhances nutrient processing, while the mosquito depends almost entirely on the pitcher plant for reproduction.

Fungi and photobionts

Lichens are formed through a close symbiotic association between two different organisms – a fungus and a photosynthetic partner, which may be a green alga, a cyanobacterium or, in some cases, both.

The fungus forms the main body of the lichen, creating a protective structure that surrounds and houses the photosynthetic partner. This structure provides support and protection from environmental stress as well as helping to retain water and minerals. In exchange for shelter and access to water and nutrients, the photosynthetic partner supplies the fungus with sugars that it can’t produce itself.

Together, the fungus and the photosynthetic partner form a single functional unit known as a lichen, which can survive in environments that would be too harsh for either organism by itself.

“Lichen have produced unique traits for living in extreme environments. An individual organism such as the fungus, algae or cyanobacteria may not be able to survive in an extreme environment such as a desert or hot spring. But because of their coevolution process, they’ve developed the ability to establish successfully in these places,” says Dr Gothamie Weerakoon, our Senior Curator of Lichens and Slime Moulds.

So, the next time you see wildlife in your local green space, think of all the other species that it might be evolving with.