We tend to think of evolution as a march through time, a climb out of the primeval ooze. In reality evolution is more like an 87 dimensional game of Snakes and Ladders (also called, as I am told by those who spent their childhood in the US of A, Chutes and Ladders). There are many directions in which you can go up and many that will take you down. One important reason for this is that the evolutionary landscape is really a stormy sea. It isn't static and it isn't wholly blind to the fate of the organism. In fact major components of the environment are evolving in concert.
We think of the environment as exerting a continuous influence resulting in evolutionary peaks, analogous to this volcano in the Andes (on the left). But the biological environment is changeable, like the clouds around the mountain.
What we're talking about this evening in the more ancient world is coevolution. The basic idea here is that a major part of the environment, imposing natural selection on an organism, is other organisms. Each interaction between species imposes natural selection on both species. If one species (species A) evolves then it is going to impose a different selection pressure on the other species (species B). If B evolves in response then the selection it imposes on A will change. And so the two species will evolve in concert.
Why do we care? Because coevolution is one of the primary drivers of biodiversity on the planet.
This may seem a bit vague and abstract so let's look at some examples. Ecologists recognize 3 major types of interactions between species. They are competition, predation, and mutualism. Let's look at each in turn.
1. Competition. When two species utilize the same resources they compete. Competition means fewer resources to go around. One response to competition is to evolve to be unlike your competitor. This is called character displacement. This is a controversial topic but some definite examples are known. One of the best documented involves two species of Darwin's finches in the Galapagos, Geospiza fuliginosa and Geospiza fortis. These species occur on several different islands in the Galapagos. When one of the two species occurs alone it has an intermediate sized beak, allowing it to eat a wide range of seeds of different sizes. When both species occur together, G. fuliginosa has a smaller beak than it would otherwise and G. fortis has a larger beak than it would otherwise.
An even more dramatic example occurs in sticklebacks, small fish of northern waters. One species, the three-spined stickleback occurs in both salt and freshwater. The ancestral form occurs in the ocean and is heavily armored. After the last ice age the ocean form has repeatedly colonized freshwater environment and each time has independently evolved a less armored freshwater form. The really cool thing is that if a second invasion of freshwater occurs in the same lake then two different forms evolve, one that lives in the open water and one that lives close to the bottom.
Two forms of sticklebacks. These specimens have been prepared to show differences in gene expression.
2. Predation Ecologists use this term in a somewhat broader fashion than most people. Basically anytime one organism eats another, that's predation. If a mosquito bites you, a tapeworm takes up residence in your gut, a cow eats some grass, those are all examples of predation.
It is common to talk of coevolutionary arms races, the idea being that prey don't 'want' to get eaten. The selection imposed by a predator results in the prey evolving in a way that reduces the chances of being eaten. For example a rabbit species might evolve longer legs. That in turn imposes selection on the predator favouring traits that increase the probability of getting some dinner.
One issue here is that many species are generalists and eat lots of things. The selection pressure imposed by any one prey item is likely to be small. However there are many examples of highly specific predator prey relationships such as parasites and their hosts.
Caterpillar under attack by the larvae of parasitic wasps. The wasps have evolved mechanisms to evade the immune systems of their prey.
Some generalists responses to predation. Crypsis (being hard to see as exemplified by the gecko and the katydid) and mimicry of eyes as exemplified by the moth.
Another group are plants and insect herbivores. Plants can't run away. They can't hide. How do they keep from getting eaten? Spines and tough tissues are one mechanism.
Another is to lace your body with poison. The euphorbia above has spines but it also has highly toxic sap. Many of our medicines and most of the recreational drugs we consume are derived from plant compounds that evolved in response to predation by insects. Insect herbivores (plant eaters) tend to be highly specialized and only eat one species of plant. The plants evolve toxins and the insects evolve resistance to the toxins, often storing them in their bodies to become toxic themselves.
Don't eat these guys!
3. Mutualisms Probably the most fascinating interactions are ones in which both species benefit. In the coral show below micro-organisms live inside the body of the coral. The zooxanthellae as these organisms are known evolved from algae-like organisms living freely in the ocean. Now they live inside another organism and exchange nutrients.
Another very common type of mutualism is animal pollination. Plants, again, are hampered in their sex lives by an inability to move about and find a mate. Instead they use animals to move pollen around. In exchange the animals get food in the form of nectar.
It is important to note that both parties in a mutualism are 'motivated' by self interest. A pollinator will be most successful if it can get as much food with as little effort as possible. Visiting flowers of many species is probably a good way to accomplish this. On the other hand the flower benefits most from pollinators that are highly species specific. Many flower characteristics have evolved to attract and allow access by certain species of insects.
The wasp is getting pollen all over its back and thus transferring between flowers. The skipper is drinking nectar but being a far less efficient pollinator.
This plant's flowers are not open but when they do open they will look and smell like rotting meat. They will attract flies who will land and pick up pollen and not get anything in return.
In the tropics in particular coevolution has shaped the biology of many plants and animals. This poses additional challenges for conservation. A plant can't survive without its pollinator and the animals that disperse its seeds. Some animals may only feed on one or two plants. Loss of a few species may lead to further losses, as ancient associations are shattered, even in protected areas until only hardy generalists are left.