The theory of evolution is the foundation upon which all of modern biology is built.

The core idea is that organisms, or living things, change over time as a result of natural selection, which acts on genes within a population. Individuals do not evolve; populations of organisms do.

The material on which evolution acts is the deoxyribonucleic acid (DNA) that serves as the heritable carrier of genetic information in all living things on Earth, from single-celled bacteria to multi-ton whales and elephants.

Organisms evolve in response to environmental challenges that would otherwise threaten a species' ability to survive by limiting its reproductive capacity.

One of those challenges is, of course, the presence of other organisms. Not only do interacting species affect each other in real time in obvious ways (for example, when a predator such as a lion kills and eats an animal it preys on), but different species can also affect the evolution of other species.

This occurs through a variety of interesting mechanisms and is known in biology parlance as coevolution.

In the mid-1800s, Charles Darwin and Alfred Wallace independently developed very similar versions of the theory of evolution, with natural selection being the primary mechanism.

Each scientist proposed that the life forms roving the Earth today had evolved from far simpler creatures, going back to a common ancestor at the dawn of life itself. That "dawn" is now understood to have been about 3.5 billion years ago, about a billion years after the birth of the planet itself.

Wallace and Darwin eventually collaborated, and in 1858 published their then-controversial ideas together.

Evolution posits that populations of organisms (not individuals) change and adapt over time as a result of inherited physical and behavioral characteristics that are passed down from parent to offspring, a system known as "descent with modification."

More formally, evolution is a change in allele frequency over time; alleles are versions of genes, so a shift in the proportion of certain genes in the population (say, genes for a darker fur color becoming more common and those for lighter fur becoming correspondingly more rare) constitutes evolution.

The mechanism that drives evolutionary change is natural selection as a result of selection pressure or pressures imposed by the environment.

Natural selection is one of many well-known but deeply misunderstood terms in the science world generally and in the realm of evolution in particular.

It is, in a basic sense, a passive process and a matter of dumb luck; at the same time, it is not simply "random," as many people appear to believe, though the seeds of natural selection are random. Confused yet? Don't be.

Changes that occur in a given environment lead to certain traits being advantageous over others.

For example, if the temperature gradually gets colder, animals of a particular species that have thicker coats thanks to favorable genes are more likely to survive and reproduce, thereby increasing the frequency of this heritable trait in the population.

Note that this is a different proposition entirely from individual animals in this population surviving because they are able to find shelter through sheer luck or ingenuity; that is unrelated to heritable traits pertaining to coat characteristics.

The critical component of natural selection is that individual organisms cannot simply will the necessary traits into existence.

They must be present in the population thanks to pre-existing genetic variations that in turn follow from chance mutations in DNA in earlier generations.

For example, if the lowest branches of leafy trees become progressively higher off the ground when a group of giraffes inhabits the area, those giraffes that happen to have longer necks will survive more easily owing to be able to meet their nutritional needs, and they will reproduce with each other to pass on the genes responsible for their long necks, which will become more prevalent in the local giraffe population.

The term coevolution is used to describe situations in which two or more species affect each other's evolution in a reciprocal manner.

The word "reciprocal" is paramount here; for coevolution to be an accurate description, it is not sufficient for one species to affect the evolution of other or others without its own evolution also being affected in way that would not occur in the absence of the co-occurring species.

In some ways, this is intuitive. Since all organisms in a particular ecosystem (the set of all organisms in a well-defined geographic area) are connected, it makes sense that the evolution of one of them would affect the evolution of others in some way or ways.

Usually, however, students are not invited to consider the evolution of a species in an interactive way, and instead they are asked to look at the interplay between a single species and its environment.

While the strictly physical characteristics of environments (e.g., temperature, topography) certainly change over time, they are nonliving systems and hence do not evolve in the biological sense of the word.

Hearkening to the basic definition of evolution, then, coevolution occurs when the evolution of one species or group influences the selective pressure, or the imperative to evolve in order to survive, of another species or group. This most often happens with groups that have close relationships within an ecosystem.

It can, however, happen to distantly related groups as the result of a sort of "domino effect," as you'll soon learn.

Examples of predator and prey interaction can shed light on everyday examples of coevolution that you are likely aware of on some level, but have perhaps not actively considered.

Plants vs. animals: If a plant species evolves a new defense against an herbivore, such an thorns or poisonous secretions, this induces a new pressure on that herbivore to select for different individuals, such as plants that remain tasty and readily edible.

In turn, these newly sought-after plants, if they are to survive, must overcome that new defense; in addition, the herbivores can evolve thanks to individuals that happen to have traits that make them resistant to such defenses (e.g., immunity to the poison in question).

Animals vs. animals: If a favorite prey of a given animal species evolves new way to escape that predator, the predator must in turn evolve a new way to catch that prey or risk dying off if it cannot find another source of food.

For example, if a cheetah cannot consistently outrun the gazelles in its ecosystem, it will ultimately perish of starvation; at the same time, if the gazelles cannot outpace the cheetahs, they too will die off.

Each of these scenarios (the second more starkly) represents a classic example of an evolutionary arms race: As one species evolves and gets faster or stronger in some way, the other must do the same or risk extinction.

Obviously, there is only so fast a given species can become, so in the end something has to give and one or more of the species involved either migrates from the area if it can, or it dies off.

• Important: The general interaction between organisms in an environment does not by itself establish the presence of a coevolutionary process; after all, almost all organisms in a given place interact in some fashion. Instead, for an example of coevolution to be established, there must exist definitive evidence that the evolution in one has triggered evolution in the other and conversely.

Predator-prey relationship coevolution: Predator-prey relationships are universal the world over; two have already been described in general terms. Predator and prey coevolution is thus easy to locate and verify in almost any ecosystem.

Cheetahs and gazelles are perhaps the most-cited example, while wolves and caribou represent another in a different, far colder part of the world.

Competitive species coevolution: In this type of coevolution, multiple organisms are vying for the same resources. This kind of coevolution can be verified with certain interventions, as is the case with salamanders in the Great Smoky Mountains of the eastern United States. When one Plethodon species is removed, the other's population grows in size and vice versa.

Mutualistic coevolution: Importantly, not all forms of coevolution are necessarily damaging to one of the species involved. In mutualistic coevolution, organisms that rely on each other for something evolve "together" thanks to unconscious cooperation – a sort of unstated negotiation or compromise. This is evident in the form of plants and the insects that pollinate those plant species.

Parasite-host coevolution: When a parasite invades a host, it does so because it has dodged the defenses of the host at that point in time. But if the host evolves in a way so that it is not drastically harmed without "evicting" the parasite outright, coevolution is in play.

Three-species predator-prey example: Lodgepole pine cone seeds in the Rocky Mountains are eaten both by certain squirrels and crossbills (a type of bird).

Some areas where lodgepole pines grow have squirrels, which can easily eat seeds out of narrow pine cones (which tend to have more seeds), but the crossbills, which cannot easily eat the seeds out of narrow pine cones, don't get as much to eat.

Other areas have only crossbills, and these groups of birds tend to have one of two beak types; the birds with straighter beaks have an easier time grabbing seeds out of narrow cones.

Wildlife biologists studying this ecosystem hypothesized that if trees coevolved based on the local predators, areas with squirrels should have yielded wider cones that were more open with fewer seeds to be found among the scales, whereas areas with birds should have yielded thicker-scaled (i.e., beak-resistant) cones.

This proved to be exactly the case.

Competitive species: Certain butterflies have evolved to taste bad to predators so that those predators avoid them. This increases the likelihood of other butterflies being eaten, adding a form of selective pressure; this pressure leads to the evolution of "mimicry," wherein other butterflies evolve to look like the ones predators have learned to avoid.

Another competitive species example is the evolution of the king snake to look almost exactly like the coral snake. Both can be aggressive toward other snakes, but the coral snake is highly venomous and not one that humans want to be around.

This is rather like someone not knowing karate, but having a reputation for being a martial-arts expert.

Mutualism: Ant-acacia tree coevolution in South America is an archetypal example of mutualistic coevolution.

The trees developed hollow thorns at their base, where nectar is secreted, likely to prevent herbivores from eating it; meanwhile, ants in the area evolved to situate their nests in these thorns where nectar is produced, but do not damage the tree apart from some relatively harmless thievery.

Host-parasite coevolution: Brood parasites are birds that have evolved to lay their eggs in other birds' nests, after which the bird that actually "owns" the nest winds up taking care of the young. This affords the brood parasites free childcare, leaving them free to devote more resources to mating and finding food.

The hosts birds, however, eventually evolve in a way that allows them to learn to recognize when a baby bird isn't their own, and also to avoid interacting with parasitic birds altogether if possible.