First some definitions: coevolution is a change in the genetic composition of one species (or group) in response to a genetic change in another. More generally, the idea of some reciprocal evolutionary change in interacting species is a strict definition of coevolution.

At first glance (or thought), it might seem that everything is involved in coevolution. This assumption might stem from the fact that virtually all organisms interact with other organisms and presumably influence their evolution in some way. But this assumption depends entirely on ones definition of the term Coevolution.

The term is usually attributed to Ehrlich and Raven's study of butterflies on plants (1964) but the term was used by others prior to 1964 and the idea was very present in the Origin of Species. Ehrlich and Raven documented the association between species of butterflies and their host plants noting that plants' secondary compounds (noxious compounds produced by the plant) determined the usage of certain plants by butterflies. The implication was that the diversity of plants and their "poisonous" secondary compounds contributed to the generation of diversity of butterfly species.

Here we have a very general observation of one group of organisms having an influence on another group of organisms. Is this coevolution? Some would argue that it is not good evidence for coevolution because the reciprocal changes have not been documented clearly. Like the issue of defining an adaptation, we should not invoke coevolution without reasonable evidence that the traits in each species were a result of or evolved from the interaction between the two species.

Lets consider plants and insects: there is little evidence to determine whether plants' secondary compounds arose for the purpose of preventing herbivores from eating plant tissue. Certain plants may have produced certain compounds as waste products and herbivores attacked those plants that they could digest. Parasites and hosts: when a parasite invades a host, it will successfully invade those hosts whose defense traits it can circumvent because of the abilities it caries at that time. Thus presence of a parasite on a host does not constitute evidence for coevolution. These criticisms are quite distinct from the opportunity for coevolution once a parasite has established itself on a host.

The main point is that any old interaction, symbiosis, mutualism, etc. is not synonymous with coevolution. In one sense there has definitely been "evolution together" but whether this fits our strict definition of coevolution needs to be determined by careful 1) observation, 2) experimentation and 3) phylogenetic analysis.

The classic analogy is the coevolutionary arms race: a plant has chemical defenses, an insect evolves the biochemistry to detoxify these compounds, the plant in turn evolves new defenses that the insect in turn "needs" to further detoxify. At present the evidence for these types of reciprocal adaptations is limited, but the suggestive evidence of plant animal interactions is widespread. An important point is the relative timing of the evolution of the various traits that appear to be part of the coevolution. If the presumed reciprocally induced, sequential traits actually evolved in the plant (host) before the insect (parasite) became associated with it, we should not call it coevolution. See different example figs. 22.6-22.7, pgs. 621-622 + text.

There are a variety of different modes of coevolution. In some cases coevolution is quite specific such as those between two cellular functions. The endosymbiont theory proposes that current day mitochondria and chloroplasts were once free-living unicellular individuals. These cells entered the cytoplasm of other cells, an example of the general phenomenon of endosymbiosis. Current-day mitochondrial and chloroplast genomes are much smaller than the genome sizes of their presumed free-living ancestors. Some of this reduction in genome size is due to the transfer of genes from organelle genomes to the nuclear genome. Thus, being in the cellular environment has influenced the evolution of organelle genomes. There is evidence that the faster rate of evolution of animal mitochondrial DNA has accelerated the rate of evolution of some of the nuclear genes that function in the mitochondria. Thus there is some evidence for reciprocal phenomena

Other modes of coevolution involve competitive interaction between two specific species. The Plethodon salamander study is a good example: two species are competing: in the Great Smoky mountains the two species compete strongly as evidenced by the fact that each species will increase population size if the other is removed. Here there is a clear reciprocal interaction between the two populations (species), each affecting the other.

[The role of competition between species, the coevolutionary responses to this competition and the consequences for the evolution of communities is illustrated in the Anolis lizard fauna of the Caribbean. There is coevolution because the competitive interactions between resident and invading species of Anolis involve reciprocal responses in the evolution of body size. These affect the structure of the lizard community as evidenced by the general pattern of there being a single species of lizard on each island.]

Character displacement also provides and example of a pattern we might interpret as the result of coevolution. Mud snails show pattern of character displacement in sympatry due presumably to competition for food items (don't confuse this with reinforcement; the selective agent here is not reduced hybrid fitness). We might call this co evolution because both species show a shift when compared to allopatric samples of each species (mean of both ~ 3.2 in allopatry vs. ~ 4.0 and ~ 2.8 in sympatry). If only one species exhibited character displacement and you were a really picky evolutionist you might not be convinced of a reciprocal response.

Another strong case is the Ant - Acacia mutualism. Here specific traits in each species appear to have evolved in response to the interaction. The ant (Pseudomyrmex species) depends on the Acacia plant for food and housing; acacia depends on ant for protection from potential herbivores (species that eat plant tissue). Specific characters of the plant appear to have evolved for the maintenance of this mutualism: 1) swollen, ~ hollow thorns (= ant home), 2) extra-floral nectaries (source of nectar outside the flower [i.e., the usual location] providing ants with food), 3) leaflet tips = Beltian bodies (= 99% of solid food for larval/adult ants). Specific characters in the ant that have evolved for the maintenance of this mutualism: 1) defense against herbivores 2) removal of fungal spores from Beltian body break-point (prevents fungal pathogens from invading plant tissues). The main point is that there are traits in both the ant and the acacia that are traits not normally found in close relatives of each that are not involved in similar mutualisms: mutualistic traits have evolved for the interaction in reciprocal fashion. See another example : fig. 22.1 & table 22.1, pg. 611.

Coevolution may be considered among broad groups of taxa, so called diffuse coevolution (such as the general coevolution between plants and insects [assuming it is real]). A nice idea, but in fact the real action must be going on between pairs of species from each group. It is true that the Pierid butterflies (family Pieridae) are associated with the plant family Cruciferae, so there may be something general about each taxon that allows the coevolution to proceed. But the true reciprocal events must be mediated at the host species-insect species level.

Mimicry presents a context were coevolutionary phenomena should be evident. Generally, we would expect that Mullerian mimicry would be more likely to exhibit reciprocal evolutionary patterns since both species involved are unpalatable and therefore have an opportunity to affect the evolution of each other's color patters. This does not mean that Batesian mimicry (one unpalatable model) will not involve coevolutionary phenomena, but the evolution of warning coloration is certainly going to be more asymmetrical since the palatable species will show a greater response to the state of the model than will the model show to the evolving state of the mimic.

The Mullerian mimics Heliconius erato and H. melpomene. illustrate both the frequency dependent nature of mimicry and the fact that each can influence the evolution of the other. One would expect that the more abundant species would be the model in a mullerian system, since it is what the selective agent (predation) is cueing on. In general H. erato is the more abundant of the two species and H. melpomene mimics the wing patterns of H. erato. In one area of overlap of the two species, H. melpomene is the more abundant and H. erato assumes the hindwing band pattern of H. melpomene (see figure below). Thus depending on local conditions, both species are influencing the adaptive responses of the other and thus fits strict definition of coevolution.

A crucial component of coevolution is phylogenetic analysis. If the cladograms of the host and the cladograms of the parasite are congruent (e.g., figs. 22.2 - 22.3, pg. 612-613) this certainly suggests coevolutionary phenomena. But again, be careful and think about it: cospeciation is just "association by descent". Have there been reciprocal phenomena?; maybe just the speciation of the host induced the speciation of the parasite and there was not parasite induced speciation of the host. One needs to know the evolutionary history before we can make firm statements about "co"evolution.