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Biol B242 - Coevolution BIOL B242 - COEVOLUTION http://www.ucl.ac.uk/~ucbhdjm/courses/b242/Coevol/Coevol.html BIOL B242 - COEVOLUTION So far ... In this course we have mainly discussed evolution within species, and evolution leading to speciation. Evolution by natural selection is caused by the interaction of populations/species with their environments. Today ... However, the environment of a species is always partly biotic. This brings up the possiblity that the "environment" itself may be evolving. Two or more species may in fact coevolve. And coevolution gives rise to some of the most interesting phenomena in nature. What is coevolution? At its most basic, coevolution is defined as evolution in two or more evolutionary entities brought about by reciprocal selective effects between the entities. The term was invented by Paul Ehrlich and Peter Raven in 1964 in a famous article: "Butterflies and plants: a study in coevolution", in which they showed how genera and families of butterflies depended for food on particular phylogenetic groupings of plants. We have already discussed some coevolutionary phenomena: For example, sex and recombination may have evolved because of a coevolutionary arms race between organisms and their parasites; the rate of evolution, and the likelihood of producing resistance to infection (in the hosts) and virulence (in the parasites) is enhanced by sex. We have also discussed sexual selection as a coevolutionary phenomenon between female choice and male secondary sexual traits. In this case, the coevolution is within a single species, but it is a kind of coevolution nonetheless. One of our problem sets involved frequency dependent selection between two types of players in an evolutionary "game". The "game theory" underlying this idea could be either between species (as in interspecific competition) or within species (different morphs of the same species) competing for a resource such as food or females. Evolutionary interactions such as this will often produce coevolution. In the rest of this lecture, we will be referring only to between-species coevolution. Because a very large part of all evolutionary biology involves coevolutionary interactions, we have to pick and choose the examples we treat. We can choose from among many types of adaptive radiation, or of parasite/host evolution (e.g. coevolution of vertebrates and their diseases). However, many of the best-studied examples which we shall discuss are to be found among the organisms with most species, insects. Coevolution and interspecific interactions Coevolution might occur in any interspecific interaction. For example: 1 of 6 20/03/2001 09:48 BIOL B242 - COEVOLUTION http://www.ucl.ac.uk/~ucbhdjm/courses/b242/Coevol/Coevol.html Interspecific competition for food or space Parasite/host interactions Predator/prey interactions Symbiosis Mutualisms However, tight interspecific interactions do not always lead to coevolution. Mimicry, for example, can be a parasite/host interaction (in Batesian mimicry) or a mutualism (Müllerian mimicry, see earlier lecture). Mimicry creates exactly the kinds of ties between species that might lead to coevolution, but in practice there are rather good reasons why adaptation may be unilateral rather than coevolutionary: Palatable Batesian mimics adapt to the unpalatable model by copying its pattern, but the model may not be able to escape its parasite. The first model individuals with a new, non-mimicked pattern would also lose the protection of their own species' warning pattern. Thus we can hypothesise that "coevolutionary chase" is an unlikely outcome of Batesian mimicry. In Müllerian mimicry the most abundant and noxious species will also be trapped by its own pattern; any individuals that mimic a rarer or less noxious species will lose the protection of their own species' pattern even though, once the new mimetic pattern became common, both species would ultimately benefit. In contrast, the rarer or less noxious species always gains by mimicking the more common or noxious species, because its own species' protection is weaker than the other's. Mutual convergence is therefore unlikely because of these difficulties for the initial mimetic variants, in spite of the fact that the outcome, once achieved, is mutualistic. Thus, mimicry is a good example showing that coevolution does not always result from interspecific interactions. In mimicry, perhaps surprisingly, the outcome seems almost always to produce unilateral adaptation by one species to the other. In general, there is much discussion about the likelihood of coevolution in cases where more than one species is involved in an evolutionary interactions. An "Ockham's Razor" approach to proving coevolution requires that we should first disprove the simpler hypothesis of unilateral adaptation. Types of coevolution Answers to the question "How likely is coevolution?" depends what you mean by coevolution! Various types have been proposed: In specific coevolution, or coevolution in the narrow sense, in which one species interacts closely with another, and changes in one species induce adaptive changes in the other, and vice-versa. In some cases, this adaptation may be polygenic; in other cases, there may be gene-for-gene coevolution, in which the mutual interactions are between individual loci in the two species. Specific coevolution may of course be short-lived, but if the interaction is very close, as in many host-parasite systems, concordant speciation or cospeciation may result; where the speciation in one form causes speciation in another. Of course, cospeciation doesn't necessarily require coevolution. For example, a very unimportant but highly host-restricted parasite may always speciate whenever its host speciates, without the parasite causing any evolutionary reaction in the host. In diffuse coevolution, also called guild coevolution, whole groups of species interact with other groups of species, leading to changes that cannot really be 2 of 6 20/03/2001 09:48 BIOL B242 - COEVOLUTION http://www.ucl.ac.uk/~ucbhdjm/courses/b242/Coevol/Coevol.html identified as examples of specific, pairwise coevolution between two species. For example, a group of plant species may be fed on by a particular family of insects, which may frequently (in evolutionary time) change hosts. The plants may evolve defensive adaptations, such as defensive chemistry, or physical defenses such as spines, which work against large numbers of the species. In time, some of the insects may be able to overcome the plant's defences, leading to further evolution by the plant, and so on. Another related type of evolution is called escape-and-radiate coevolution. Here, an evolutionary innovation by either partner in a coevolutionary interaction enables an adaptive radiation, or speciation due to the availability of ecological opportunity. For example, it is easy to imagine that this could be a result of the diffuse kind of herbivore-plant coevolution described above. It is interesting that Ehrlich and Raven almost certainly did not mean specific coevolution in their original paper about the evolutionary interactions between butterflies and their host plants. Some people today even go so far as to say that they were not talking about coevolution at all. Concordant and non-concordant phylogenies Phylogenies are very useful in the study of coevolution. If the phylogenies of two closely associated groups, such as host and parasite, are concordant (see overhead), this may imply: That cospeciation has occurred, or That one of the groups (often the parasite) has "colonized" the other (the host). Here, host shifts by the parasite may well correspond to the host phylogeny, but only because closely related hosts are similar, and liable to colonization by closely-related parasites. In other cases, phylogenies may not be concordant, because the parasite may be able to switch between host lineages fairly frequently (see examples on overheads). Host/parasite and predator/prey coevolution However, as we have seen, even contemporaneous cospeciation with concordant phylogenies does not prove that two lineages have coevolved. Instead, we can look at individual adaptations of the interacting species to get an idea of whether coevolution has taken place. Here are some examples: Defences of plants against herbivores Plants have many complex chemicals, called "secondary chemicals", which are not obviously used in normal metabolism. Ehrlich and Raven and others subsequently interpreted this "secondary chemistry" as an example of defensive adaptation by the plants. Many of these compounds (for instance, tannins and other phenolic compounds, alkaloids like nicotine, cocaine, opiates and THC, or cyanogenic glycosides) are highly toxic. Many animals such as insects have adapted to feeding exclusively on plants with particular defensive chemistry. If the plants evolved secondary chemistry to avoid insects, and insects evolved to handle the plant chemistry, then plant/insect coevolution has occurred. However, critics argue that: phytophagous insects are usually rare, and therefore do not pose a threat to their host plants 3 of 6 20/03/2001 09:48 BIOL B242 - COEVOLUTION http://www.ucl.ac.uk/~ucbhdjm/courses/b242/Coevol/Coevol.html secondary chemistry may be a byproduct of normal metabolic processes, rather than necessarily defensive To find evidence for coevolution, we must show that specific poisons or other defenses work against specific insects, or that they become less necessary when the insects are not present. Ant-acacias. Good evidence for
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