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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:
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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
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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
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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 insect/plant coevolution is found in the Central American plant
known as "bullshorn Acacia", Acacia cornigera. This plant is similar to other members of the
genus Acacia (thorn trees in the pea family), in that it has large spines which presumably protect it
against mammalian herbivores (another example of coevolution, presumably against mammalian
browsers). However, it lacks the cyanogenic glycosides (cyanide-producing chemicals) found in
related Acacia and the thorns in this species are particularly large and hollow, and provides
shelter to a species of Pseudomyrmex ant. The plant also provides proteinaceous food bodies on
the tips of the leaflets, which sustain the ant colonies. These ants are particularly nasty (I can tell
you from personal experience!), and are well able to deter even mammals with their wasp-like
stings. It has been shown experimentally that the ants will also remove any caterpillars from the
leaves that they patrol. The ants even remove vines and plants from around the base of the tree,
creating a bare patch on the soil. Plants of the bullshorn Acacia which have not been occupied by
ant colonies are heavily attacked by herbivores and often have vines growing in the branches.
Related Acacia species lack hollow thorns and food bodies, and do not have specific associations
with ants. They also have many cyanogenic glycosides in their leaves. This data strongly
supports the idea that the bullshorn Acacia has evolved a close, mutualistic association with the
ants in order to protect themselves from herbivores (and also plant competitors). It also supports
the idea that the cyanogenic glycosides found in other species have a defensive role; a role which
has been taken over by Pseudomyrmex in the bullshorn Acacia.
Egg mimicry in Passiflora. Similarly, we have already given examples of egg-mimicry in
Passiflora, which protects plants against species of Heliconius butterflies. Female Heliconius
avoid laying eggs on plants already occupied by eggs, because first instar larvae of Heliconius are
highly cannibalistic; the plants exploit this habit of Heliconius by creating fake yellow eggs as
deciduous buds, stipule tips, or as part of the "extrafloral nectaries" on young leaves. Clearly, the
plant, whose defenses of cyanogenic glycosides, alkaloids, and a host of other secondary
compounts, have been breached by Heliconius, has counterevolved new defenses against this
genus.
Predator-prey coevolution
Predators have obviously evolved to exploit their prey, with hunting ability being at a premium.
Mammalian predators, for example, must be fast, strong and cunning enough to be able to catch
their prey. It is almost as obvious that prey have evolved to protect themselves from predators.
They may have a variety of defenses:
Large size and strength
Protective coverings such as shells or hard bony plates
Defensive weapons, such as stings or horns
Defensive coloration (see mimicry lecture)
Unpalatability and nastiness
Clearly these features also represent examples of coevolution. We have already pointed out that
it is thought unlikely that mimics coevolve with their models in Batesian or Müllerian mimicry
systems. These mimicry systems are nonetheless examples of coevolution, but with their
predators rather than with their mimics!
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Some very specialized coevolved pollination systems
Two of the most famous are figs and fig-wasps, and Yucca and Yucca moths (Tegeticula).
In both cases, the larvae are seed/flower eaters, which reduce the fertility of the flowers or
inflorescences they infest.
In both cases, the plant is completely dependent on its herbivore for pollination. The arrangement
is therefore a tightly coevolved mutualism, in which the plant relies exclusively on the insect for
pollination, and the insect relies exclusively on the plant for food.
In the case of the Yucca moth the mutualism has sometimes broken down, and some clades of
the moth have reverted to a parasitic mode of life -- they oviposit in the plant, but do not pollinate
-- the ancestral condition for the moths.
These examples are interesting because they represent cases where mutualisms have become
so specific that they almost rival the ancient prokaryotic mutualisms of mitochondria and
chloroplasts with archaebacterial cells, to produce what we now know as eukaryotes.
Coevolutionary competitive interactions and adaptive radiation
It is an ecological principle (Gause's principle) that related species must differ in some part of their
ecology. If two species have identical or nearly identical resources, competitive exclusion will
result, and the less well adapted species will go extinct.
If this is true, and it probably is, the reverse should also occur. If a species colonizes an area
where its competitors do not occur, then it may experience ecological release, and grow to very
large population sizes. Not only that, the colonists may also experience disruptive selection,
followed by speciation. The process can be repeated for multiple species, which evolve apart from
one other to form an adaptive radiation.
Many examples of this principle are known in island colonists. For example, we have already
come across the Darwin's finches of the Galapagos islands, which have evolved into a whole
range of seed-feeding and insectivorous forms. A similar, although much more diverse radiation
occurs in the Hawaiian archipelago: the Hawaiian honeycreepers.
Sometimes, the islands are "ecological islands" rather than actual islands. A number of lakes in
the North temperate zone were left behind during the retreat of the ice. These lakes have in the
last 10,000 years been colonized by a variety of fish. In many cases of stickleback and the trout
family, multiple forms have now been produced in each lake or large fresh water body.
Sticklebacks in Canada (Gasterosteus) often produce benthic (deep water) and limnetic (shallow
water) forms (see overhead), which appear to have specialized feeding differences. These forms
also keep to their own habitat, and may mate assortatively.
Similarly, the Atlantic char (Salvelinus) in Thingvallavatn, Iceland's largest lake, have produced
no less than FOUR different trophic forms; similar examples are known from Norway and Ireland
for other salmonids.
Adaptations leading to ecological release, and "escape and radiate" coevolution
As well as the colonization of new habitat, the possession of a unique adaptation may also allow
adaptive radiation to colonize a new "adaptive zone" opened up as a result. There is good
evidence for this:
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A massive phylogenetic study of beetles by Brian Farrell showed that new adaptations for
herbivory on flowering plants led to massive amounts of speciation. Most of the diversity of
species of beetles is in the herbivorous clades (overhead).
The evolution of resin- or latex-bearing canals allowed plants carrying them a more rapid
speciation rate than among sister taxa that lacked these adaptations. Latex and resin is a
physical defence against herbivorous insects.
Conclusions
Evolutionary interactions between species, and coevolution show that the complexity of genetic
evolution goes on increasing, even beyond the species level. Coevolution represents an area
where genetics, ecology, phylogeny all interact. To understand the evolution of life fully, the
interactions between individuals and species must be explored at many levels.
One thing is clear; the majority the diversity of life and life forms is not just due to adaptation to
static environments; biotic interactions are probably much more important. The biotic environment
is itself constantly evolving, leading to orders of magnitude more diversity possible than could be
produced by evolutionary adaptation to simple physical conditions.
Further reading
Ehrlich, PR, Raven, PH 1964. Butterflies and plants: a study in coevolution. Evolution 18,
586-608.
Farrell, BD 1998. "Inordinate fondness" explained: why are there so many beetles? Science 281,
555-559.
Futuyma, DJ 1998. Evolutionary Biology. Chapter on coevolution.
Thompson, JN 1994. The Coevolutionary Process. Chicago University Press.
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