Lecture 17: Coevolution
Coevolution: The joint evolution of two or more interacting species, each of which evolves in response
to selection imposed by the other.
• Coevolution is an important process of both micro- and macroevolutionary change
Three types of coevolution:
i. Specific coevolution: two species evolve in response to each other.
ii. Guild coevolution: multiple species coevolve, and their effects are not
iii. Escape-and-radiate coevolution: A species escapes predation or parasitism after
evolving a major defense, then diversifies. Latter a different predator or parasite
adapts to the host clade and diversifies . Ex latex in plants that is poisonous to
insects escapes predation and radiates like a motherfucker. Sometime in the
future the insects evolve to counteract the latex
2) Phylogenetic detection of Coevolution
Figure 19.2 The phylogeny of endosymbiotic bacteria included under the name Buchnera aphidicola is
perfectly congruent with that of their aphid hosts. Coevolution their phylogenies correspond to one
another and look alike. The bacteria has almost the same phylogentic tree as its aphid host. That’s
because the bacteria live in the host and diverse endosymbiotically with their hosts. Especially in
Figure 19.3 (A) A phylogeny of specialized feather lice is mostly congruent with that of their hosts. These
feather lice don’t correspond to the phylogeny of their host . that’s because the lice jump from one host
to the other. Host species mizing so they are not correlated to one another.
3) Processes of Coevolution
We will consider processes of coevolution in 4 types ofrelationships between species
A. Coevolution of enemies and victims B. Mutualism
3.1) Coevolution of enemies and victims: Predator-prey coevolution
• Newt and garter snakes and TTX
o Rough-skinned newt (Taricha granulosa) has the potent neurotoxin tetrodotoxin (TTX) in
o Most populations have high levels of the toxin (typically, a single newt has enough toxin
to kill 25,000 mice).
o Some populations, like those from Vancouver Island, lack the toxin.
o Resistance to the toxin in the garter snake Thamnophis sirtalis was measured in garter
snake populations where the newt is absent, where it is present but lacks TTX, and
where it is present but has high levels of TTX.
o Garter snake populations showed high levels of resistance to the toxin only where
occurred sympatrically with high TTX newt populations.
• A fledgling common cuckoo being fed by its foster parent, a much smaller reed warbler. (B)
Mimetic egg polymorphism in the European cuckoo
o Each common cuckoo population contains several different genotypes that prefer differ
o Each genotype lay eggs closely resembling those of their preferred hosts
o Other individuals lay non-mimetic eggs
o Host species vary in their ability to recognize mimetic eggs.
• Davies and Brooke 1998 place artificial cuckoo eggs in a variety of host species nests and followed the
fate of the eggs.
• Species not parasitized by cuckoos: rarely rejected eggs
• Commonly parasitized species: high levels of egg rejection.
• Rarely parasitized species: equally high levels of egg rejection.: THIS IS WEIRD?
• These results suggest that rarely parasitized species probably once served as common hosts
(sometime in the past), and their ability to reject cuckoo eggs has selected against the cuckoo
genotypes that parasitize these species. 3.1) Coevolution of enemies and victims: Coevolution in plants and herbivores
• The fruit of the Japanese camellia has a thick, woody fruit wall (pericarp) that encloses the
• North we have smaller fruit: shorter rostrums and south we have big fruit and the weevils have
• The larva of the camellia weevil feed exclusively on this species.
• Adult weevils bore a hole through the pericarp into the seed chamber using their rostrum
(mandible) and deposit their eggs.
• There is a latitudinal gradient in rostrum length and pericarp thickness with southern
populations having exaggerated traits.
Joju and Sota (2006) determined weevils success in
boring through to the seed chamber as a function of
their rostrum length relative to the thickness of a fruits
(D) shows what combinations of rostrum length and
pericarp thickness would result in a 50% success rate
for the weevil.
Northern populations are ahead in this conflict – their
rostra are long enough to ensure a success rate well
over 50% - whereas in the south, the plant population
is ahead with pericarps thick enough to reduce the
These species may be involved in an “arms race”,
although the reason for the thinner pericarp in the
north is not clear.
3.1) Coevolution of enemies and victims: Infectious disease and evolution of parasite virulence
QUESTION: What would be the optimal level of virulence for a parasite?
High virulence : too much will kill the host
Slow Virulence: 3.1) Coevolution of enemies and victims: Infectious disease and evolution of parasite virulence
a. How Virulent should a parasite be?
• Rabbits introduced to Australia:
a. Multiplied like a motherfucker
b. So to counteract this increase they introduced myxoma virus to kill off the
• myxoma virus introduced to reduce rabbit numbers
• Result: Myxoma: south America virus so this Australian rabbits did not have
adaptation against the virus. First year : Virelnt grade of 1 : bby 1964 : virulent
grade declined to 3 or 4. Same virus was introduced in France and Britain and
similar decline in virulence over time. What happened was that this high virulence
was not optimal for the virus .
• Mutualisms: interactions between species that benefit individuals of both species
• In The Origin of Species, Darwin challenged his readers to find one example of a species
having been modified solely for the benefit of another species. So far, no one has met
• Mutualisms exemplify not altruism, but reciprocal exploitation, in which each species
obtains something from the other.
• Some mutualisms, for example, may have originally started out as parasitic or other types
of exploitative relationships which evolved to become benign and eventually mutualistic.
• Yuccas, pollinated by yucca moths are one such example.
Most of the basal species of yucca moth do not pollinate and are thus parasites.
A few species in the basal genera (like Greya) incidentally pollinate flowers in which
they lay eggs. Such species may have lead to a transition from a parasitic to a
Active pollination, and thus a mutualistic relationship, evolved in the ancestor of
Tegeticula and Parategeticula.
Several Tegeticula have evolved to become cheaters and no longer actively
This brings up the issue of conflict within mutualisms.: “Cheaters”
2. Within mutualisms there is always the potential for conflict. A genotype that “cheats” (by
exploiting its partner without paying the cost of providing a benefit) is likely to have a
3. “Cheaters” don’t always have a selective advantage. In some cases, mechanisms are in place
to punish “cheaters”. For example, in the case of the yucca species, the yucca moth is the sole pollinator.
Typically a moth lays only a few eggs in each flower, so that only a few seeds of each
fruit are consumed by the larva. This leaves some seeds which can then germinate.
Fruit in which many eggs have been laid are more likely to be aborted by the plant,
than fruit with only a few eggs. Larva in aborted fruit all die.
Thus there is a real selective incentive for a female moth not to “cheat” by laying to
many eggs in each flower.
Hawkmoth doesn’t like having pollen on his face. Don’t think of mutualism as
organisms helping each other out. Its like they have both positive effects
Orchid had long spur and hawkmoth have long tongues
Both species are mutually exploiting each other and have evolved th