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Lecture 17

BIOC50H3 Lecture Notes - Lecture 17: Myxoma Virus, Garter Snake, Common Cuckoo


Department
Biological Sciences
Course Code
BIOC50H3
Professor
Jason Weir
Lecture
17

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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
independent.
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
endosymbiotic species.
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

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B. Mutualism
C. Competition
D. Mimicry
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
its skin.
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.
o
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
hosts.
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.

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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
seeds.
North we have smaller fruit: shorter rostrums and south we have big fruit and the weevils have
longer rostrums.
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.
Studies:
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
pericarp (C)
(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
weevil’s success.
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:
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