Coevolution and Mutualisms
Species interactions may be
mutually beneficial
In some cases, these interactions
may lead to coevolution among
participants
What is coevolution?
Evolutionary change in one species
causes evolutionary change in
the other
Species 1
Five points about coevolution
and specialization
1). Outcomes unlikely to be equal
2). Coevolution may occur in one
population, but not another
3). Population-level variation in
degree of specialization
Species 2
(Thompson 1994. The Coevolutionary Process)
Five points about coevolution
and specialization
4). Variation in response leads to
geographic mosaic of
coevolution
5). Temporal variation in mosaic
as well.
Coevolution difficult to document
Many interactions are highly
specialized but not coevolutionary
Evolutionary response may be to
generalized type of organism:
diffuse coevolution.
May also be specifically between
2 species- “narrow sense”
(Thompson 1994. The Coevolutionary Process)
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Example: rabbit control in Australia
European rabbits introduced into
Australia, spread rapidly
Gene-for-gene mechanisms of
coevolution
1. single-allele complementary
system
Introduction of myxoma virus
Coevolved relationship between
resistance and virulence
Mutualisms
2. elicitor-receptor model
3 types of mutualisms
•Three basic types of mutualistic
interactions
1. Trophic (obligatory)
•May or may not involve coevolution
2. Defensive (both facultative and
obligatory)
•Obligate versus facultative mutualism
•Cheaters
Trophic mutualisms
3.Dispersive (some obligatory,
most not)
Defensive Mutualisms
Copyright: Hilton Pond Center
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Dispersive Mutualisms
Pollination versus seed dispersal
Pollination systems sometimes
coevolved, seed dispersal
systems are not
Very great degree of specialization
in some pollinator systems
Seed dispersal: mutualistic but
not coevolved
Why not coevolved?
No specialization
Sites suitable for seeds not
predictable
No “payment on delivery”
Pollination: mutualistic, not
always coevolved
Target specificity
Need for predictability
Advantages to non-specialization
(Wheelwright and Orians 1982)
“How to be a fig”
A stunning example of coevolution…
Two paths to pollination (and
seed dispersal)
Deception
Photo by Ken Gardiner
Reward
Photo by Babs and Bert Wells
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Trade-off in mutualisms
How do you maximize your benefit
for your cost?
The plant perspective…
Mutualisms: how do they work??
•Theory poorly developed
•Apparent evolutionary instability
•Cheaters
The animal perspective…
(Ferriere et al. 2002)
Specialization and phylogeny
Specialization may already exist“preadaptation”
Phylogeny can often explain preadaptations and intermediate steps
Implications?....
Summary
•Mutualisms offer benefits to both
participants
•Mutualisms have enormous impact
on natural communities
•Coevolution can occur in mutualisms
but appears rare, hard to
demonstrate
•Mutualisms represent a cost-benefit
balance between participants
Ungraded homework
Assignment
What are two ways plants can attract
pollinators, and what is the cost or
risk associated with each strategy?
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