Transcript 2.coevolution

Coevolution: Patterns and Processes
1. Introduction
Definition ?
• mutually induced evolutionary change between two or more
species or populations
• reciprocal evolutionary change in interacting species; the partial
coordination of non-mixing gene pools over evolutionary time
• Key questions
– Do competitors evolve in response to each other?
– Do predators (or parasites) evolve in response to prey (or
hosts)?
– Does coevolution promote biodiversity (ie. lead to speciation
and radiation) ?
2. Historical context
• First indication in Darwin’s (1859) ‘entangled bank’
– ‘mutual relations’, ‘mutual adaptations’, ‘co-adaptations’
– First indication of ‘arms race’ ideas of coevolution
3. Coevolution as reciprocal evolutionary change (insects)
• Ehrlich and Raven (1964) (butterflies on plants)
• Described process, and linked it to process of speciation
• The ‘original’ process (Ehrlich and Raven)
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Plants produce novel secondary compounds via mutation and recomb
These compounds reduce rates of herbivory, so are favoured by NS
Plants containing these compounds radiate, free from herbivores
Novel mutation in insect population reduces toxicity to compound
Insects enter new ‘adaptive zone’ and radiate
• The evidence
– 1) Observation (but take care!)
– 2) Empirical studies (Berenbaum and the parsnip web worms)
– 3) Phylogenetic studies (cospeciation)
Incongruence versus cospeciation
Host
Parasite
incongruence
(not cospeciation, e.g. host switching)
congruence
(cospeciation)
Phoebetria palpebrata
Diomedea chrysostoma
Diomedea bulleri
79 Paraclisis diomedeae
78 Paraclisis diomedeae
77 Paraclisis diomedeae
81 Paraclisis hyalina
Diomedea epomophora
82 Paraclisis hyalina
Diomedea exulans
80 Paraclisis giganticola
Diomedea irrorata
83 Paraclisis miriceps
Diomedea nigripes
Diomedea immutabilis
76 Paraclisis confidens
0.1
Gophers and Lice
O. hispidus
G. chapini
O. underwoodi
G. setzeri
O. cavator
G. panamensis
O. cherriei
G. cherriei
O. heterodus
Z. trichopus
P. bulleri
G. costaricensis
G. trichopi
G. nadleri
G. expansus
C. castanops
C. merriami
G. bursarius (b)
G. geomydis
G. oklahomensis
G. bursarius (a)
G. ewingi
G. breviceps
G. texanus
G. personatus
T. bottae
T. talpoides
G. actuosi
G. perotensis
G. thomomyus
T. minor
T. barbarae
• Counter-evidence
– Functions of secondary plant compounds ?
– Coevolution does not necessarily equal cospeciation
4. Requirements for coevolution
• Identify mutual traits
– Variation among individuals
– Heritable
– Variation in trait is linked to variation in fitness
• *Verify traits are mutually induced*
• Lab studies demonstrating natural selection on traits
5. Case studies - Garter snakes and newts
Skin toxin (tetrodotoxin,TTX) in newts vs. resistance to toxin in
snakes (Brodie and Brodie and others…).
Natural history (general):
• TTX produced by newts (and some others!) - powerful skin toxin
that interferes with nerve function
• some populations of garter snakes contain individuals that can
detoxify TTX
• ’resistance’ measured on a race track
• both reciprocal traits are heritable
• allopatric snakes are highly susceptible to TTX
• sympatric snakes show close phenotypic matching
Natural history (specific):
• tube-shaped proteins on snake nerve cells control rate of sodium
ion in-flow
• TTX enters through a hole on the surface of the protein, severely
interfering with rate of flow
• sodium channel genes are highly conserved, but not in snakes
• some snakes don’t allow TTX to bind to the protein (but
probably at a cost)
• molecular arms race involving toxification/detoxification
genes
In regions of sympatry,
concentration of toxin
is matched to level of
resistance (ie. phenotypic matching).
Snake pops without
Newts are highly susceptible to TTX.
Coevolutionary hotspots,
surrounded by regions
where reciprocal selection
is less intense.
• Conclusion
– example of predator/prey coevolution
– strong trait-for-trait coevolution
– strong evidence for arms race model of coevolution
6. Types of Coevolution
• Gene-for-gene coevolution
– Each host gene affecting resistance is matched by a parasite gene that
affects attack (or virulence)
– e.g. Flor (1950’s) flax/rust fungi system
- genetic crosses involving 2 strains of flax and 2 strains of rust
- host resistance and susceptibility were Mendelian
- parasite virulence was Mendelian
- genetic systems were perfectly complimentary
– Link to Ehrlich and Raven (1964)
– Commonness of gene for gene coevolution?
• Escape and radiation coevolution
– includes periods when intimate interactions do not occur
– e.g. Berenbaum’s parsnip webworm
– e.g. catipillars on milkweed
• Guild or diffuse coevolution
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Coevol occurs among groups of species, not pairs
pollinators and flowering plants
fruit-eating birds and fleshy-fruited plants
evolution of immune systems, evolution of parasite deception
• e.g. immunoglobulin diversity (i.e. humoral immunity)
7. The geographic mosaic of coevolution (J. Thompson)
• outcomes of interactions vary among populations
– coevolutionary hotspots (where there is reciprocal selection)
– coevolutionary coldspots (no reciprocal selection)
Lithophragma and Greya moths, Thompson, 2002
Conclusions
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Outcome varied spatially AND yearly
Reciprocal selection varies over time and space
Relevance to conservation biology
Parallels to snake/newt system
8. Summary
• Coevolution is an umbrella term, aimed to describe various outcomes of
evolving interactions
• Diversity of outcomes within given interactions
• High spatial and temporal variation = fluctuating selection pressures
• “look for coevolutionary vortices within an evolutionary stream”