4. In 1964, Ehrlich and Raven used the concept of reciproca

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COEVOLUTION

• Defined as mutually induced evolutionary change
  between two or more species or populations
• Think of coevolution as reciprocal evolutionary
  change in interacting species the partial
  coordination of non-mixing gene pools over
  evolutionary time

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Darwins Origin of Species

• Darwin did not use the word coevolution but he
  did use coadaptation several times, and he
  thought of coadaptation as reciprocal change

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Four independent approaches were developed for
the study of reciprocal evolutionary change

• 1. Flor (1942) developed the concept of
  gene-for-gene interactions
• 2. Modes (1958) paper on gene-for-gene
  interactions, entitled A Mathematical model for
  the co-evolution of obligate parasites and their
  hosts, was the first explicit mathematical model
  of coevolution

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Approaches to Reciprocal Evolutionary Change

• 3. In 1961, Pimentel developed the hypothesis of
  genetic feedback by which reciprocal genetic
  changes could regulate populations of interacting
  species
• 4. In 1964, Ehrlich and Raven used the concept of
  reciprocal change much more broadly to link
  adaptation and speciation in interacting species

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When is it Coevolution?

• Janzen stated that the lack of an original
  definition of coevolution and the example chosen
  by Ehrlich and Raven led to misleading uses of
  the term
• For example, its commonly assumed that a pair of
  species whose traits are mutualistically
  congruent have coevolved

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Mutually induced evolution must be demonstrated

• That is, the central problem in coevolutionary
  studies is to understand the ecological and
  genetic conditions that permit interacting
  species to undergo repeated bouts of reciprocal
  genetic change specifically because of the
  interaction

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Modes of Coevolution

• Coevolution is an umbrella for a variety of
  mechanisms and outcomes of reciprocal
  evolutionary change

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Modes of Coevolution

• Gene-for-gene coevolution
• Specific coevolution
• Guild or diffuse coevolution
• Diversifying coevolution
• Escape and radiation coevolution

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Gene-for-Gene Coevolution

• For each gene causing resistance in a host there
  is a corresponding (matching) gene for virulence
  in the parasite or pathogen

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A resistant (incompatible) reaction depends upon
both the presence of a gene for resistance (R) in
the host and the corresponding gene for
avirulence (V) in the parasite or pathogen
Host Genotype Pathogen Genotype RR rr
V V VVRR Incompatible VVrr Compatible v v vvRR
Compatible vvrr Compatible Incompatible -
avirulent Compatible - virulent
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5 other possible combinations VvRR, VvRr, Vvrr,
VVRr, vvRr are generally indistinguishable from
other compatible or incompatible reactions,
because resistance is usually dominant to
susceptibility and avirulence dominant to
virulence
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Demonstrated Gene-for-Gene Interactions - 44
examples

• 25 (11) rust fungi
• 11 (5) bunts
• 9 (4) powdery mildews
• 7 (3) downy mildews

• 14 (6) viruses
• 2 (1) an insect
• 2 (1) a nematode
• 23 (10) other fungi
• 7 (3) bacteria

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Most coevolution between species will probably
not fit the gene-for-gene hypothesis

• Most of the behavioral, morphological,
  physiological and life history characters
  affecting species interactions are likely to be
  polygenic

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Rapidity of Evolution in Housefly Populations
Exposed to Varying Levels of Toxicants

• Pimentel, D. and A.C. Bellotti. 1976.
  Parasite-host population systems and genetic
  stability. The American Naturalist

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Toxicants included

• citric acid .125M
• copper sulfate .007
• magnesium nitrate .150
• sodium chloride .325
• ammonium phosphate .080
• potassium hydroxide .325

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Experimental Design

• 4 ml glass culture tube with 0.25M sucrose and a
  single toxicant
• Each caused 80 mortality when exposed for 96hr
• Survivors were fed, allowed to lay eggs, and
  larvae were allowed to pupate
• Six populations exposed to a single toxicant and
  one to all six

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Experimental Design
4-4-4 4-4-4
2-6-5-1-4-3 5-4-1-6-3-2 3-1-6-5-2-4 6-2-4-3-5-1 1-
3-2-4-6-5 4-5-3-2-1-6
1-1-1 1-1-1
5-5-5 5-5-5
2-2-2 2-2-2
6-6-6 6-6-6
3-3-3 3-3-3
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Results
100
Single
Survival
50
Diverse
0
2 4 6 8 10 12 14 16
Generations
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Conclusions

• Genetic stability (preventing the parasite from
  overcoming host resistance) is possible when
• 1. There is suitable genetic diversity in the
  host population
• 2. Ample gene flow between parasite colonies (34
  gene flow)
• 3. High selection coefficients (80) on the
  parasite population from the host population

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Specific Coevolution (Coadaptation)

• Very close coevolution, or coadaptation, between
  two species occurs without a gene-for-gene
  relationship
• Less close genetic associations are likely to be
  far more common and include close mutalisms,
  symbioses and divergence of traits in competing
  species

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Example Bulls-Horn Acacia Ants, Acacia
cornigera/ Pseudomyrmex ferruginea
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Specific Coevolution

• Demonstrating specific coevolution between two
  species is not a simple task
• The problem for any particular interaction is to
  demonstrate that both species have evolved in
  response to the interaction

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Guild or Diffuse Coevolution

• Reciprocal evolutionary change may occur among
  groups of species rather than pairs of species
• Examples might include the interactions between
  pollinators and flowering plants, the
  interactions between frugivorous birds and
  fleshy-fruited plants and some mimicry complexes

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Guild or Diffuse Coevolution

• The concept of guild coevolution is important
  because it emphasizes that the evolutionary unit
  of an interaction may be broader than a pair of
  species
• Its a useful heuristic tool for thinking about
  how interactions change and link together groups
  of species within communities - only useful when
  restricted to interactions in which we can see
  how natural selection is shaping reciprocal
  change in groups of species

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Kodric-Brown, A. and J.H. Brown. 1979.
Competition between distantly related taxa in the
coevolution of plants and pollinators. American
Zoologist
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Convergent evolution between hummingbirds and
flowering plants in western North America
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Diversifying Coevolution

• Can be defined as reciprocal evolution between
  species in which the interaction causes at least
  one of the species to become subdivided into two
  or more reproductively isolated populations
• If gene flow between populations is sufficiently
  restricted, speciation may result as a
  consequence of populations adapting to different
  local ecological conditions as the interactions
  evolve in different ways

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Diversifying Coevolution

• Reproductive isolation can be the direct result
  of one species exerting significant direct
  control over either the movement of gametes in
  the other species or the success of matings among
  subgroups in the other species - may produce
  speciation in one or both of the interacting
  species
• Two kinds of interaction may best fit the
  conditions of diversifying coevolution plants
  pollinators, hosts intracellular symbionts

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Speciation in Crossbills

• J.W. Smith and C. W. Benkman. 2007. A
• coevolutionary arms race causes
• ecological speciation in crossbills.
• American Naturalist 169455-465.

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Escape and Radiation Coevolution

• A specific form of how guild coevolution may
  involve both adaptation and speciation
• It differs from other concepts of coevolution in
  that it explicitly includes periods during which
  the interaction between the taxa does not occur -
  initially formulated by Ehrlich Raven (1964)

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Five Steps

• 1. Plants produce novel secondary compounds
  through mutation and recombination
• 2. The novel chemical compounds reduce the
  palatability of these plants to insects, and are
  therefore favored by natural selection
• 3. Plants with these new compounds undergo
  evolutionary radiation into a new adaptive zone
  in which they are free of their former herbivores

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Five Steps

• 4. A novel mutation or recombinant appears in an
  insect population that permits individuals to
  overcome the new plant secondary compounds
• 5. These insects enter a new adaptive zone and
  radiate in numbers of species onto the plants
  containing the novel secondary compounds, forming
  a new taxon of herbivores

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An Example

• Berenbaum, M. 1980. Coumarins and caterpillars a
  case for coevolution. Evolution

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1. Plants produce novel secondary compounds
through mutation and recombination
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Novel Secondary Substances

• Paracoumaric acid
• Hydroxycoumarins
• Linear furanocoumarins
• Angular furanocoumarins

• 100 plant families
• 31 plant families
• 8 plant families
• 2 plant families

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2. New secondary substances alter the suitability
of plants as food for insects

• Increasing toxicity from hydroxycoumarins to
  angular furanocoumarins
• Generalist like the Southern Army Worm can feed
  on artificial diets of hydroxycoumarins but not
  furanocoumarins
• Angular furanocoumarins are toxic to insects that
  feed successfully on linear furanocoumarins -
  like the Black Swallowtail Butterfly

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3. Plants undergo evolutionary radiation

• That plants underwent evolutionary radiation as a
  result of producing linear and angular
  furanocoumarins is suggested by patterns of
  species diversification within the Umbelliferae

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Among three subfamilies of the Umbelliferae the
one producing any type of coumarin is the most
speciose

• Apioideae 1,950 species coumarins
• Hydrocotyloideae 320 species
  no coumarins
• Saniculoideae 250 species
  no coumarins

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4. Insects evolve resistanceBehavioral or
Biochemical

• Leaf-rolling behavior by Parsnip Webworm -
  facilitated the adoption of phototoxic plants as
  hosts
• In the presence of UV light, the Black
  Swallowtail can feed on xanthotoxin, a linear
  furanocoumarin, with a concentration 10X greater
  than that which kills 100 of the polyphagous
  Southern Army Worm

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Insect Resistance

• The Black Swallowtail suffers reduced fecundity
  from feeding on angular furanocoumarins
• The Short-tailed Swallowtail feeds almost
  exclusively on plants containing angular
  furanocoumarins

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5. Adapted insects can enter a new adaptive zone
and undergo an evolutionary radiation