Adaptation to environmental gradients: A simulation and some observations on Littorina saxatilis

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Adaptation to environmental gradientsA
simulation and some observations on Littorina
saxatilis

• Roger ButlinAnimal and Plant SciencesThe
  University of Sheffield

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Adaptation to environmental gradients

• Two outstanding questions
• What limits spread?
• Can local adaptation lead to speciation?

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Outline

• An individual-based simulation
• Masakado Kawata, Tohoku University
• Jon Bridle, Institute of Zoology
• Empirical progress with the periwinkle, Littorina
  saxatilis
• John Grahame and Henry Wood, University of Leeds
• First a bit about the nature of margins

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Some range margins in Britain
Cirsium eriophorum
Gentianella campestris
Pinguicula vulgaris
Trollius europaeus
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Patchiness is on a range of scalesCD Thomas and
coworkers
Plebejus argus
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Central to marginal gene flow
Asymmetric gene flow
Tolerance limit
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Kirkpatrick and Barton 1997
From Case Taper 2000
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Kirkpatrick and Barton 1997
Rate of change of environment
Bbs/(r(2Vs)0.5)
Genetic potential for adaptation AG/(2Vsr)
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Barton 2001

• Genetic variance imposes costs in centre of range
  and benefits at margins
• If variance is allowed to evolve, adaptation to
  arbitrarily steep gradients is possible
• Therefore, something is missing

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An individual-based modelderived from Kawata 2002
Habitat8000x1000
Environmentalgradient
Spatial parameters dispersal distance,
interaction distance, mating distance
stabilizing selection
Fitness W 2 r (1 - N/K) - (Ux-z)2/2Vs
phenotype of female
intrinsic rate of increase
optimum at x
equilibrium density
competitors
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Typical outcomes
Spread dispersal 200 mating 150
No spread dispersal 700 mating 150
Extinct
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Expected clines etc
Total dispersal 100 Mating distance 150 Carrying
capacity 25
Width and variance closer to expectation, and
effect of LD increases,as clines overlapmore
with higherdispersal.
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Variance at centre and margin
Phenotype
Variance
Density
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Effect of carrying capacity
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Population density
cc25
cc12
cc5
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Mating area effect
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Allee effect at margins
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Contribution of Allee effect
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Biased gene flow at high mating distance
Mating distance600
Enhances migrational load in marginal populations
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Ve and mutation

• Adding environmental variance
• No effect on central density (depends on G not
  h2, Kirkpatrick and Barton 1997)
• Critical dispersal value for spread
  increases(depends on h2 and intensity of
  selection (Vp/Vs), KB97)
• Increasing mutation rate
• Little effect on critical dispersal
  value(because effect on G small compared to
  dispersal)
• Does increase upper critical mating
  distance(because of biased gene flow after
  selection?)

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Simulation summary

• Finite population size does influence ability to
  adapt to an environmental gradient
• Mating/gamete dispersal as well as offspring
  dispersal need to be considered
• Narrow parameter space for limited range
• Not critical because of steepening gradients at
  boundaries?