Transcript Speciation along an environmental gradient

Speciation along Environmental Gradients
Michael Doebeli and Ulf Dieckmann
Resource Competition
competition
function a(s-s0)
s0
resource
distribution k(s)
resource
gradient s
Dynamics of population sizes ni of strategy si
d
1
ni  r ni [1 
a ( si  s j )n j ]

j
dt
k ( si )
Resultant Pairwise Invasibility Plots
 With k = k0 N(0,σk) and a = N(0,σa) we obtain
–
+
–
+
Resident trait s
 Evolutionary Stability
for σa < σk
Mutant trait s‘
Mutant trait s‘
for σa > σk
–
+
+
–
Resident trait s
 Evolutionary Branching
Evolutionary Branching
Branching point
Convergence to disruptive selection
Sexual Cohesion Can Prevent Branching
Mating Character
Sexual Evolutionary Branching
Assortative Mating
Random Mating
Disassortative Mating
Evolutionary Branching Point
This mechanism also works when assortative mating is based on a marker character
and when evolutionary branching is driven by interspecific interactions.
Spatial Gradient
Trait value at maximum
Maximum of carrying capacity varies with location
Spatial location
Spatial Evolutionary Branching
Assortative
Mating
Random
Mating
Disassortative
Mating
Spatial Gradient
(vertical direction is ecologically neutral)
Evolutionary Branching Point
Trait value
Spatial Evolutionary Branching
Time
Spatial location
Spatial Gradient Facilitates Branching
Migration scale
Branching range for non-spatial model
Extra branching range due to gradient
1
σa / σk
Intermediate Slopes Are Most Speciation-Prone
Migration scale
No branching expected for non-spatial model σa = 
Extra branching range due to gradient
1
g σ s / σk
Full Characterization
Summary
 Spatial gradients can greatly facilitate branching
 Sympatric speciation processes may lead to patterns of
species abutment
 The responsible mechanism is not isolation by distance
but instead local adaptation leading to frequencydependent disruptive selection
 Intermediate slopes appear to be most speciation-prone
 Sympatric speciation processes may lead to patterns of
peripheral speciation
Peripheral Speciation?
Optimal
phenotype
Expected speciation zone?
Species
range