Jan 04, 2016
Sasha GimelfarbSasha Gimelfarbdied on May 11, 2004died on May 11, 2004
A Multilocus Analysis of A Multilocus Analysis of Frequency-Dependent Selection Frequency-Dependent Selection
on a on a QuantitativeQuantitative Trait Trait
Reinhard BürgerDepartment of Mathematics, University of Vienna
Frequency-dependent selection
has been invoked in the explanation of evolutionaryphenomena such as
• Evolution of behavioral traits
• Maintenance of high levels of genetic variation
• Ecological character divergence
• Reproductive isolation and sympatric speciation
Frequency-Dependent Selection Caused by
Intraspecific Competition
Intraspecific competition mediated by a quantitative trait under stabilizing selection:
• Bulmer (1974, 1980)
• Slatkin (1979, 1984)
• Christiansen & Loeschcke (1980), Loeschcke & Christiansen (1984)
• Clarke et al (1988), Mani et al (1990)
• Doebeli (1996), Dieckmann & Doebeli (1999)
• Matessi, Gimelfarb, & Gavrilets (2001)
• Bolnick & Doebeli (2003)
• Bürger (2002a,b), Bürger & Gimelfarb (2004)
The General Model
• A randomly mating, diploid population with discrete generations and equivalent sexes is considered.
• Its size is large enough that random genetic drift can be ignored.
• Viability selection acts on a quantitative trait.
• Environmental effects are ignored (in particular, GxE interaction). Therefore, the genotypic value can be identified with the phenotype.
Ecological Assumptions
Fitness is determined by two components:
• by stabilizing selection on a quantitative trait,
and
• by competition among individuals of similar trait value,
,)(1)( 2 gsgS
.)(1),( 2hgahg
The strength of competition experienced byphenotype g (= genotypic value) for a givendistribution P of phenotypes is
where and VA denote the mean and (additive
genetic) variance of P.
],)[(1
)(),()(
A2 Vgga
hPhgg hP
g
If stabilizing selection acts independently ofcompetition, the fitness of an individual withphenotype g can be written as
),())(()( gSgNFgW P
where F(N) describes population growth accordingto N´=NF(N). (F may be as in the discrete logistic,the Ricker, the Beverton-Holt, the Hassell, or theMaynard Smith model.)
For weak selection ( , f = a/s constant),this fitness function is approximated by
,]))[((
)(1)()(
)(
A2
2
VggNs
gsNFgW
where is a compound measure for the strengthof frequency and density dependence relative tostabilizing selection, i.e., .
)(N
0)( 2 sO
fN NFNNF)(
)(')(
Genetic Assumptions
• The trait is determined by additive contributions from n diploid loci, i.e., there is neither dominance nor epistasis.
• At each locus there are two alleles. The allelic effects at locus i are -i/2 and i/2. (This is general because the scaling constants can be absorbed by the position of the optimum and the strength of selection.)
Genetical and Ecological Dynamics
pi , pi´ : frequencies of gamete i in consecutive generations
Wjk : fitness of zygote consisting of gametes j, k
R(jki): probability that a jk-individual produces a
gamete of type i through recombination
W
kjkjjki ijkRppWWp
,
1)( '
WNN '
: mean fitness
Issues and problems to be addressed
• What aspects of genetics and what aspects of ecology are relevant, and under what conditions?
• When does FDS have important consequences for the genetic structure of a population?
• How does FDS affect the genetic structure?
• How much genetic variation is maintained by this kind of FDS?
Numerical Results from a Statistical Approach (with A. Gimelfarb)
Strength of frequency dependence, f
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
Pol
ymor
phis
m
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
n = 2, = 0n = 3, = 0n = 4, = 0
Figure, poly, th=0
Strength of frequency dependence, f
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
Pol
ymor
phis
m
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
n = 2, = 0n = 3, = 0n = 4, = 0n = 2, = 0.25n = 3, = 0.25n = 4, = 0.25
Figure, poly,
th=0, 0.75
Figure: poly
Strength of frequency dependence, f
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Pol
ymor
phis
m
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0n = 2, r = 0.5n = 3, r = 0.5n = 4, r = 0.5n = 2, 0 < r < 0.01n = 3, 0 < r < 0.01n = 4, 0 < r < 0.01
Stabilizing Complicated DisruptiveSelection:
Quad. App.:
The Weak-Selection or
Linkage-Equilibrium Approximation
• The structure is the same as in Turelli and Barton 2004 (but ). The proofs of the results below use their results.
• The population is assumed to be in demographic equilibrium, i.e., N and η are treated as constants.
• All models of intraspecific competition and stabilizing selection I know have the above differential equation as their weak-selection approximation.
• ‘Arbitrary‘ population regulation, i.e., with a unique stable carrying capacity, is admitted.
iv
General Conclusions
• The amount and properties of variation maintained depend in a nearly threshold-like way on , the strength of frequency and density dependence relative to stabilizing selection.
• This critical value is independent of the number of loci and, apparently, also of the linkage map.
Weak FDS
• If more than two loci contribute to the trait, then weak frequency dependence ( < 1) can maintain significantly more genetic variance than pure stabilizing selection, but still not much. The more loci, the larger this effect.
• FDS of such strength does not induce a qualitative change in the equilibrium structure relative to pure stabilizing selection.
• Such FDS does not lead to disruptive selection, rather, stabilizing selection prevails.
Strong FDS
• Strong FDS ( > 1) causes a qualitative change in the genetic structure of a population by inducing highly polymorphic equilibria in positive linkage disequilibrium.
• In parallel, such FDS induces strong disruptive selection, the fitness differences between phenotypes maintained in the population being much larger than under pure stabilizing selection.
Disruptive Selection
• Therefore, disruptive selection should be easy to detect empirically whenever FDS is strong enough to affect the equilibrium structure qualitatively.
• Its strength (the curvature of the fitness function at equilibrium) is s( – 1).
When Genetics Matters
• The degree of polymorphism maintained by strong FDS depends on the number of loci and the distribution of their effects.
• Models based on popular symmetry assumptions, such as equal locus effects or symmetric selection, are often not representative (they maintain more polymorphism).
• Linkage becomes important only if tight. It produces clustering of the phenotypic distribution. Otherwise, the LE-approximation does a very good job.
Outlook
• Include assortative mating to study the conditions leading to divergence within a population (work in progress K. Schneider).
• Determine the conditions under which sympatric speciation is induced by intraspecific competition.