Supporting Information 2. Analytical approximation of selection coefficients
generated by host-parasite coevolution.
Intuitively, host-parasite coevolution in which offspring are biased towards encountering
parasites transmitted by their own mothers should result in both genotypic and similarity
selection. Here I describe how to calculate the selection generated in an one-locus host-
parasite model in terms of the selection coefficients used in the single-species model
presented in the main text (i.e., equation 1).
First, I determine the relationship between the selection coefficients and the
fitnesses of different offspring/mother combinations. Let wi,j be the fitness of an offspring
of genotype i from a mother of genotype j. Using the single-species fitness function
given by equation 1, multiplied by a scaling factor K, we obtain a set of equations relating
fitnesses to selection coefficients: wAA,AA = K(1 A), wAA,Aa = K(1 a), wAa,AA = K(1
a), wAa,Aa = K(1 a)(1 a)(1 A)(1 A,a), wAa,aa = K(1 a)(1 a)2
A,a), waa,Aa = K((1 a)2
+ A)(1 a), and waa,aa = K((1 a)2
+ A)(1 a)2