We study the evolution of gene frequencies in a population living in R d , modelled by the spatial Λ-Fleming-Viot process with natural selection ([BEV10], [EVY14]). We suppose that the population is divided into two genetic types, a and A, and consider the proportion of the population which is of type a at each spatial location. If we let both the selection intensity and the fraction of individuals replaced during reproduction events tend to zero, the process can be rescaled so as to converge to the solution to a reaction-diffusion equation (typically the Fisher-KPP equation, [EVY14]). We show that the rescaled fluctuations converge in distribution to the solution to a linear stochastic partial differential equation. Depending on whether offspring dispersal is only local or if large scale extinction-recolonization events are allowed to take place, the limiting equation is either the stochastic heat equation with a linear drift term driven by space-time white noise or the corresponding fractional heat equation driven by a coloured noise which is white in time. If individuals are diploid (i.e. either AA, Aa or aa) and if natural selection favours heterozygous (Aa) individuals, a stable intermediate gene frequency is maintained in the population. We give estimates for the asymptotic effect of random fluctuations around the equilibrium frequency on the local average fitness in the population. In particular, we find that the size of this effect -known as the drift load -depends crucially on the dimension d of the space in which the population evolves, and is reduced relative to the case without spatial structure.AMS 2010 subject classifications. Primary: 60G57 60F05 60J25 92D10. Secondary:
60G15.2. Update q as in (6).Note that if we let w = |B(x, r)| −1 B(x,r) q t − (z) dz, then at a neutral reproduction event, P (k = a) = w and at a selective event, P (k = a) = w − F (w).Remark. The existence of a unique Ξ-valued process following these dynamics under condition (4) is proved in [EVY14, Theorem 1.2] in the special case F (w) = w(1 − w) (in the neutral case s = 0, this was done in [BEV10]). In our general case, the condition on w − F (w) allows us to define a dual process and hence prove existence and uniqueness in the same way as in [EVY14].We shall consider two different distributions µ for the radii of events, i) the fixed radius case : µ(dr) = δ R (dr) for some R > 0, ii) the stable radius case : µ(dr) = 1 r≥1 r d+α+1 dr for a fixed α ∈ (0, 2 ∧ d).