The stable co-existence of two haploid genotypes or two species is studied in a spatially heterogeneous
environment submitted to a mixture of soft selection (within-patch regulation) and
hard selection (outside-patch regulation) and where two kinds of resource are available. This is
analysed both at an ecological time-scale (short term) and at an evolutionary time-scale (long
term). At an ecological scale, we show that co-existence is very unlikely if the two competitors
are symmetrical specialists exploiting different resources. In this case, the most favourable conditions
are met when the two resources are equally available, a situation that should favour
generalists at an evolutionary scale. Alternatively, low within-patch density dependence (soft
selection) enhances the co-existence between two slightly different specialists of the most available
resource. This results from the opposing forces that are acting in hard and soft regulation
modes. In the case of unbalanced accessibility to the two resources, hard selection favours the
most specialized genotype, whereas soft selection strongly favours the less specialized one. Our
results suggest that competition for different resources may be difficult to demonstrate in the
wild even when it is a key factor in the maintenance of adaptive diversity. At an evolutionary
scale, a monomorphic invasive evolutionarily stable strategy (ESS) always exists. When a linear
trade-off exists between survival in one habitat versus that in another, this ESS lies between
an absolute adjustment of survival to niche size (for mainly soft-regulated populations) and
absolute survival (specialization) in a single niche (for mainly hard-regulated populations). This
suggests that environments in agreement with the assumptions of such models should lead to an
absence of adaptive variation in the long term.