Evol Ecol Res 2: 981-995 (2000)     Full PDF if your library subscribes.

Adaptive diversity in heterogeneous environments for populations regulated by a mixture of soft and hard selection

T. de Meeûs¹ and J. Goudet²

¹Laboratoire de Parasitologie Comparée, UMR 5555 CNRS, Université Montpellier II, Place E. Bataillon, 34095 Montpellier Cedex 05, France and ²Institut de Zoologie et d’Ecologie Animale, Université de Lausanne, Bâtiment de Biologie, 1015 Lausanne, Switzerland

Address all correspondence to Thierry de Meeûs, Centre d’Etude sur le Polymorphisme des Micro-organismes (CEPM), UMR 9926 CNRS-IRD, Centre IRD, 911 Avenue d’Agropolis, 34032 Montpellier Cedex 01, France.
e-mail: demeeus@cepm.mpl.ird.fr

ABSTRACT

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.

Keywords: adaptive polymorphism, co-existence, density dependence, hard selection, heterogeneous environments, invasive ESS, population regulation, soft selection.

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