Spatial Niche Packing, Character Displacement and Adaptive Speciation Along an Environmental Gradient

In this study, we examine the ecology and adaptive dynamics of an asexually reproducing population, migrating along an environmental gradient. The living conditions are optimal at the central location and deteriorate outwards. The different strategies are optimized to the ecological conditions of different locations. The control parameters are the migration and the tolerance of the strategies towards the environmental condition (location). Locally, population growth is logistic and selection is frequency-independent, corresponding to the case of a single limiting resource. The behavior of the population is modeled by numerically integrated reaction-diffusion equations as well as by individual-based simulations. Limiting similarity, spatial niche segregation and character displacement are demonstrated, analogous to resource-heterogeneity based niche partitioning. Pairwise invasibility analysis reveals a convergent stable singular strategy optimized to the central, optimal location. It is evolutionary stable if the migration rate and the tolerance are large. Decreasing migration or decreasing tolerance bifurcates the singular strategy to an evolutionary branching point. Individual-based simulation of evolution confirms that, in the case of branching singularity, evolution converges to this singular strategy and branches there. Depending on the environmental tolerance, further branching may occur. The branching evolution in the asexual model is interpreted as a sign that the ecology of an environmental gradient is prone to adaptive geographic speciation