  The evolutionary and ecological processes behind the origin of species are
among the most fundamental problems in biology. In fact, many theoretical
hypothesis on different type of speciation have been proposed. In particular,
models of sympatric speciation leading to the formation of new species without
geographical isolation, are based on the niche hypothesis: the diversification
of the population is induced by the competition for a limited set of the
available resources. On the other hand, neutral models of evolution have shown
that stochastic forces are sufficient to generate coexistence of different
species. In this work, we bring this dichotomy to the context of species
formation, and we study how neutral and niche forces contribute to sympatric
speciation in a model ecosystem. In particular, we study the evolution of a
population of individuals with asexual reproduction whose inherited characters
or phenotypes are specified by both niche-based and neutral traits. We analyse
the stationary state of the dynamics, and study the distribution of individuals
in the whole space of possible phenotypes. We show, both by numerical
simulations and analytics, that there is a non-trivial coupling between neutral
and niche forces induced by stochastic effects in the evolution of the
population that allows the formation of clusters (i.e., species) in the
phenotypic space. Our framework can be generalised also to sexual reproduction
or other type of population dynamics.
