A model of sympatric speciation through assortative mating

Abstract

A microscopic model is developed, within the frame of the theory of quantitative traits, to study the combined effect of competition and assortativity on the sympatric speciation process, i.e. speciation in the absence of geographical barriers. Two components of fitness are considered: a static one that describes adaptation to environmental factors not related to the population itself, and a dynamic one that accounts for interactions between organisms, e.g. competition. A simulated annealing technique was applied in order to speed up simulations. The simulations show that both in the case of flat and steep static fitness landscapes, competition and assortativity do exert a synergistic effect on speciation. We also show that competition acts as a stabilizing force against extinction due to random sampling in a finite population. Finally, evidence is shown that speciation can be seen as a phase transition. The problem.The notion of speciation in biology refers to the splitting of an original species into two fertile, yet reproductively isolated strains. The allopatric theory, which is currently accepted by the majority of biologists, claims that a geographic barrier is needed in order to break the gene flow so as to allow two strains to evolve a complete reproductive isolation. On the other hand, many evidences and experimental data have been reported in recent years strongly suggesting the possibility of a sympatric mechanism of speciation. For example, the comparison of mythocondrial DNA sequences of cytochrome b performed by Schlieven and others [1], showed the monophyletic origin of cichlid species living in some volcanic lakes of western Africa. The main features of these lakes are the environmental homogeneity and the absence of microgeographical barriers. It is thus possible that the present diversity is the result of several events of sympatric speciation. An increasing number of studies referring both to animal and plant species lend further support to this hypothesis [2,3,4,5,6,7,8,9].The key element for sympatric speciation is assortative mating that is, mating must be allowed only between individuals whose phenotypic distance does not exceed a given threshold. In * Electronic address: franco.bagnoli@unifi.it † Electronic address: carlo@dma.unifi.it 1 fact, consider a population characterized by a bimodal distribution for an ecological character determining adaptation to the environment: in a regime of random mating the crossings between individuals of the two humps will produce intermediate phenotypes so that the distribution will never split. Two interesting theories have been developed to explain the evolution of assortativity. In Kondrashov and Kondrashov's theory [10] disruptive selection (for instance determined by a bimodal resource distribution) splits the population in two distinct ecological types that are later stabilized by the evolution of assortative mating. The theory of evolutionary branching developed by Doebeli and Dieckmann [11] is more general in that it does not require dis...

A microscopic model of evolution of recombination

“…The simulations also show (in agreement with the findings in our study on sympatric speciation [29]) that competition acts as a stabilizing force allowing the survival of strains in regions of the phenotypic space where the low static fitness level is counteracted by a low competition pressure.…”

Evolution of biodiversity and sympatric speciation through competition in a unimodal distribution of resources

“…As already mentioned, positive assortative mating plays a basic role in the path towards speciation. This is obtained by allowing mating only if the distance in phenotype space between two partners is smaller than some predefined threshold a [18]. It is interesting to notice that several models (for example in [11]) implement some mechanism that, by normalizing mating probabilities, guarantees an equal reproductive success for all individuals.…”