Computer simulation and mathematical modeling of the interactions between ecological and sexual selection to reveal the mechanism of sympatric speciation

In modern evolutionary theory, the mechanism of sympatric speciation is still an unsolved mystery. Ecological niche specialization and the subsequent premating reproductive isolation (RI) are the putative initial steps of sympatric speciation, even though the exact mechanism of how the former leads to the latter remains unclear. This study aims to develop a simple, intuitive, and realistic model ecosystem that uses the fewest variables possible but is still comprehensive enough to uncover the fundamental mechanisms of sympatric speciation. Applying the concept of an adaptive fitness landscape, the study used an individual-based computer simulation to investigate how ecological selection and sexual selection may interact to produce sympatric premating RI. Mathematical models were derived to describe the nonlinear adaptive dynamics of the system. A user-friendly computer application was able to solve, numerically, all the trajectories, fixed points, and bifurcation points of the models and display the solutions as phase portraits. The findings reveal that under favorable conditions, fixed points may emerge in the systems phase portraits to create stable mating-bias-allele polymorphisms and varying degrees of premating RI between niche ecotypes. Solving the nonlinear equations of the mathematical models establishes the precise parametric values required to produce such fixed points. It also specifies the conditions necessary for high-mating-bias mutant alleles to invade and produce, or enhance, premating RI. Restricting migration between niches impedes mutant invasion in parapatric and allopatric populations. Multi-locus mating traits may produce hybrids that prevent speciation. Simulation results of a gonochoric, multi-niche system demonstrate that linkage disequilibrium can emerge spontaneously among ecological, male-trait, and female-preference genotypes to produce premating RI. From these results, a five-stage mechanism of sympatric speciation is formulated: Initially, disruptive ecological selection creates selection pressure for high-mating-bias alleles to invade and rapidly establish premating RI. This then paves the way for the recruitment of late-stage mechanisms, such as adaptive coupling and post-zygotic isolation, to complete the speciation process. Applying nonlinear dynamics theories to model and analyze sympatric speciation has yielded novel findings that support and extend the results of prior studies. These new discoveries may help to overcome the theoretical objections to sympatric speciation and establish it as a prevalent mode of speciation in nature.