Extinction and hybridization in a neutral model of speciation

Evolution is usually pictured as a tree where ancient species branch into new ones and eventually disappear. In this simplified view, the balance between speciation and extinction fully determines the diversity of life. Hybridization, however, introduces another level of complexity, allowing neighboring branches of the tree to interact, mixing their genetic content. This generates further diversity leading to reticulated trees. In this paper we study processes of speciation, extinction and hybridization using a genetically and spatially explicit neutral model of diversification. The model is based on the Derrida-Higgs formulation, where the genome of haploid individuals is represented by binary strings and reproduction is constrained by genetic similarity. Tracking all events of speciation, extinction and hybridization throughout the evolutionary process allows us to compute complete and exact phylogenetic trees. We found that genome size played a key role in these processes, increasing the extinction rate and decreasing the hybridization rate. Only in the limit of large genomes the simplified picture of a branching tree is recovered. Most hybridization events occurred between relatively abundant species, discarding lack of sexual partners or small population sizes as potential causes. We found that hybridization occurred mostly because of opportunity (genetic similarity and spatial proximity) between recently branched species, when the number of accumulated mutations is not yet too large.