  The speciation model proposed by Derrida and Higgs demonstrated that a
sexually reproducing population can split into different species in the absence
of natural selection or any type of geographic isolation, provided that mating
is assortative and the number of genes involved in the process is infinite.
Here we revisit this model and simulate it for finite genomes, focusing on the
question of how many genes it actually takes to trigger neutral sympatric
speciation. We find that, for typical parameters used in the original model, it
takes of the order of $10^5$ genes. We compare the results with a similar
spatially explicit model where about 100 genes suffice for speciation. We show
that when the number of genes is small the species that emerge are strongly
segregated in space. For larger number of genes, on the other hand, the spatial
structure of the population is less important and the species distribution
overlap considerably.
