Assortative mating may split a population even in the absence of natural se
lection. Here, we study when this happens if mating depends on one or two q
uantitative trails. Not surprisingly, the modes of assortative mating that
can cause sympatric speciation without selection are rather strict. However
, some of them may occur in nature. Slow elimination of intermediate indivi
duals caused by the gradual tightening of assortative mating, which evolves
owing to relatively weak disruptive selection, provides the alternative sc
enario for sympatric speciation, in addition to fast elimination of interme
diate individuals as a result of the direct action of strong disruptive sel
ection under an invariant mode of assortative mating. Even when assortative
mating alone cannot split an initially coherent population, it may be able
to prevent the merging of species after their secondary contact.