A gradual drifting away of a species from its previous form happens on small islands or in tiny populations precisely because of effects somewhat analogous to Muller’s ratchet: the chance extinction of some forms and the chance prosperity of other, mutated forms. In larger populations the process that hinders this is sex itself, for an innovation is donated to the rest of the species and quickly lost in the crowd. In island populations sex cannot do this precisely because the population is so inbred.
