Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

A large proportion of these extensive genome restructurings cause partial or complete sterility in their bearers or at least form very effective interspecies barriers.However, polyploidization is simultaneously a mechanism that enables hybrid speciation.If two species cross, the zygotes cannot normally develop, because the gametes of the two species contain different sets of chromosomes.Thus, during cell division, they cannot form regular pairs of homologous chromosomes, so that the two chromosome sets divide unevenly into the daughter cells.As a consequence, most cells are not viable.However, if polyploidization occurs prior to hybridization, either autopolyploidization as a consequence of mitosis, which is not followed by cellular division, or alopolyploidization as a  consequence of fusion, e.g. of two diploid cells derived from two different  species in a single tetraploid cell or, more frequently, through a triploid intermediate stage (see XXI.5.3), the situation is substantially more favourable.If, for example, two tetraploid organisms cross together, their gametes already contain pairs of homologous chromosomes, so that a quite regular dividing spindles are formed during cell division and the chromosomes can be divided completely evenly amongst the daughter cells.Examples of hybrid speciation are encountered very frequently in some families of plants and also in animals with parthenogenetic reproduction, for example in some daphnia (Daphnia) (Dufresne & Hebert 1994).