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

In sexual reproduction, the accumulation of a large number of lethal or more or less semi-lethal recessive alleles can represent an evolutionary trap; these alleles occur with low frequency in the gene pool of every diploid organism but cannot accumulate in haploid organisms. These mutations are active only in the homozygote or hemizygote state and thus are not very influential in an outbred population. However, as soon as a diploid organism begins to reproduce asexually, its offspring become homozygote in the given recessive lethal and semi-lethal genes, which results, at the very least, in reduction of their fitness. Consequently, only a small percentage of mutants can go back from sexual reproduction to asexual reproduction. Transition from asexual reproduction to sexual reproduction (permitting the persistence and accumulation of recessive lethal mutations), similar to transition from haploidy to diploidy, can be a one-way route for more complex organisms, a sort of evolutionary trap, in which most species of organisms finally end up (Bernstein et al. 1985; Crow 1994).