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

Advantage of sexual reproduction Reproduction is one of the properties of living systems that is essential for biological evolution.However, sexual reproduction is a modern product of this evolution, in a certain sense a luxury product and, as far as the mechanism of its emergence goes, a somewhat problematic product.Simultaneously, this type of reproduction clearly predominates in nature at the present time, at least in the number of species in which it occurs.It has been estimated that, amongst multicellular eukaryotes, only one species in a thousand is parthenogenetic (unisexual), i.e. reproduces through unfertilized eggs, or asexual, i.e. reproduces (entirely) without producing sex cells, for example through somatic clones (Simon et al. 2003).Asexual reproduction (agamogenesis or monogony) as an sole means of reproduction is encountered only in some groups of unicellular organisms (even here, however, research in the future will, in a great many cases, lead to the discovery of sexual processes) and also as a secondarily formed means of adjusting to specific living conditions and strategies in various unrelated taxa.In a number of fauna taxa, some form of parthenogenesis, i.e. some form of reproduction through unfertilized eggs, emerges as a consequence of parasitization by vertically transmitted microparasites, such as bacteria of the Wolbachia genus (Koivisto & Braig 2003).If these parasites cannot be transferred to progeny by sperm, they are very frequently capable of “manipulating” the female so that she does not produce – from their point of view – valueless males, but rather multiplies only parthenogenetically.Similarly in plants, parthenogenesis (denoted here by the term agamospermy)frequently emerges through the activity of “selfish genes” occurring in the genome of mitochondria and plastids, i.e. organelles transferred to progeny through eggs but not pollen in “higher plants”.
            The fact that asexual reproduction constitutes a secondary state in most taxa can be derived from the fact that sexual species occur amongst the relatives of asexual species and the asexual species represent only mutually isolated terminal twigs on the phylogenetic trees, i.e. individual species and genera, but not extensive branches of mutually diverse higher taxa, i.e. families, orders, classes and phyla.An exception mentioned in the evolutionary literature is the class of bdelloid rotifers (Bdelloidea, phylum Rotifera), a taxon with a phylogenetic age of at least 35 – 40 million years, whose 360 described species, divided into four families and 18 orders, apparently reproduce apomictically without participation by males.Analysis of the genome of four members of this group indicated that their originally diploid genome apparently changed over time to a genome that was functionally haploid and that sexual reproduction most probably never occurs in their life cycle (Welch & Meselson 2000).However, the newest results again throw doubt on this conclusion and indicate that recombination (and thus apparently also sex) occurs with very low frequency even here {9956}.Because the members of other classes of rotifers reproduce sexually, it is certain that asexual reproduction occurred secondarily in bdelloid rotifers.
While the disadvantages of sexual reproduction are mostly quite obvious, this is not as unambiguously true of its advantages.Simultaneously, the fact that, for a great many species and entire large taxa, such as birds and animals, it is the only means of reproduction, indicates that it should be an evolutionarily extremely advantageous process.The fact that asexually reproducing mutants do not gradually predominate in populations of sexually reproducing species also indicates that this is also an evolutionarily stable strategy (ESS) and that, in a population of sexually reproducing individuals, an asexually reproducing mutant is apparently in some way “penalized”, placed at an evolutionary disadvantage.
            At the present time, a number of hypotheses attempt to explain the emergence and persistence of sexual reproduction.The first group of hypotheses assumes that sexual reproduction increases the evolutionary potential of the particular biological species.The second group of hypotheses assumes that sexual reproduction brings an advantage to its carrier, i.e. increases its direct fitness or inclusive fitness.The third group of hypotheses corresponds to a model that assumes that sexual reproduction can be maintained by the mechanism of an evolutionary trap and does not bring its carriers any advantage, while the fourth group assumes that sexual reproduction could have been forced on organisms from outside, and provides an advantage to other subjects of biological evolution at their expense. See alsoNegative heritability of fitness model,  Lottery model,  Elbow room hypothesis, Repairing mutations hypothesis, Stopping the Muller's ratchet hypothesis, Simultaneous selection hypothesis, Genetic elite hypothesis, Sisyphean genotypes hypothesis, Tangled bank hypothesis,  Mills of God model, Evolutionary trap hypothesis, Evolutionary constraints hypothesis, and Manipulation hypothesis of the origin of sexuality.