XIII.1 Sexual reproduction substantially predominates in contemporary eukaryotic organisms.

Reproductionis 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.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.