The sex or sexual reproduction is a variation of the propagation of eukaryotes (animals with nucleus), in which alternating nuclei fuse together ( karyogamy ), wherein the number of chromosomes in the nucleus (the ploidy level ) is doubled, and in one particular form of nuclear division , the meiosis or reduction division, the number of chromosomes is halved again. This change between a haploid and a diploid phase is known as the nuclear phase change .
Most eukaryotes form special gametes (sex cells ) which fuse to form a zygote during fertilization , in which karyogamy then takes place. In humans and in most other eukaryotes, the gametes are differentiated as female and male. In many cases (e.g. with many algae ) they do not differ externally, which is why one does not speak of sexes, but of mating types , of which more than two can be present. Fungi do not form gametes; With them fertilization takes place in that hyphae fuse apically (at the tips) or special multinucleated sexual organs ( gametangia ) are formed for this purpose (see mushrooms # Sexual reproduction ).
In addition to bisexual or bisexual reproduction, unisexual or unisexual reproduction also occurs in many eukaryotes, in which new individuals emerge from female sex cells without fertilization ( parthenogenesis or apomixis ). A distinction is made between these two forms of sexual reproduction, asexual or asexual reproduction , in which the degree of ploidy remains unchanged.
Sexual reproduction is at least occasional in most eukaryotes. The recombination of hereditary factors ( genes ) from generation to generation increases the genetic diversity within the population . Almost always, but not necessarily, it is associated with an increase (increase in the number of individuals).
Since the division of the chromosomes in meiosis happens randomly, the genes of the parents are mixed on to the offspring during sexual reproduction. This results in a large number of gene combinations in the offspring, which in the sense of the theory of evolution can be newly adapted individuals. The associated faster rate of adaptation is more of an advantage under adverse environmental conditions with high selection pressure.
The high number of possible combinations of parental genes in the offspring is seen as an evolutionary advantage of sexual reproduction (compared to asexual reproduction). In addition, it is advantageous if an individual carries two genes of the same type ( alleles ) so that a disadvantageous mutation in one gene does not always have to come into play for the individual, since there is a chance that the function will be replaced by the other gene . The trisomies in humans, however, indicate that this does not always have to be associated with advantages.
Sexual reproduction and genetic recombination reduce the likelihood of Muller's ratchet ; H. the accumulation of harmful mutations over time in asexual reproduction. This will u. a. supported by findings in water fleas, which can reproduce both sexually and asexually.
Ultimately, it reduces competition between beneficial alleles on different genes. Two mutations developed independently of one another can unite at some point in an individual, while one of the two is inevitably suppressed during asexual reproduction ("clonal interference").
It is not known exactly why sexual reproduction developed in nature in the first place. After all, asexual reproduction is much less energy-consuming: no second (male) sex is required; it must be generated no major germ cells and finally with respect to the proliferation superior sexual reproduction. Evolutionary biologist John Maynard Smith worked out that after four generations of parthenogenesis, a female will have produced almost four times more offspring than a sexually reproducing couple. Asexual reproduction and self-fertilization also have the advantage that they work even with very low population densities. If foreign gametes are missing (lack of pollination , low population density), some animal and plant species switch to these types of reproduction. If the density of a population falls below a certain limit, there is a risk of the species becoming extinct in species that are compulsorily reproduced by xenogamy (cross-fertilization) .
- Sarah P. Otto: Sexual Reproduction and the Evolution of Sex . In: Nature Education 1 (2008), 182.
- Matthew R. Goddard, H. Charles J. Godfray, Austin Burt: Sex increases the efficacy of natural selection in experimental yeast populations . In: Nature . tape 434 , no. 7033 , March 31, 2005, ISSN 0028-0836 , p. 636–640 , doi : 10.1038 / nature03405 ( nature.com [accessed October 26, 2017]).
- Susanne Paland, Michael Lynch: Transitions to Asexuality Result in Excess Amino Acid Substitutions . In: Science . tape 311 , no. 5763 , February 17, 2006, ISSN 0036-8075 , p. 990–992 , doi : 10.1126 / science.1118152 , PMID 16484491 ( sciencemag.org [accessed October 26, 2017]).
- John Maynard Smith: The evolution of sex . Cambridge Univ. Press, Cambridge 1978, ISBN 978-0-521-29302-0 .