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medical physiology :: Why Sex?
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by:
Johan H Koeslag
Medical Physiology
Stellenbosch University
PO Box 19063
Tygerberg, 7505.
South Africa
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INTRODUCTION
Sex is the exchange of genetic material between individuals. Gender and reproduction are widespread but incidental associations with sex. Unicellular organisms can exchange genetic material more or less at any time during their life cycles. For multicellular organisms the best, and most practically feasible time to exchange genetic material is at the one cell stage - hence the almost universal association of sex with reproduction in higher organisms.
THE EVOLUTIONARY PROBLEM
Having two genders is, however, a major unsolved evolutionary problem. The reason for this is that among genderless creatures (i.e. hermaphrodites and asexuals) all individuals can produce offspring. When there are males, then only half of the members of the community can produce offspring. Thus hermaphrodites and asexuals would be expected to produce twice as many offspring (and four times as many grand-offspring) as bi-gender sexual creatures do. Thus, if an hermaphrodite were to arise (by mutation) in a population of male and female creatures, then her progeny would rapidly replace the others, relegating the production of male and female offspring to something of the past. In technical jargon, hermaphrodites and asexual creatures have an inherent "two-fold selective advantage" over sexual creatures that produce male and female offspring. Despite this major evolutionary disadvantage, bi-gender sex is extremely common among multicellular organisms.
ORIGIN OF SEX
Primitive sex may have arisen for any number of possible reasons. Hickey and Rose1 have, for instance, suggested that sex resulted from DNA parasites that induced their hosts (i.e. the infected individuals) to exchange genetic material with their neighbours to promote the parasite's horizontal spread through the population. Once sex became established it conveyed a small selective advantage over asex because, whereas an asexual creature will pass all of its mutations on to everyone of its offspring, in sexual creatures the mutations are spread unevenly between the offspring. Some inherit none, while others get all of the mutations from both parents. Those that inherit an excess of mutations will die or, at least, reproduce less well than those that have fewer or no mutations. Sex is therefore a mechanism for shedding mutations. This advantage is, however, not great enough to compensate for the two-fold disadvantage of producing males.
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If creatures exchange genetic material, it is obviously in their best interests for the DNA they gain through the exchange to be of at least the same quality as their own, or better. Sexual creatures would therefore be expected to be choosey about their sexual partners; they should prefer fit partners over unfit partners. However, fitness is something that can only be recognized in retrospect. It is, by definition, the relative propensity to produce offspring and grand-offspring. At any one moment it is therefore impossible to predict which phenotypic features (physical and behavioural attributes) are fitter than their alternatives.
Thus, if, in the diagram below, the white, blue, yellow and red dots in generation 1 represent alternative nose colours in reindeer, then, in generation 1, it is totally impossible to even hazard a guess as to which colour is the best for a reindeer's nose.
After a few generations, however, there can be no doubt that red is selectively more advantageous than any of the other nose colours.
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Thus, it is only possible to tell which phenotypic features have proven themselves to be fit over the past several generations. If a given feature is common, and its alternatives are rare, then it is a fairly good bet that the common feature is fitter than any of the others (or, has been up until now). Sexual creatures would therefore be expected to prefer common features over unusual or rare features. This is known as koinophilia2,3,4,5. It is defined as a preference for mates sporting a preponderance of common, or average features.
Koinophilia has, as one of its inevitable side effects, the effect that sexual creatures will avoid mates displaying mutant features. Since most mutations are disadvantageous, this, in itself, would probably make sexual creatures obligatory koinophiliacs. In computer simulations to study competition between asexual and sexual reproduction, with and without koinophilia it could be shown that bi-gender, koinophilic sex could indeed outcompete asex if mutations with random fitnesses occurred randomly.6,7 The advantage of koinophilia was greatest when populations were large, the incidence of mildly disadvantageous mutations was high, and beneficial mutations were rare. Under these circumstances, koinophilic bi-gender sex could resist replacement by asexuals for over 10 000 generations (when the simulations were voluntarily stopped), even when one sexual mutated into a parthenogen every generation.
The reason that koinophilia provides its greatest advantage when the mutations are only mildly disadvantageous is that these are the mutations that are, by definition, cleared only very slowly by natural selection. They tend therefore to accumulate in large numbers in asexual creatures. (Highly disadvantageous mutations, which kill early in the life cycle, are weeded out by natural selection and therefore tend not to accumulate in the way that mildly disadvantageous mutations do.) Mild mutations obviously also tend to accumulate in sexual creatures, but koinophilia weeds them out with the same vigour that natural selection reserves for lethal and near-lethal mutations.
KOINOPHILIA EXPLAINS THE DISTRIBUTION OF SEX AND ASEX
Thus, the koinophilia hypothesis explains not only how bi-gender sex can resist replacement by asex, but also the reasons for the exceptions, which are not randomly distributed in nature. Asex is more common among unicellular organisms than among the more complex creatures such as the vertebrates. This is almost certainly due to the relative rates at which mutations appear in the two types of creature. Koinophilia is at its most effective when mutations occur frequently, which they are more likely to do in megagenomic creatures than in organisms with relatively fewer genes. Furthermore, asex tends to be especially common in small isolated populations (e.g. fish in isolated river systems8, and lizards and damselflies on small islands9,10), or where population densities are very low in arid environments, such as at high latitudes, on mountains or in deserts.11,12 In these environments, the choice of mates is not only limited, but the effectiveness of koinophilia as a genetic error detecting devise may be severely compromised. Genetic drift confounds the very principle on which it operates: commonness loses its association with fitness. In a computer simulation of such low population density conditions, with otherwise identical parameters (mutation rates etc.) as the before, the population was always replaced by asex within a few dozen generations.6,7
CAVEAT
It should be noted, however, that these considerations only indicate that koinophilic sexual creatures can afford to produce male offspring. They do not explain what evolutionary pressures cause them to do so (i.e. we do not know the advantage of producing males, and, in particular, we do not understand why the practice of producing males is so widespread in nature). Hermaphrodites enjoy the two-fold selective advantage of not producing males, and, being sexual, almost certainly reap all the benefits of koinophilia as well. So, why are they not more common than they are?
Last updated: 16 July 2007
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REFERENCES
1. HICKEY, D.A., ROSE, M.R. (1988) The role of gene transfer in the evolution of eukaryotic sex. In "The Evolution of Sex" Ed: MICHOD, R.E., LEVIN, B.R. Sunderland, Massachusetts: Sinauer. pp. 161-175.
2. KOESLAG, J.H. (1990). Koinophilia groups sexual creatures into species, promotes stasis, and stabilizes social behaviour. J. theor. Biol. 144, 15-35.
3. KOESLAG, J.H. (2003). Evolution of cooperation: cooperation defeats defection in the cornfield model. J. theor. Biol. 224, 399-410.
4. KOESLAG, J.H. (1997). Sex, the prisoner's dilemma game, and the evolutionary inevitability of cooperation. J. theor. Biol. 189, 53-61.
5. KOESLAG, J.H. (1995). On the engine of speciation. J. theor. Biol. 177, 401-409.
6. KOESLAG, P.D., KOESLAG, J.H. (1994). Koinophilia stabilizes bi-gender sexual reproduction against asex in an unchanging environment. J. theor. Biol. 166, 251-260.
7. KOESLAG, J.H., KOESLAG, P.D. (1993). Evolutionarily stable meiotic sex. J. Heredity 84, 396-399.
8. VRIJENHOEK, R., PFEILER, E. (1997). Differential survival of sexual and asexual Poeciliopsis during environmental stress. Evolution 51, 1593-1600.
9. INEICH, I (1999). Spatio-temporal analysis of the unisexual-bisexual Lepidodactylus lugubris complex (Reptilia, Gekkonidae). In "Tropical Island Herpetofauna." Ed: OTA, H. London: Elsevier. pp. 199-227.
10. CORDERO RIVERA, A., LORENZO CARBALLA, M.O., UTZERI, C., VIERA, V. (2005). Parthenogenetic Ischnura hastata (Say, wisespread in the Azores (Zygoptera: Coenagrionidae). Odonatologica 34, 1-9.
11. BELL, G. (1982). "The Masterpiece of Nature: the Evolution and Genetics of Sexuality." Berkley: University of California Press.
12. LEWIS, W.M. (1987). The cost of sex. In "The Evolution of Sex and its Consequences." Ed: STEARN, S.C. Basel: Birkhäuser. pp. 33-57.
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