Since hypotheses for the origins of sex are difficult to test experimentally, most current work has focused on the maintenance of sexual reproduction. Sexual reproduction must offer significant fitness advantages to a species because despite the two-fold cost of sex, it dominates among multicellular forms of life, implying that the fitness of the offspring produced outweighs the costs. The following five theories can be split into three categories: Genetic variation, Adaptation and Mutation. The first two theories focus on genetic variation, the third on adaptation and finally the last two on mutations. These models are only a selection of the ones available.
1. Lottery Principle
First proposed by George Williams and suggests that sexual reproduction introduced genetic reproduction in order to enable genes to survive in changing or novel environments. Williams uses the lottery analogy, breeding asexually is like buying many tickets but with the same number whereas sexual reproduction is more making do with fewer tickets but having different numbers (Williams, 1975). Since sex introduces variability, organisms would have an improved chance of producing offspring that will survive if they reproduce a range of types rather than merely more of the same. Asexual reproduction is poorly equipped to adapt to rapidly changing conditions because the offspring are essentially clones of the parents and therefore possess less genetic variation. Parasites are used to provide an illustration of this principle (Hamilton, 1980; Janovy et al., 1992). When a host is first invaded, usually the parasite reproduces asexually, but once the niche is filled new offspring are produced using sexual production and will leave to infect new hosts. On the side of this theory it helps to explain why organisms such as aphids choose to multiply asexually when environmental conditions are stable, but switch to sexual reproduction when facing a changing future. One of the difficulties against this theory is the diversity of the species. It suggests that sex would be favoured by a variable environment, however when looking at the global distribution of sex, it is seen that where environments are stable, sexual reproduction is most common; whereas asexual reproduction is most common in areas where the environment is unstable (Case & Taper, 1986; Dagg, 2016).
2. Tangled Bank
This theory proposes that sex evolved in order to prepare offspring for the world around them. It was developed from the lottery principle by Michael Ghiselin. The term comes from Darwin’s book The Origin of Species, where it refers to the wide assortment of organisms that are all competing for light, space and food on a ‘tangled bank’. This concept states that in any given environment where there exists intense competition for space, food, and resources, a premium is placed on diversification(Ghiselin,1974; Pound et al., 2002;Scheu & Drossel, 2007). Although once popular, the theory faces several problems. The theory would predict a greater interest in sex among animals that produce lots of small offspring that compete with one another. Sex is invariably associated with organisms that produce a few large offspring (K-selection), whereas organisms producing small offspring (r-selection) frequently engage in parthenogenesis. In addition, the evidence from the fossil record suggests that species go for vast periods of geologic time without changing much. Some argue that bacteria ‘evolved’ in such a fashion as to ultimately be responsible for sexual reproduction. But in light of this model it becomes inexplicable why bacteria themselves remain virtually unchanged for billions of years of Earth history. It should also be noted that we still see organisms today reproducing asexually as well as organisms that reproduce sexually (Song et al., 2011).
3. Red Queen Hypothesis
Now this theory is viewed as one of the most promising explanations of the evolutions of sex. It was first proposed by Leigh Van Valen in 1973, whilst studying marine fossils. He founded that the probability of a family of marine organisms becoming extinct at one time has no relation to how long it has already survived. The name of this theory is taken from Alice in Wonderland where the Red Queen's catchphrase was "It takes all the running you can do to stay in the same place" which is a good description of co-evolution between competing species. The application of this theory to the problem of the maintenance of sex is summed up by the phrase "genetic arms race" (Ochoa & Jaffe, 1999). Prey, predators and parasites all co-evolve with each other, hoping that any tiny advantage gained by favourable variation can give an edge over close competitors, predators, prey or parasites. No single species progresses too far ahead because genetic variation among the progeny of sexual reproduction provides all species with a mechanism to improve. Species which cannot keep up will become extinct. This hypothesis gains support from other studies too. For example, the comparative approach to sexual reproduction (Bell, 1982; Bell & Maynard-Smith, 1987; Sinervo & Svensson, 2002). Sickle cell anaemia where the mutation that has independently occurred, at least four times in human history, with the same gene involved, indicating that this gene must have been preserved for some reason and parasitic pressure caused it to be manifested (Lively, 1990; Otto, 2008). Lastly the industrial melanism shown by the peppered moth is an excellent example of this process in action. Without this mutation there would be a chance that the species may have become extinct (Otto & Nuismer, 2004).
4. Muller's Ratchet
In genetics, this hypothesis explains how functionally important genes may be lost when organism's genes are only transmitted vertically, without recombination caused by sex (Muller, 1932; Muller, 1964). Exclusive vertical transfer occurs when the organism is an endosymbiont, e.g. a bacterial endosymbiont of insects, which is only transmitted to offspring from the mother. Another example is mitochondria and chloroplasts, which are also transmitted only vertically (Lynch & Blanchard, 1998) . Muller's ratchet applies to any deleterious mutation that occurs in a vertically transmitted organism (Havird et al., 2015). It may be that the mutation is deleterious, but not lethal. Furthermore, the organism with the mutation may have another advantageous mutation or its host may have an advantageous mutation, that will lead to their survival despite the deleterious mutation. There is no possibility for genetic mixing, so the organism's descendants have no opportunity to receive a good copy of the gene. If the other mutation is advantageous, their survival means that the deleterious mutation persists. These deleterious mutations resemble the operation of a ratchet, in that the organism can never go back. A deleterious mutation coupled with an advantageous one can be undone in organisms with sexual reproduction (Moran, 1996).
5. DNA Repair
Michod proposes that sex evolved as a mechanism of DNA repair. He argues that meiosis and crossing over can function to repair and or potentially mask both kinds of genetic errors, damage and mutation. Genetic changes that mechanistically hinder the separation and or linking of DNA strands are considered damage because replication is not possible without repair. Mutations, however, do not interfere with replication and cannot be recognized by repair enzymes and thus this type of error can only be corrected by a backup chromosome or crossing over (Michod 1995). DNA can be damaged in at least two different ways. It can be damaged in situ by ionising radiation, mutagenic chemicals or a mutation can occur through errors of replication. This is sometimes best thought as change rather than damage as it can alter the genetic code. The second way errors can occur is during the replication process. If the damage is on a single-strand it can be made right by enzymes using the other correct strand as a template, however double-strand damage is more serious; here the cell may die. Repair on a damaged double-strand can be repaired during the crossing over in meiosis. In asexual organisms, any mutation that occur in one generation will be passed on to the subsequent generation. In this case as asexual organisms accumulate mutation they eventually face the prospect of being unable to reproduce or unviable. As sexual reproduction allows most plants and animals to produce offspring with good copies of two genes via crossover and this then helps eliminates the effect of DNA damage as it is less likely to be passed onto the next generation as it must first appear in the genes of both parents (Keightley & Otto 2006). Computer simulations (Howard & Lively, 1994) suggest that rates of deleterious mutations, selection pressure against these mutations, parasite transmission rate, and parasite virulence interact to maintain the prevalence of sex in populations.
References:
Bell, G., 1982. The Masterpiece of Nature:: The Evolution and Genetics of Sexuality. CUP Archive.
Bell, G. & Maynard Smith, J.1987. Short-term selection for recombination among mutually antagonistic species. Nature, 328 : 66-68
Case, T.J. and Taper, M.L., 1986. On the coexistence and coevolution of asexual and sexual competitors. Evolution, pp.366-387.
Dagg, J., 2016. The Maintenance of Sex and David Lack's Principle. bioRxiv, p.035832.
Ghiselin, M.T., 1974. A radical solution to the species problem. Systematic Biology, 23(4), pp.536-544.
Hamilton W.D 1980 Sex versus non-sex versus parasite. Oikos. 35, 282–290
Havird, J.C., Hall, M.D. and Dowling, D.K., 2015. The evolution of sex: a new hypothesis based on mitochondrial mutational erosion. Bioessays,37(9), pp.951-958.
Janovy Jr, J., Clopton, R.E. and Percival, T.J., 1992. The roles of ecological and evolutionary influences in providing structure to parasite species assemblages. The Journal of parasitology, pp.630-640.
Keightley, P.D. and Otto, S.P., 2006. Interference among deleterious mutations favours sex and recombination in finite populations. Nature,443(7107), pp.89-92.
Lively, C.M., 1990. Red Queen hypothesis supported by parasitism in sexual and clonal fish. Nature, 344, p.26.
Lively, C. M., and R. S. Howard. 1994. Selection by parasites for clonal diversity and mixed mating. R. Soc. B 346:271-281
Lynch M, Blanchard JL. 1998. Deleterious mutation accumulation in organelle genomes. Genetica 102–103: 29–39.
Michod, R. E. 1995. Eros and Evolution: A Natural Philosophy of Sex. Addison-Wesley Publishing, 241pp
Muller H.J 1964 The relation of recombination to mutational advance. Mutat. Res. 1, 2–9
Muller, H.J. 1932 Some Genetic Aspects of Sex. American Naturalist 66:118-138
Moran, N.A. 1996, Accelerated evolution and Muller’s ratchet in endosymbiotic bacteria,Proceedings of the National Academy of Sciences USA, 93, pp. 2873-2878.
Ochoa, G. and Jaffe, K. 1999. On sex, mate selection and the Red Queen. J. Theor. Biol 199: 1-9
Otto S.P, Nuismer S.L 2004 Species interactions and the evolution of sex. Science. 304, 1018–1020
Otto, S., 2008. Sexual reproduction and the evolution of sex. Nature education, (1), p.182.
Pound, G.E., Doncaster, C.P. and Cox, S.J., 2002. A Lotka–Volterra model of coexistence between a sexual population and multiple asexual clones. Journal of theoretical biology, 217(4), pp.535-545.
Scheu, S. and Drossel, B., 2007. Sexual reproduction prevails in a world of structured resources in short supply. Biological Sciences, 274(1614), pp.1225-1231.
Sinervo, B. and Svensson, E., 2002. Correlational selection and the evolution of genomic architecture. Heredity, 89(5), pp.329-338.
Song, Y., Drossel, B. and Scheu, S., 2011. Tangled Bank dismissed too early. Oikos, 120(11), pp.1601-1607.
Williams, G.C., 1975. Sex and evolution (No. 8). Princeton University Press.
