Do we know why animals have evolved to have sex for reproduction?

Question

I've been reading 13 Things That Don't Make Sense: The Most Intriguing Scientific Mysteries of Our Time (2010) and Chapter 10 concentrates on the subject of Sex. The author argues that we do not know why humans, or animals, have evolved to reproduce sexually rather than asexually.

Asexual reproduction is far more efficient according to the author, who backs his claims up by citing Richard Dawkins amongst others.

He hasn’t done it yet. In his 2004 book The Ancestor’s Tale, he again admits defeat over the origin of sex. “To do justice to all the theories would take a book—it has already taken several . . . Yet no definitive verdict has emerged.” In the end, he settles for discussing a consequence of sexual reproduction, rather than explaining its origin. The question of what is so good about sex is one that “betters scientists than I have spent book after book failing to answer,” Dawkins admits.

He suggests we have Ptolemaic view of sexual reproduction, which has been shaped over the centuries (quote needed)

Has any progress been made to explain why animals reproduce sexually? Or is it an incorrect assertion in the first place, and we've known for a while?

Answer 1

TL;DR: Sexual reproduction, by providing avenue for recombination of genes, assists in mitigating harmful mutations.

Neiman, Gery Hehman, Joseph T. Miller, John M. Logsdon, Jr., and Douglas R. Taylor. "Accelerated Mutation Accumulation in Asexual Lineages of a Freshwater Snail". Molecular Biology and Evolution, 2009; DOI: 10.1093/molbev/msp300

Abstract

Sexual reproduction is both extremely costly and widespread relative to asexual reproduction, meaning that it must also confer profound advantages in order to persist. One theorized benefit of sex is that it facilitates the clearance of harmful mutations, which would accumulate more rapidly in the absence of recombination. The extent to which ineffective purifying selection and mutation accumulation are direct consequences of asexuality and whether the accelerated buildup of harmful mutations in asexuals can occur rapidly enough to maintain sex within natural populations, however, remain as open questions. We addressed key components of these questions by estimating the rate of mutation accumulation in the mitochondrial genomes of multiple sexual and asexual representatives of Potamopyrgus antipodarum, a New Zealand snail characterized by mixed sexual/asexual populations. We found that increased mutation accumulation is associated with asexuality and occurs rapidly enough to be detected in recently derived asexual lineages of P. antipodarum. Our results demonstrate that increased mutation accumulation in asexuals can differentially affect coexisting and ecologically similar sexual and asexual lineages.

The accelerated rate of mutation accumulation observed in asexual P. antipodarum provides some of the most direct evidence to date for a link between asexuality and mutation accumulation and implies that mutational buildup could be rapid enough to contribute to the short-term evolutionary mechanisms that favor sexual reproduction.

Answer 2

John Holland's "Adaptation in Natural and Artificial Systems" discusses in great deal the algorithmic basis of evolution. Apropos the question, he says:

One key to understanding [...] lies in observing what happens to a small set of alleles under [a reproductive strategy]'s action. In particular, what happens if an adjacent set of alleles appears in several different chromosomes of above-average fitness and not elsewhere? Because each of the chromosomes will be duplicated an above-average number of times, the given alleles will occupy an increased proportion of the population after the duplication phase. This increased proportion will of course result whether or not the alleles have anything to do with the above-average fitness. The appearance of the alleles in the extra-fit chromosomes might be happenstance, but it is equally true that any correlation between the given selection of alleles and above-average fitness will be exploited by this action. Moreover, the more varied the chromosomes containing the alleles, the less likely it is that the alleles and above-average fitness are uncorrelated.

(Holland, 1992, pg. 15) emphasis added

In other words, recombination plays a larger role than is generally understood. Most people think in terms of small mutations leading to specific changes because that fits with how an intentional design is improved. But in fact its recombination, acting across the entire genome and in the context of a large population, that improves the average fitness of the next generation.

Sexual reproduction, although expensive and not the only conceivable route to massive recombination, provides far more recombination than what gets from asexual reproduction.

Answer 3

Modelling sex vs asexual with respect to parasites showed in 1980 that sexual reproduction

...in certain states of cycling sexual species easily obtain higher long-term geometric mean fitness than any competing monotypic asexual species or mixture of such.

And this was confirmed in landmark experiment in 2011 published in Science. Here a worm (C. elegans.) which normally reproduced both sexually and asexually was modified to isolate a strain which only reproduces sexually, and one which only reproduces asexually.

They then infected the worms with a bacterial parasite (S. marcescens) which can proved lethal within 24hours if the worm is not resistant. Both the worm and the bacteria can evolve resistance/virulence over time.

In this experiment the sexually reproducing worms had a much lower mortality than the asexually reproducing species when infected with the bacteria. This effect was much greater after being co-cultured with the bacteria for 10 generations, where ~70% of the asexual worms died while only ~18% of the sexually reproducing worms died after 30 generations. The asexually reproducing lines all went extinct after 20 generations.

In summary, we found that obligately selfing lineages were driven to extinction when confronted with a coevolving parasite. These results are consistent with the macroevolutionary aspects of the Red Queen hypothesis, as originally formulated by Van Valen (Van Valen L. Evol Theory. 1973;1:1–30.)

So yes, we do know at least one reason why sex has evolved; coping with parasites. As with all evolutionary rationalisation, this may not be the initial reason or the most important reason it evolved.

Answer 4

There's another theory that sexual reproduction is an adaptation to resist parasites.

Abstract:

Darwinian theory has yet to explain adequately the fact of sex. If males provide little or no aid to offspring, a high (up to 2-fold) extra average fitness has to emerge as a property of a sexual parentage if sex is to be stable. The advantage must presumably come from recombination but has been hard to identify. It may well lie in the necessity to recombine defenses to defeat numerous parasites. A model demonstrating this works best for contesting hosts whose defense polymorphisms are constrained to low mutation rates. A review of the literature shows that the predictions of parasite coevolution fit well with the known ecology of sex. Moreover, parasite coevolution is superior to previous models of the evolution of sex by supporting the stability of sex under the following challenging conditions: very low fecundity, realistic patterns of genotype fitness and changing environment, and frequent mutation to parthenogenesis, even while sex pays the full 2-fold cost.

Wikipedia's article on the Red Queen's Hypothesis explains further:

By contrast, a Red-Queen-type theory that organisms are running cyclic arms races with their parasites can explain the utility of sexual reproduction at the level of the gene by positing that the role of sex is to preserve genes that are currently disadvantageous, but that will become advantageous against the background of a likely future population of parasites.