Sex: Evolutionary Theory

Note: I tried to simplify this matter as much as possible, but there are some things that one must have at least a tentative grasp of before reading. E-mail me or comment if Wikipedia is too badly written: meiosis, mitosis, ploidy, chromosome, recombination, linkage.

As all my girlfriends have known, sex is weird. In fact, it’s a fundamental problem for evolutionary theory, and has been called the “queen of problems in evolutionary biology”. It’s the most common form of reproduction when in theory, it should not be; it also evolved relatively early in eukaryotic history, between 2 and 3.5 billion years ago.

The first issue you run into is the evolutionary cost of sex. Simply said, asexual organisms can, in theory, produce double the amount of offspring as a sexual organism – consider a population of 100 asexuals, each capable of producing offspring, and compare it to a population of 50 females and 50 males. Therefore, sexual reproduction must have some sort of short-term advantage to balance this extreme inequality.

The root cause of this (and dilemma of its own) is the nature of meiosis itself. Why replicate and then divide twice, when unreplicated chromosomes could just pair up, move to opposite poles of the cell and form two haploid nuclei?

Rising above the cellular level, we also have the evolutionary cost of finding and attracting mates (think of grasshopppers: they stridulate very loudly to attract mates just so they can reproduce. However, they noise will also attract all the predators.) There is also the risk of STDs and danger in sex (think of rhinos who violently rape the females – most of them die during the act).

Of course, those problems only concern the maintenance of sexual reproduction, but there must be some way to account for its origin. The most elegant explanation would be to say that the very first forms of sexual reproduction did not have the problems, i.e. there was a hypothetical type of one-step meiosis, eliminating the evolutionary double cost of males. Without proof, however, this is wishful thinking at best.

So basically, the expectation is that in a mixture of asexuals and sexuals, the asexual population will wipe out the sexual one simply by outcompeting them.

Obviously, sexual reproduction has advantages. The main one is that it can hide bad mutations, which are typically recessive. This is a natural consequence of having a diploid genome and it’s a great advantage over asexual organisms, who are mathematically more open to lethal mutations. Does this alone account for the presence of sex? In evolutionary terms, is the cost of being exposed to all sorts of deleterious mutations higher than the double cost of meiosis and males?

In a very simplified model, yes it is – meaning sexual reproduction is favoured for being ‘cheaper’. However, there are many caveats, the most significant being polyploidy. With multiple copies of each gene, reaching a homozygous state expressing the deadly mutations is much harder. In the short-term, polyploidy protects asexual organisms from their disadvantage. However, this is also not an ideal situation: polyploidy means more mutations in total since there are more genes, and that’s never good in the long run; this issue is compounded by the rampant linkage inherent with the lack of recombination.

Let me explain that last statement. Alleles are never distributed independently of each other – they are always associated genetically, by their positions on the chromosome. This is called linkage. With sex and recombination, these links are constantly broken and new ones are made, allowing for new possibilities, i.e. more genetic diversity. But again, we run into a paradox: good associations must be selected for by natural selection, or else sex is just a roulette, not a mechanism for natural selection. But recombination is more or less random. But this is solved once you move out of the confines of a set laboratory/mathematical model: in nature, nothing is static. A good link today may be horribly disadvantageous for the next generation.

This theory that sex is advantageous because of more genetic diversity was first proposed by Weismann (one of the strongest early supporters of Darwinian evolution) in 1889. Summarising it, he said that sex made natural selection more efficient by increasing the genetic diversity, allowing bad genetic combinations to be weeded out, leaving populations with good genetic make-ups. * See Appendix A for a correction by Joachim.

But it’s still a very risky business, begging the question: how could something that was so obviously advantageous before (genetic stability) get so decidedly replaced and open the way to danger (recombination)?

The majority of mutations are neutral, i.e. they don’t affect anything. Of those mutations that do have an effect, most of them are deleterious, with only a small fraction of them being positive. This begs the question of why the mutation rate doesn’t just evolve to zero; I consider this to be a philosophical question (in other words, detached from reality and mired in vague, nonsensical thinking), since there can be no realistic way for having no mutations. It’s an inevitable part of life. However, a reduction in mutation rate can be imagined – and computational models have shown that sexual populations evolve it. This would necessitate the development of novel physiological processes (either at the organismal or more likely at the cellular level), the cost of which has to be paid back by the benefits brought by the lower mutation rates.

However, this distinction I’ve drawn may be completely wrong. The possibility that sex was actually a primitive state in the earliest life forms, before there were genetic architectures or ‘genes’, exists; however, the only proof of this comes from theoretical biological mathematics, not experiments or observation and therefore suffers from a severe lack of realism. What we do know for sure is that the genetic system that controls sex was present in the last common ancestor of plants, animals and fungi – it is a relatively conserved sequence (although it has been lost and replaced several times in individual lineages, notably the arthropods and placozoans).

Obviously, I have simplified the matter quite a lot. In reality, there are many feedbacks and links and it’s not a simple matter of mutation vs sexuality; as we’ve seen, sexuality first evolved as a means to prevent mutations, but then developed to encourage them, thereby negating the advantage it had at the beginning. However, since it was already established, it did not go away – on the contrary, sex is a true evolutionary success story: disadvantageous except on longer time scales and rife with potential of screwing up. However, it is that exact same potential to screw up that allows natural selection to act so strongly in sexual reproduction and what allows only the fittest to survive.

The evolution of sex is a perfect example of a Red Queen model. Those who have read Through the Looking Glass will probably know this quote from the Red Queen: “It takes all the running you can do, just to stay in the same place.” Interpret that as an evolutionary metaphor and it means that you need to evolve just to survive – i.e. mutations are essential. The reasoning behind this is that the environment is the largest selection pressure there is, and only those populations with the greatest diversity of genes can withstand whatever comes against them. And what better way to increase genetic diversity than mixing up the genome in every new generation?

It’s important to note that by ‘environment’, I don’t only mean abiotic factors. Parasites and pathogens also belong to the environment – basically, anything that an organism can interact with, including coevolutionary antagonists. Think of it in agricultural terms: having a crop made completely of clones will certainly die of disease, whereas the genetically diverse crop will be safer and much more reliable in the long run.

But I still haven’t answered how sex may have first come about. The very first cells were haploid. Diploid cells may have arisen when two of them joined together (accidentally?). Alternatively, it somehow didn’t divide after DNA replication. These are not just imagined, some fungi (the group with the most diverse reproduction types) have similar mechanisms! But then you run into a problem: chromosomes. When two nonhomologous chromosomes come together during cell division, you get recombination, but completely different genes getting swapped. This is absolutely horrible – and the reason why meiosis may have evolved! It forces only homologous chromosomes to pair up, thus acting as a defence.

And that’s the dry theory of sex. As you’ve all realised, my time is extremely tight these days. I will try as hard as I can to update more regularly! I will tally up what people are more interested in: more on sex, with examples from real organisms, or change of topic for the next post.

* Appendix A

CORRECTION by Joachim in the comments:

The problem of Weismann was that he rejected the inheritance of acquired variation. Therefore, he needed another source of heritable variation, in order to have a theory in which natural selection kept going in the first place. He did not yet know about mutations being the ultimate source of heritable variation and seized sexual reproduction as this ultimate source.

That is also why Fisher and Muller both rejected Weismann’s idea that sex is an inevitable prerequisite for natural selection. Mutations had been discovered in the meanwhile.

For Darwin and Fleming Jenkin, sex was a problem in combination with blending inheritance. For Weismann it was the solution in combination with his rejection of the inheritance of acquired variation. For Fisher and Muller it became an extravagance, because mutation provided heritable variation that should suffice for natural selection to occur in the first place.

Further Reading:

There are three classic books on the evolution of sex, each written from a different point of view due to disciplinary bias, and each offering unique insights that can be merged together to get a complete picture of this topic – remember that it is still a thorny issue, no matter how rosy I made it sound here.

Williams uses known examples from nature to try and explain the evolution of sex. While it’s as realistic and precise as we can get, it’s not very general – only applicable for the taxa discussed.

John Maynard Smith is a theoretical evolutionary biologist – one who uses plenty of maths and builds models to explain evolution (game theory, for example). As such, his book is very generalist and as precise as mathematics can get you. But at the cost of realism: the situationsin a model are almost never replicable in nature. However, they do give us a framework to work in.

If you’re only going to read one book about the evolution of sex, it must be this one. Bell uses Williams-type observations to review all known sexual and asexual life cycles. He does eschew Maynard Smith-ish models and values realism above all – like a true biologist. It’s general and extremely realistic, but admittedly lacks the precision of Williams’ taxon-specificity and Maynard Smith’s maths. Still, this is the best starting block (and ending block for most!)

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