Why sex exists, when it costs so much

An organism that reproduces without a partner passes on all of its genes. One that reproduces sexually passes on half, and has to find someone first. On the arithmetic, sex should have been outcompeted long ago. It is instead nearly universal among complex life, and explaining that is one of the genuinely unsettled problems in biology.

The two-fold cost

John Maynard Smith set the problem out plainly. Consider a population reproducing sexually, and introduce a variant that reproduces on its own instead.

Each sexual pair produces offspring carrying half of each parent's genes. The asexual individual produces offspring carrying all of hers, and needs no partner to do it. Per unit of reproductive effort she contributes roughly twice as much inheritance to the next generation. That advantage compounds every generation, and it should sweep through the population quickly.

On top of that, sex means finding a mate, competing for one, and the risks that come with both. Recombination also breaks up combinations of genes that were working perfectly well together. The costs are real and easy to count, which is what makes the persistence of sex such a sharp problem.

What the benefit has to be

Whatever sex provides has to be worth more than a two-fold disadvantage, every generation, indefinitely. Two explanations have survived scrutiny, and they are probably both partly right.

Clearing out harmful mutations. In an asexual lineage, every mutation that arises stays in that line permanently, because the only way to lose it is for the whole line to die. Damage accumulates and cannot be reversed, a process known as Muller's ratchet, which only turns one way. Recombination lets offspring be assembled that carry fewer harmful variants than either parent, so the ratchet can be undone.

The Red Queen. Parasites and hosts are in a permanent arms race, and parasites adapt fast because their generations are short. An asexual host produces offspring genetically identical to herself, and a parasite that has solved her has solved all of them. Sexual reproduction produces offspring with novel combinations, so the target moves every generation. The name comes from Lewis Carroll: running as hard as you can to stay in the same place.

The Red Queen explanation has the strongest direct evidence. Populations under heavy parasite pressure tend to be sexual, and related populations in low-parasite environments more often lose it. Where a species does both, sexual reproduction reliably rises when parasites do.

Neither is the whole answer

It is worth being honest that this remains open. Both mechanisms are demonstrated in specific systems and neither has been shown to be generally sufficient to pay a two-fold cost everywhere sex occurs. Ancient asexual lineages that should have accumulated fatal damage and did not are a standing problem. This is a live question, not a solved one with a settled answer that popular accounts have merely simplified.

A world where the question is settled by fiat

Reproduction here is asexual. An organism crosses a threshold, pays part of its own energy, and an offspring appears carrying a mutated copy of its genes and its network weights. There is one parent. No partner is sought, no recombination occurs, and no genetic material is ever combined from two sources.

That is a design decision, not a finding, and it has consequences worth naming. Muller's ratchet applies here in full: a lineage carrying a harmful mutation has no mechanism for shedding it except the lineage dying out. There is also no parasite pressure of the kind the Red Queen describes, since nothing here has been shown to specialise on the genetic identity of a host.

So this world cannot answer the question this essay is about. It can only illustrate one side of it, which it does by running the clonal case continuously and letting the consequences show up in the lineage records. The mutation model and how lineages are tracked are on genome, inheritance and mutation.