Sex as contraception

In Idle Theory, reproduction is asexual. The creatures are born full-sized, and they start working immediately to produce another copy of themselves. They replicate themselves at regular intervals.

Using the pool simulation model, it was found that if this reproductive work rate (P_r) was allowed to vary randomly, so that some grazers reproduced faster, others slower, then slow reproducers were regularly selected, and the grazers reproduced more and more slowly. The reason this happened was because fast reproducers worked harder - and were consequently less idle - than slow reproducers, and they tended not survive the periodic crises during which all the creatures worked harder.

If predators were introduced to catch and eat the grazers, then reproduction rates increased. The reason for this was that fast reproducers had a better chance of reproducing before being caught by a predator than slow reproducers, so fast reproducers tended to predominate.

Ideally, the creatures should be able to respond rapidly to whatever circumstance they find themselves in. If they have no predators, they should be able to slow down reproduction. If they come under predator attack, they should be able to increase reproduction. But how can an asexual replicator, which pumps out another copy of itself at regular intervals, be contrived to control its reproduction.

The proposal here is that sexual reproduction allows reproduction rates to be increased or decreased by varying the ratio of males to females. If most offspring are female, the population will increase. If most offspring are male, the population will decrease.

Mitosis and Meiosis

In nature, most reproduction is asexual. Cells grow and divide in two (mitosis). Each cell is a replica of its parent, carrying the same complement of genes. Large multicellular creatures are almost entirely made up of cells produced in this way.

But mitosis isn't the only way that cells can divide. In meiotic division, cells divide not into two, but into four. Each of these cells has only half the complement of genes of the parent cell. These "haploid" cells don't grow and divide like other cells. They can only do so if they can acquire a full set of genes by fusing with another haploid cell. Only when they have the full set, can they start growing and dividing.

The first point here is that meiosis stops cell replication, producing numbers of non-dividing cells. Any cell that divides meiotically will have no further descendants, unless they fuse somehow with another haploid cell to regain a full set of genes. Seen this way, meiosis is a kind of "contraception". Mitotically reproducing populations could limit their numbers by switching from mitosis to meiosis.

Sexual Reproduction

Sexual reproduction happens when two haploid cells combine to produce a "diploid" cell with a full set of genes, and this cell can then start to divide mitotically. In sexually reproducing populations, males provide one half of the genes, and females the other half.

But a whole obstacle course has to be negotiated for sexual reproduction to work.

1. Males and females have to find each other.
2. Both males and females have to be fertile - carrying viable haploid cells.
3. Male sex cells (sperm) have to be transferred from male to female.
4. The sperm must successfully unite with the female sex cells (eggs).
5. The multicellular creature produced by the mitotic division of the fertilised egg must successfully grow to become an adult male or female.

Given this tremendous obstacle course, most sex cells never get to unite with another sex cell, and grow and divide into an entire multicellular creature.

But this is the main point of sexual reproduction: it is so inefficient, so likely to fall at some hurdle, that it restricts the reproduction of the creatures. Natural selection favoured sexual reproduction precisely because it restricted reproduction.

The Benefits of Sexual Reproduction

In biology, sex is a puzzle because biologists always assume that the creatures are trying to reproduce as rapidly as possible. Seen that way, sexually-reproducing females waste half their energy producing males, while asexually reproducing females devote all their energy to producing more females. But if the problem of reproduction is not how to speed it up, but how to slow it down, then the intricacies of sexual reproduction begin to make sense.

Sexual reproduction has other advantages over asexual reproduction than slowing reproduction rates. Since a sexually reproducing population is always exchanging genes within itself, beneficial gene variants can be rapidly dispersed through the population. In asexual reproduction, this can't happen.

If the vegetative (asexual) descendants of an individual contain 10 different mutations, then there are 10 different genotypes. If the 10 lines could cross sexually, 3 different genotypes would be possible at each locus (AA,Aa,aa) giving a total of 3¹⁰ or 59,049 different genotypes.
(John Maynard Smith. The Theory of Evolution. Ch 12.)

But sexual reproduction also offers the possibility of controlling reproduction by varying the ratio of the sexes. If it is taken that males produce large numbers of sperm, and a single fertile male can fertilise a large number of females, then if the offspring are mostly females, population will rise rapidly if the few males present can fertilise all the females. Conversely, if the offspring are mostly males, then population will fall if, on average, each female leaves less than one female offspring, because the number of females will steadily fall.

This offers, in principal at least, a way for sexually reproducing populations to tailor their reproduction rate to their circumstances. Where a population is under predator attack, a rise in the proportion of females born will act to increase population growth to counteract the losses to predators. But where a population's food resources are restricted, population can be reduced or stabilized by decreasing the proportion of females.

The ability to vary the sex ratio may be absent. But in the case of crocodiles, the sex of their offspring is determined by the depth at which eggs are laid below ground level. Thus crococdiles, whether or not they make use of this control, have the means to vary the ratio of the sexes.