Gender division


For what the term 'gender' means in this essay, see the 'gender meaning' essay.

Gender origin

This essay assumes that distinct genders exist - and asks why they exist in separate bodies - as opposed to organisms being hermaphrodites.

The issue of why genders exist in the first place is dealt with by another essay.

Matt Ridley on gender division

The question of why organisms are not hermaphrodites has been asked and answered by Matt Ridley:

Why are people not hermaphrodites?

- The Red Queen, Matt Ridley, Chapter 4, p.100

Matt correctly observes that there is an ecological difference between a mobile creatures and immobile ones on this front:

Were I a plant, the question might not arise: most plants are hermaphrodites.

There is general pattern for mobile creatures to be dioecious (with separate genders) and sessile creatures like plants and barnacles to be hermaphroditic.

- The Red Queen, Matt Ridley, Chapter 4, p.100

His answer is that it is down to the organelles.

The answer lies in those muttering organelles left behind at the gate when the sperm entered into to the egg.

In a male, any gene in an organelles is in a cul-de-sac, because it will be left behind if in the sperm.

All of the organelles in your body and all of the genes from them came from your mother none came from your father.

This is bad news for the genes, whose life's work remember, is to pass into the next generation.

Every man is a dead end for organelle genes.

Not surprisingly there is a temptation for such genes to invent solutions to their difficulty (i.e. those that do solve the problem has spread at the expense of that do not).

The most attractive solution to for an organelle gene in a hermaphrodite is to divert all of the owner's resources into female reproduction and away from male.

This is not pure fantasy hermaphrodites are in a state of constant battle up against rebellious organelle genes trying to destroy their male parts.

Male killer genes have been found in more than 140 species of plant. They grow flowers but the male anthers are stunted or withered.

Seed but no pollen is produced.

Invariably of the cause of this sterility is a gene that lies inside an organelle, not a nuclear gene.

By killing the anthers the rebellious gene diverts more of the plant resources into female seed, through which it can be inherited.

- The Red Queen, Matt Ridley, Chapter 4, p.101

How does this explain why few animals are hermaphrodites while many plants are?

The same logic does not apply to animals, many of which are not hermaphrodites. It pays an organelle gene to kill males only if by doing so some energy or resources is diverted to the sisters of the killed males and hence the mayor of killing is rarer.

In hermaphroditic plants, if the male function dies, the female function of the plant grows more vigorously or produces more seed.

But a male-killer gene in, say, a mouse, by killing the males in a brood benefits those mices sister's not at all.

Killing males because they are an evolutionary cul-de-sac for organelles would be pure spite.

- The Red Queen, Matt Ridley, Chapter 4, p.100

At this point some problems arise:

Firstly, it is not true that killing males in mice does not benefit their future sisters.

The key to reallocating resources from males to females is to kill the males early, before they are even born. Then the resources which would have been spent on their pregnancy are available for other purposes. There are numerous segreation distorters that work in just this way in animals.

However, there is also a much more serious problem:

Ridley is trying to explain why separate genders arise more frequently in mobile creatures - like animals - than they do in immobile ones - like plants.

However the difference between plants and animals he is invoking is the very fact that they have their males and females in separate bodies - so it is harder to transfer resources between them after they are born.

This argument has the form of a classical logical fallacy, known as assuming what you are trying to prove.

If you are trying to show why animals have separated genders, it is not legitimate to assume the fact that they have separated genders as a step in your argument.

This observation is fatal to the argument presented.

Wirt Atmar on gender division

Another explanation for the existence of separate males has been offered by Wirt Atmar.

In his essay [On The Role Of Males] he offers an explaination of why male organisms exist.

Wirt's theory is that males exist in order to help eliminate germ-line errors.

To quote from the abstract:

A primary reason for the existence of males in a bisexual species may be to act as a pre-zygotic filter of gene defects. Males appear to be an evolved auxiliary sexual caste that may be culled at less cost to the reproductive success of a species than by allowing both maternal and paternal lines of inheritance to be culled uniformly.

A variety of genetical and behavioural mechanisms promote and exaggerate a general physiological fragility in male animals not apparent in females, haplodiploidy being the most obvious.

However, explicit genetical mechanisms, such as haplodiploidy, which overtly expose error, may not be wholly necessary. The evolution of appropriately pugnacious combative behaviour in the diploid male may be sufficient.

- [On The Role Of Males]

The theory is an unusual one - in that it proposes a species-level benefit for a trait - having an unmutated germ line.

However, reformulating it so that the benefits involved accrue to individuals may be possible:

One way in which I imagine this could be done is via female choice. It benefits a female to mate with the best quality (i.e. least mutated) male she can find. Females get to choose on whatever grounds they like - and so perhaps they will base their decisions on traits that show a large variance in the male population, in the hope of picking the best male possible. The long-term, cumulative result of such choices could easily be large variations in male fitnesses.

The theory invokes male combat - which leads to males failing to reproduce.

Sexual selection also has considerable power to increase the intensity of selection on genes in male bodies.

Between them, these ideas have some potential to explain the ecological observation that many plants are hermaphrodites, while many animals have divided sexes - since plants tend to lack male combat and female choice.

The argument seems to predict that males will tend to be smaller and more numerous than females (unless they do a lot of fighting). Male animals are often smaller and more numerous than females. However, this seems to be true much less frequently of plants - a potential problem for the theory. Why don't plants have males (and make them small) more frequently? Perhaps the explanation is that the female choice mechanism described above is what is behind the whole phenomenon in the first place - and most plants can't easily choose their mates.

Wirt's argument seems too focused on eliminating mutations to be the whole story to me.

If having a high variance in fitness is indeed part of what it means to be a male, I can't help thinking that there are also some other aspects.

Why are the genders separated?

Since I have not been convinced that previous authors have completely dealt with this issue, what is the reason why we are not hermaphrodites?

The answer seems to me to be that male and female are often different jobs - and bundling them together into the same body prevents organisms from playing either role properly.

The bodies of pure males can differ from those of pure females in a large number of ways:

  • Distributing male seed effectively can benefit from the distributors being small, mobile, inexpensive and numerous.

  • A strategy of collecting nearby females together and killing off any other males who come near benefits from males being large and well armoured with shields and weapons.

  • Males are sometimes bright and showy - as a consequence of sexual selection via female choice.

Such strategies often involve morphological specialisations.

They benefit from sexual dimorphism, and are hampered if both male and female parts must share the same somatic body.

This theory explains why plants have divided sexes less frequently than animals do: these kinds of strategies are not available to such an extent to plants, who typically cannot move, do not fight with other males, and can't choose their mates.

It also explains why creatures like barnacles are usually hermaphrodites: they too are immobile, and can't fight with other males, get to nearby females faster, or choose who they have sex with.

The idea that role differences are involved is highly compatible with the phenomena of protandry and protogyny - where an organism starts male and turns female when it increases in size - or starts female and turns male when it gets bigger.

Theory origin

As far as I am aware, I developed this theory.

It was first published in 2002 [3, 4, 5, 6, 7, 8, 9, 10]

Any other references to previous relevant work would be warmly appreciated.


  1. The Red Queen, Matt Ridley, Chapter 4

  2. On The Role Of Males - Wirt Atmar

  3. Tim Tyler,, 2002, Jun

  4. Tim Tyler,, 2003, Jan

  5. Tim Tyler,, 2003, Jan

  6. Tim Tyler,, 2003, Oct

  7. Tim Tyler,, 2004, Feb

  8. Tim Tyler,, 2005, Jan

  9. Tim Tyler,, 2006, Dec

  10. Tim Tyler,, 2006, Dec

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