Last week I was discussing the Test Achats ruling from the European Court of Justice with a couple of colleagues.  (For those interested it means that insurers won’t be able to set premiums based on a person’s gender from December 2012.)

Eventually this discussion rambled off topic and led to a surprising question from one of them: is a female defined as the sex in a species that gives birth?

As i explained, the answer is actually that the female is the sex with the larger gametes,  ie the sex that produces eggs rather than sperm.  However, there are some species which reproduce sexually but have sexes with equal sized gametes; they are isogamous.  These species have two or more sexes none of which is labelled “male” or “female”.

How does that work?  Well if you think of a species with three sexes (A, B and C) then each sex can reproduce with either one of the other two sexes.  This actually leads to evolutionary pressure to produce an ever greater number of sexes.  An organism of sex C can reproduce with 2/3rds of the other organisms.  It won’t be able to reproduce with sex C organisms as a wway to prevent self-fertilisation.  If, following a mutation, an organism arises that is a new sex (sex D) it will be able to reproduce with any other organism in its species as it will be the only sex D organism around.  Eventually, due to this advantage, the number of sex D organisms will rise until there is an equal proportion of each sex and there will be evolutionary pressure for a fifth sex to arise.  There are some species out there with hundreds of sexes with the number growing all the time presumably.

Thinking about this, what surprised me was that this isn’t a topic that was covered in my GCSE biology.  I’d read about it much later.  Yet I think it’s highly interesting, not too complex to understand and illustrates evolution in action.  All of which make me think that it’s something that should be taught.

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