Why do we have 2 sexes? It’s something we take for granted, but is there an evolutionary reason for it? The standard evolutionary arguments of ‘survival of the fittest’ don’t seem to easily apply here, but that doesn’t necessarily mean it is just an accident of history. There is a story1 behind it, and it’s beautiful. (I came across this story in Matt Ridley’s book ‘The Red Queen: Sex and the Evolution of Human Nature’ which I highly recommend.)
The story starts in a time when our ancestors are single celled organisms, and sex (i.e. mixing of DNA) has just been invented. (The story of why sex was invented is for another day). In its most basic form, two cells have sex by fusing2 together. This is all fine, except for the fact that the mitochondria3 in the two parent cells don’t get along very well when they are mixed together – reason being that this situation selects for aggressive mitochondria. So you end up with a war of attrition4 5 when two cells with aggressive mitochondria fuse, and this impoverishes the whole cell. Clearly this is no good for anyone and it is in the nuclear genes’ interests to stop this from happening, so they invent a system whereby only one of the cells in the fusion is allowed to contribute the mitochondria and the other is forced to leave them behind. So you end up with two different types of cell, the ones which donate the mitochondria (the females6) and the ones which don’t (the males), and that is the initial asymmetry which is the origin of two sexes!7
But while that explains why there needs to be two separate sex cells, it still doesn’t explain why these two different sex cells have to be produced in separate bodies in larger organisms. Why can’t everyone have both male and female parts i.e. be hermaphrodites? This would surely make more sense because then anyone can mate with anyone. Indeed most plants are hermaphrodites so this is clearly a natural option in some cases. Why would any species diverge from this? Well if you think from the point of view of the mitochondria, they care a lot more about the female producing parts of the organism than the male producing parts because that’s their route to the next generation, so any mitochondria that can invent a way of killing the male producing parts of the organism would do quite well. So if this occurs, you get pure females in the population as well as just hermaphrodites (and there are species of plants alive today which are in this equilibrium8).
In such a situation maleness is now a bit more of a rarity and it could pay for some organisms to abandon their female parts and concentrate their efforts on being good males. This would add pure males to the mix too. But in a mix of females, males and hermaphrodites, hermaphrodites are the losers because they spread themselves too thinly and can’t compete against the pure males and the pure females, so it’s not surprising that you end up with just males and females. And once this position is reached it is quite stable – although the mitochondria in the males aren’t happy because it is a dead end for them, there is no longer a benefit to them of killing the male producing parts9, because there is no longer an alternative route for them, so there’s no going back. Hence the stable split of males and females!10
At this stage the only difference between the male and female is the sex cells which they produce, and there need not be any other differences. But since the female sex cells provide the mitochondria, it makes sense if they provide everything (except half the genes) and so they tend to be larger and less mobile. As a result it makes sense for the male sex cells to come to them, for example by being injected into the female body (although it need not be this way round – in seahorses the female has the penis). And from this it usually follows that females make a larger parental investment than the males do. So as you can see, more significant differences between the sexes start to arise naturally from the initial asymmetry in the sex cells, and indeed a lot of the differences between male and female that we are familiar with today can be traced back to this, in particular to the asymmetry in parental investment, but that is also a story for another day.
You may be wondering what reason we have to believe this story? We can not go back in time and prove that our ancestors followed this route, but we can still find a lot of evidence pointing to a story like this from the current natural world, for example in the existence of species at all ‘stages’ of this story currently alive today (see footnotes). The main value of this story to me though is not how historically accurate it is, but rather that it gives a possible explanation and quite a convincing and interesting one too.
- I came across this story in Matt Ridley’s book ‘The Red Queen: Sex and the Evolution of Human Nature’ which I highly recommend. ↩
- The alternative is sex by conjugation (interchange of DNA), as occurs in bacteria and fungi, which doesn’t lead to genders being required. ↩
- The same argument applies to all organelles in the cell that have their own DNA – mitochondria, chloroplasts, bacteria etc. ↩
- This can be observed today (between chloroplasts) in a species of alga called Chlamydomonas. ↩
- This is a typical example of a ‘Tragedy of the Commons’ situation. ↩
- So in particular this means the mitochondria in your cells you got from your mum which she got from her mum, which she got from her mum, and so on like this all the way down the maternal line! ↩
- Incidentally it doesn’t follow that there has to be exactly two sex cells – two sex cells is the simplest solution but there can be more as long as there is some sort of system which determines who gives up the mitochondria on fusion (a form of slime mould which does sex by fusion has a hierarchical system of 13 genders). ↩
- Male-killer genes have been found in more than a hundred and forty species of plant. ↩
- … Unless there is competition between the young in the brood (as then there is some indirect benefit to the mitochondria in the females), as is the case in some species of insect where male-killer genes have been observed. ↩
- It is usually in mobile species where this gender split, as opposed to hermaphroditism, occurs – presumably because mobility makes male specialisation pay more. But it’s not just plants which are hermaphrodites, for example earthworms are (and equally not all plants are hermaphrodites). ↩
RSS - Posts