Imagine sex worked like this:
You’ve been feeling bad lately, getting sick a lot. You’re not at your best. You find someone who seems to be in better shape. One thing leads to another and you wind up acquiring body fluids from the other party – and picking up some new genes from them. The new genes help a lot in fighting off infection. You’re feeling better now.
Reproduction? That’s another matter, nothing directly to do with sex. When you reproduce, your offspring will carry all the genes you happen to have at the moment.
Also, I forgot to mention that you’re neither male or female – the gene exchange could have gone in the other direction if you’d both been in the mood. And your partner in the adventure above might not even have been the same species as you.
This is more or less how bacteria work out sex. (Joshua Lederberg got the Nobel Prize for figuring this out.) Eukaryotes (you’re one of them) mostly do it differently, combining sex and reproduction. It’s the story you learned in high school about passing on half your genes to a gamete (sex cell), which joins with another gamete to make a new organism.
Most eukaryotes also have two sexes. The best theory we have about why that got started goes like this: Most of the DNA in a eukaryote cell is in the nucleus. But a small fraction is in the mitochondria, little powerhouses outside the nucleus that started out as bacteria, and got domesticated. Imagine that two gametes join together, and combine two sets of mitochondria. There’s a potential conflict here. Suppose your mitochondria have a mutation that lets them clobber your partner’s mitochondria. This is good (evolutionarily speaking) for the winning mitochondria, but very likely to be bad for the cell as a whole. Better for the cell as a whole is if one gamete, acting on instructions from the nucleus, preemptively clobbers all their own mitochondria, so that all the mitochondria come from just the other gamete. This is the beginning of what will eventually lead to a distinction between sperm and eggs, pollen and ovules, male and female. Which means you got all your mitochondrial DNA from your mom, something that will turn out to be important when we look later in the year at geneticists unraveling human prehistory. This is also an example of how selection at one level (within cells) can conflict with selection at another level (between cells). We’ll see such multilevel selection again and again, for example in the evolution of complex human societies.
Sex has to be highly advantageous, although we’re not sure exactly what the advantage is. When eukaryote species give it up, they don’t seem to last long. Dandelions reproduce asexually: based on what we see in other organisms, they probably won’t be around for long, evolutionarily speaking. There’s one mysterious exception, tiny animals called bdelloid rotifers which have been reproducing asexually for tens of millions of years . For readers who are not bdelloid rotifers: Happy Valentine’s Day tomorrow! We’ll have an appropriate evolutionary post up tomorrow.