The Evolution of Sex
Sex is confusing and not only in the figuring out what goes where and who, if indeed anyone, you want to do it with. It’s very existence is a problem that has puzzled evolutionary biologists for centuries. For all intents and purposes sex shouldn’t exist, let alone be intrinsic to phyla across the planet. So why is sex so confusing? What about it is so paradoxical?
Well let’s start by defining sex, in the simplest terms sex is the fusion of two gametes (the fancy word for sex cells aka sperm and egg) from separate parents. Simple right? Well it’s surprisingly complicated and costly to everyone involved, which is a problem for any organism as using up energy for reproduction means you have less for ensuring your own survival. Not only do you have to produce gametes, but you have to find a mate and convince them to mate with you and let’s not get started on the child care…
Reproducing via sex is slower and produces fewer young which can produce children (aka daughters or as I fondly call them ‘baby birthers’). When compared to asexual reproduction which incurs very little cost and is faster with the added bonus that all your young are ‘daughters’ so they can all produce young of their own, sexual reproduction seems pretty lame. This phenomenon is known as ‘the two fold cost of sex’ or more simply, the production of males means you produce young at a slower rate in comparison to asexually reproducing organisms.
So sex doesn’t seem to be a great evolutionary strategy to produce young, in fact it seems inefficient and expensive! What benefits could possibly outweigh the negatives of reproducing sexually? Well sex doesn’t mean that you produce more offspring so your young can’t out compete other organisms by numbers alone. What else is important? Did someone say genetic variation? Because if you did you’d be on the right track or at least one of the more accepted theories for this phenomenon.
Genetic variation is created in three different ways, random mutation, segregation and recombination. I think we’ve all heard of random mutation, which comes down to a change in the DNA sequence however, this kind of genetic variation isn’t very stable. Randomly changing a bit of DNA doesn’t always go down so well and in most cases it doesn’t, so we can say that while random mutations are a source of genetic variation they don’t contribute much to the variation we see in nature. Okay, so what exactly is segregation and recombination? Well they both occur through the arduous process of sex, segregation is the mixing around of chromosomes during gamete production and recombination is the switching of bits of chromosomes during gamete production. It’s a might more stable than just changing a chunk of your DNA sequence and of course the fusion of two parents different gametes incurs variation also in their young.
Well that was quite a chunky paragraph, but I hope you understood it, if not the fault is all mine. You may now be wondering what the hell genetic variation has to do with fitness and how on earth it could possibly rescue the mess that is sexual reproduction. Well the theory goes that more variation means there’s less chance of a population going extinct due to a bunch of inbreeding, like if a disease sweeps through a population everyone is more likely to be susceptible and less likely to be a lucky git with beneficial variation which creates immunity if you’re all inbred. It’s suggested that genetic variation allows the creation of novel genotypes of higher fitness. Meaning that with sex you can evolve quickly to adapt to harsh environments via natural selection of beneficial variants and survive effectively, something that asexual populations struggle to do. Alternatively in a population plagued by a parasite the occurrence of a novel variant that’s immune to the parasite would lead to selection for that variant whose genes would spread throughout the population. Therefore if these assumptions of the effect of sex and the increased fitness of novel variants holds true, the evolution of sexual reproduction might not be all that confounding.
So there you have it, some of the main arguments for why sex evolved which don’t really explain why it evolved at all. It’s an exciting area to be working in or to work in the future as it is such a huge unanswered question, the opportunity for research is there for the taking. So sex exists despite the costs to individual fitness and it’s less than superior reproductive rate. It may indeed increase genetic variation and so prevent (in some cases) inbreeding depression or improve survival under harsh conditions. However, we still can’t explain how sex came about or why it stuck around to basically take over most animal’s reproductive strategies. We must always remember that evolution has no foresight, so while these theories make a good argument for why sex stuck around they aren’t universal and indeed do not apply to many sexually reproducing organisms on our little blue planet.