Why Is Sex a Thing?

Here’s a sanitized version of the transcript, with inappropriate language and references removed:

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Hey smart people, Joe here. A lot of us probably remember when we had that talk.

Dad: Hey son, we need to talk…

Kid: This is not happening.

Dad: …about a certain topic. [Show open] This isn’t going to be like that. (hopefully) But it is going to surprise you.

First, a seemingly simple question: What is the purpose of sex? If you were to ask a bunch of people “what’s the purpose of sex, biologically speaking?” they would probably say “reproduction.” I mean, that’s how you and I got here, right? But that’s not the whole story. Biologically speaking, “sex” isn’t what you might be picturing.

We all know that passing genes on to the next generation is essentially the whole point of life. Survive to reproduce and get your genetic information into some offspring so they can go on and do the same thing. There are two primary ways to accomplish that:

Method 1: Divide and copy all your information over to a genetically identical offspring.

Or Method 2: First do some complex cell division, then spend time and energy looking for a potential partner, attract that partner, and while you’re mating, try not to get eaten by a predator. If you survive and get lucky, you mix a random half of your genes with a random half of their genes to make some genetically diverse offspring.

One of these methods is obviously much simpler than the other. “Sex” refers to just this one part of the process: taking part of the genetic instructions from two organisms and combining them into one complete set of genetic instructions. Plenty of living things reproduce without sex—like microbes that bud off new copies of themselves, or flatworms that can grow into whole new individuals. Even some fish and reptiles are capable of reproducing without a mate.

So, sex isn’t required for reproduction. Then what is it for? The truth is, why sex exists is one of the big unanswered questions in evolutionary biology. If you think sex is hard to figure out in your life, take comfort in the fact that science hasn’t totally figured it out either.

What makes sex so puzzling is that it’s so costly. I’m not talking about dinners and dates; I’m talking about biological costs. For starters, sex is slow. In the time it takes, say, a bacterium to copy itself and divide, we’ve barely gotten ready for a date. Finding a mate can be really hard. For a tiny bug or small ocean creature, searching for a mate can feel a lot like searching for a needle in a haystack.

In fact, that’s why so many organisms have both male and female parts—less searching. But slowness and loneliness aren’t actually the biggest costs of sex. Surprisingly, it’s the cost of having males in general.

Consider a model in which every female has two offspring. In a species that reproduces asexually, every individual female and all their offspring are also female. In a sexually reproducing species, she has one male and one female on average. The asexual population grows twice as fast as the sexual population. In other words, sex is twice as costly as no sex! This is because, in a sexual species, females spend half their resources having sons, who can’t make offspring themselves. Yes, males contribute their genes in sex, but in most species throughout nature, males don’t do that much else. Males are costly.

But one of the strangest costs of sex is that it can disrupt favorable combinations of genes. Imagine you walk into a poker tournament and offer all the winners at each table this opportunity: For the next round, you can keep your current winning hand, or you can shuffle your cards with another player. Surely the winners would choose to hold onto the cards they have because they’re winning hands! They have combinations of cards that work well together. Shuffling winning hands together is more likely to make each of them worse, not better.

What determines if you win a poker game is a combination of cards working together, not the individual cards. And that’s how it works with genes too. It’s the combination of genes working together in a particular environment that decides whether an individual will survive and reproduce… or fail. And sex forces you to shuffle your hand.

To make sex cells, a special kind of cell division happens in the testes or ovaries called meiosis. In eukaryotic cells like ours, the DNA lives in the nucleus, bundled in structures called chromosomes. Those chromosomes come in pairs, one from mom and one from dad. Early in meiosis, each chromosome swaps chunks with its partner to make new mixed-up chromosomes. Then, the newly crossed-over partners split up so each sex cell randomly gets just one mixed-up chromosome from each pair. That way, when male and female sex cells come together to form the next generation, the offspring have one complete set of chromosomes instead of two.

In a species like ours, with 23 sets of chromosome pairs, there are over 8 million different ways your mixed-up chromosome pairs can split up during segregation! Add in the mixing of recombination, and that’s exponentially even more genetic diversity. Which is why you’re unique.

So, sex is a complex way to get your genes to the next generation. It’s slow, it is costly, and it can break up winning genetic combinations. But why is sex so common? Only about 1 in a thousand animal species and only 1% of flowering plants are exclusively asexual. From fleas to trees, pretty much every eukaryote reproduces sexually.

It’s a paradox. There must be a reason, a huge advantage to sex that more than makes up for the costs. What is it? Shuffling up your winning hand may actually be the winning strategy because nature doesn’t always play by the same rules.

Sex creates genetic diversity! But remember, for an organism that’s already well adapted to its environment, shuffling seems counterproductive. Here’s the thing: the rules of life’s game don’t stay the same. Environments change, and what was a winning hand today might not be tomorrow.

So if we think about our poker tournament again, imagine now that the rules of the game change every hand, so you don’t know if a straight flush will be a good hand or a bad hand until the next round starts. Now do you want to change cards? Remember that asexual reproducers are stuck with the hand they’re dealt.

Which might seem good at first, but over time, as the rules of the game change, the filter of natural selection changes too. They gradually accumulate random mutations, too. If you’re asexual, what was an advantage at first can quickly turn into a disadvantage.

So once you realize that nature is constantly changing the rules of the game in an organism’s environment, sex makes a bit more sense, especially when that game involves parasites. Whether a parasite can infect a host has a lot to do with genetics. Your cells and your body have genetically programmed locks that parasites are trying to unlock.

In a population where hosts have the same genes, like in asexuals, any parasite that evolves to pick that lock can bring down the entire population. But by shuffling genes, like in sexual populations, every genetic lock that a parasite sees is a lock that it’s never seen before.

The same reasoning applies to any relationship where species are evolving together: predators and prey, viruses and their hosts. Biologist Leigh Van Valen called this idea the Red Queen hypothesis. The name “Red Queen” comes from “Through the Looking Glass,” where the Red Queen tells Alice that it takes all the running you can do to keep in the same place.

When the environment and its challenges are constantly changing, the constant shuffling of genes in sex is the key to long-term survival. Scientists have even seen this play out in real life. In small desert pools in Mexico, sexual fish species compete alongside asexual species. Scientists noticed that the asexual ones carried more parasites, just as the Red Queen predicts. The sexual fish are a moving target for the parasites.

When a drought dried up these ponds, the sexual fish lost their parasite resistance because they had lost their genetic diversity, just like the asexuals. Until the scientists seeded the pond with sexual fish from nearby ponds, and their genetic diversity returned.

Species don’t stop evolving just because they’ve succeeded in their environment. Ever wonder why species like peacocks have these huge ornaments that are expensive to grow or make them easier targets for predators? The Red Queen may explain this too. What if these ornaments are attractive to mates because they display how healthy and strong an animal is? In other words, if you’ve got a healthy mix of parasite-resistant genes, you’re more likely to mate.

These visual displays are a way for sex to keep some winning genetic combinations together. Sex is costly, but it gives the next generation a better chance of surviving a challenging environment. Some organisms, including many plants and some algae, clone themselves asexually some of the time but reproduce sexually when their environment presents a challenge.

Switching back and forth between reproductive modes seems ideal, so why can’t we reproduce asexually? Well, in addition to eventually running out of resources, organisms don’t get to decide how to reproduce. All of these mechanisms, trade-offs, costs, and advantages are coded into an organism’s biology by their environments and their ancestors’ history.

It can be tempting to say, “sex evolved to create genetic diversity,” but that’s not quite right. Sex evolved randomly, like everything else, at some still-unknown point in the distant history of life. The resulting genetic diversity made the organisms that did it a little bit more fit and a little bit more likely to pass on their genes to the next generation.

And that’s what matters in this game. It’s not fate. It’s evolution. Stay curious.

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This version maintains the educational content while removing any inappropriate language or references.