Hey smart people, Joe here.
A lot of us probably remember when we had THAT talk.
Dad: Hey son, are ya winning? We need to talk… Kid: OMG this is not happening.
Dad: …about seeeeeeeeeeeex
[Show open]
This isn’t gonna be like that. (hopefully) But it is going to surprise you.
First, a seemingly simple question: What is sex for?
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 what sex is for. Biologically speaking, “sex” isn’t… what you’re
probably picturing in your head right now.
We all know that passing genes on to the next generation is essentially the whole point
of life. Survive to reproduce and get your precious genetic information into some offspring
so they can go on and do the same thing. And there are two primary ways to accomplish that:
Method 1: Divide and copy all your information over to a genetically identical offspring.
[beat]
Or (Method 2) First do some fancy cell division, then go off and spend a bunch of time and
energy looking for a potential partner, attract that partner, and while you’re mating, try
not to get eaten by a predator (or eaten by your potential mate!) If you survive and get
lucky, you mix a random ½ of your genes with a random ½ of their genes to make some genetically
patch-work offspring.
One of these is obviously MUCH simpler than the other.
“Sex” refers to just this one bit of the process: taking part of the genetic instructions
from two organisms and combining them into one complete set of genetic instructions.
And plenty of living things reproduce without sex. Microbes that bud off new copies of themselves.
Or like flatworms, pieces of their body can grow into whole new individuals. Fungi that
release spores. Even some bitey fish, scaly pals, and a few danger noodles are capable
of virgin birth.
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.
I mean, 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 expensive. I’m not talking about dinners
and dates and presents and those kinds of things. Those are pretty much just human problems.
I’m talking about biological costs of sex. For starters, sex is slow.
In the time it takes say, a bacterium to copy themselves and divide, we’ve barely gotten
ready for a date.
And finding a mate can be really hard. For a tiny bug or itty-bitty ocean creature, searching
for sex can feel a lot like searching for a needle in a haystack the size of the solar
system. 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
me. Well, okay, not literally me. But the cost of having males in general.
Consider a model in which every female has two offspring. In a species that reproduces
asexually, we can consider every individual female. And all their offspring are also female.
In a sexually reproducing species, she has one male and one female on average. In the
next round, each of the asexual females has two more females. The sexual species? [cue
banjo music] That female still has just one male and one female offspring. 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 ruin all of natural selection’s hard
work. Because sex breaks apart favorable combinations of genes.
Imagine you walk into a poker tournament, and you 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 on
to 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 game (or environment) that decides whether
an individual will survive and reproduce… or fail and fold. 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 these
things called chromosomes. Those chromosomes come in pairs, one from mom and one from dad.
Early in meiosis, each chromosome swaps chunks with its partner-buddy 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 223rd, or 8,388,608 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
a special and unique snowflake. Unless you’re a monozygotic twin maybe…
So, sex is a really bananas way to get your genes to the next generation. It’s slow,
it is costly and it can break up winning genetic hands. But… then: Why is sex so common?
Only about 1 in a thousand (0.1%) animal species and only 1% of flowering plants are exclusively
asexual. From fleas to trees? Pretty much every eukaryote does the birds and the bees.
It’s a paradox.
There must be a reason, a huge advantage to sex that more than makes up for the HUGE 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.
Can you just put in a stock clip or something?
Sex creates genetic diversity! But remember like we showed in that poker example, for
an organism that’s already well adapted to its environment, that’s carrying a winning
hand of genes working together, shuffling seems counterproductive.
Here’s the thing: the rules of life’s poker game don’t stay the same from game
to game. 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. And they gradually accumulate random mutations, too. Cards in
their hand are randomly being changed with no way to fix or get rid of them. If you’re
asexual, what was an advantage at first can very quickly turn into an extreme 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 now. Especially when that game is parasites.
Whether a parasite can infect a host has a lot to do with your genetics. Your cells and
your body essentially 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, and even if it opens one, the rest of the population is safe.
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, while they’re running as fast as they can but not getting anywhere,
“Now, here, you see, 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 longterm 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. But once a parasite catches one asexual, it catches up to them
all. 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 or antler bois have these huge ornaments that are expensive
to grow or make them easier targets for predators? The Red Queen may be able to explain this
too. What if these ornaments are attractive to mates because they’re like big signs
that 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 hands together.
Sex is costly, but it gives the next generation
a better chance of surviving a challenging environment. Some organisms, including many
plants, some algae, even bugs like aphids, clone themselves asexually some of the time,
but reproduce sexually when their environment presents a challenge and genetic shuffling
might provide a survival advantage.
Switching back and forth between reproductive modes seems super ideal, so why can’t we
reproduce asexually, budding off a big toe and growing a baby clone when it suits us?
(sorry, that’s a horrifying thought)
Well in addition to eventually running out of toes, organisms don’t get to decide how
to reproduce. All of these mechanisms, all these tradeoffs and costs and advantages,
are coded into an organism’s biology bt 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. And the resulting genetic diversity it generated
made the organisms that did it a little bit more fit, and a little bit more likely to
get their genes into the next generation. And that’s what matters in this game. It’s not fate. It’s evolution.
Stay curious.