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Let’s talk about sex - evolutionary explanations for the persistance of sexual reproduction · MicGyver Media

• 0:00
  SPEAKER_00: Why sex?
• 0:01
  SPEAKER_00: Perhaps the simplest yet deepest question about sexual reproduction is why it exists at all.
• 0:07
  SPEAKER_00: The answer seems obvious at first.
• 0:09
  SPEAKER_00: Sex exists to produce offspring.
• 0:12
  SPEAKER_00: On second thought, however, the question becomes far more intriguing.
• 0:16
  SPEAKER_00: As the biologist August Weissman pointed out back in 1889,
• 0:20
  SPEAKER_00: sex is not the only way to reproduce.
• 0:23
  SPEAKER_00: Plenty of organisms bud or grow from cuttings or lay eggs that develop
• 0:27
  SPEAKER_00: without ever being fertilized.
• 0:29
  SPEAKER_00: And on paper, those asexual options should win.
• 0:33
  SPEAKER_00: I'm Mads Mikhaelsen,
• 0:34
  SPEAKER_00: and this is an adapted audio version of a scientific review I wrote as
• 0:38
  SPEAKER_00: a biology student.
• 0:39
  SPEAKER_00: It's called Let's Talk About Sex,
• 0:41
  SPEAKER_00: and it tries to make sense of why sex persists despite its great costs.
• 0:46
  SPEAKER_00: Part one, the paradox.
• 0:48
  SPEAKER_00: Let's start with those costs.
• 0:50
  SPEAKER_00: An asexual organism doesn't have to find a mate or attract one.
• 0:54
  SPEAKER_00: It also avoids sexually transmitted diseases and the other risks of mating.
• 0:59
  SPEAKER_00: And then there is the reshuffling.
• 1:01
  SPEAKER_00: Sex systematically breaks up genetic combinations that have already proven they work.
• 1:06
  SPEAKER_00: You survived long enough to reproduce,
• 1:08
  SPEAKER_00: and your reward is that your winning hand gets reshuffled and dealt again.
• 1:12
  SPEAKER_00: The biggest cost of all, though, is what John Maynard Smith called the twofold cost of males.
• 1:18
  SPEAKER_00: A sexual female spends half her effort on sons, and sons can't bear young themselves.
• 1:23
  SPEAKER_00: An asexual female has only daughters, each one a clone that can reproduce on her own.
• 1:29
  SPEAKER_00: All else equal, her share of the population doubles every generation.
• 1:33
  SPEAKER_00: You see the same cost on the gene level.
• 1:36
  SPEAKER_00: A sexual parent passes on 50% of its genes to each offspring.
• 1:40
  SPEAKER_00: A cloner passes on 100%.
• 1:42
  SPEAKER_00: So sex should lose.
• 1:45
  SPEAKER_00: And yet it's almost everywhere among complex life.
• 1:48
  SPEAKER_00: Only about 1% of plant species, and 1 in 1,000 animal species, reproduce purely by cloning.
• 1:55
  SPEAKER_00: And most of those asexual species are young on an evolutionary timeline.
• 1:59
  SPEAKER_00: They tend to be twigs on the tree of life, and short ones at that.
• 2:03
  SPEAKER_00: So there must be major advantages to sex.
• 2:06
  SPEAKER_00: What are they?
• 2:07
  SPEAKER_00: One caveat before we go on.
• 2:09
  SPEAKER_00: There are really two questions here.
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  SPEAKER_00: How sex began and why it persists.
• 2:13
  SPEAKER_00: This episode is only about the second.
• 2:16
  SPEAKER_00: Part 2.
• 2:17
  SPEAKER_00: Good for the group.
• 2:18
  SPEAKER_00: First, a quick definition.
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  SPEAKER_00: Biologically, sex consists of two things.
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  SPEAKER_00: Recombination,
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  SPEAKER_00: where your chromosomes swap segments when you make sex cells,
• 2:28
  SPEAKER_00: and outcrossing, where those cells fuse with someone else's.
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  SPEAKER_00: In short, sex is a machine for mixing genes.
• 2:35
  SPEAKER_00: It does so systematically across the entire genome.
• 2:38
  SPEAKER_00: But why?
• 2:40
  SPEAKER_00: The earliest answer is August Weissmann's.
• 2:42
  SPEAKER_00: Sex generates variation for natural selection to act upon.
• 2:46
  SPEAKER_00: Any given shuffle is at least as likely to make a bad combination as a good one,
• 2:50
  SPEAKER_00: so no individual gains.
• 2:52
  SPEAKER_00: But the population does, because selection weeds out the bad combinations and keeps the good.
• 2:57
  SPEAKER_00: Later, this was nicknamed the Vicar of Bray hypothesis,
• 3:00
  SPEAKER_00: after a clergyman in an English folk song who changed his creed every time
• 3:04
  SPEAKER_00: the monarch changed, and so kept his job.
• 3:06
  SPEAKER_00: It's worth stressing that sex does not create new genes.
• 3:10
  SPEAKER_00: Mutation does that.
• 3:11
  SPEAKER_00: Sex only deals new hands from the same deck.
• 3:15
  SPEAKER_00: In the 1930s, Ronald Fisher and Hermann Muller turned Weissmann's idea into mathematics.
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  SPEAKER_00: Their argument is easiest to see as a picture.
• 3:23
  SPEAKER_00: Imagine a river flowing from left to right.
• 3:25
  SPEAKER_00: That's a population moving through time.
• 3:28
  SPEAKER_00: Somewhere upstream, a useful mutation appears in one individual.
• 3:31
  SPEAKER_00: Call it A.
• 3:33
  SPEAKER_00: Around the same time, a different useful mutation, B, appears in another individual.
• 3:37
  SPEAKER_00: In a sexual population, the two streams widen, meet and merge.
• 3:42
  SPEAKER_00: Within a few generations,
• 3:43
  SPEAKER_00: A and B end up in the same body, and that combination takes over the whole river.
• 3:48
  SPEAKER_00: In an asexual population there is no merging.
• 3:51
  SPEAKER_00: The A clones and the B clones each spread until they run into each other.
• 3:55
  SPEAKER_00: Then they compete.
• 3:57
  SPEAKER_00: Say the B stream gets squeezed thinner and thinner and dries up.
• 4:00
  SPEAKER_00: B was a good mutation and it's gone.
• 4:03
  SPEAKER_00: The population only gets A and B together later if the B mutation happens to arise
• 4:08
  SPEAKER_00: a second time, by luck, inside an A carrying individual.
• 4:12
  SPEAKER_00: So without sex, evolution has to make the same invention twice.
• 4:16
  SPEAKER_00: Good ideas in different lineages Mueller then turned from good mutations
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  SPEAKER_00: to bad ones, which is fair since most mutations that do anything are harmful.
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  SPEAKER_00: His idea is known as Muller's Ratchet.
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  SPEAKER_00: In a clonal lineage, a child inherits every mutation its parent had plus any new ones.
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  SPEAKER_00: The mutational load can go up and it can never go down.
• 4:46
  SPEAKER_00: Click, click, click.
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  SPEAKER_00: Sex escapes the ratchet because recombination can bring the undamaged versions
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  SPEAKER_00: of genes back together in one individual.
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  SPEAKER_00: The ratchet turns fastest when mutation rates are high,
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  SPEAKER_00: genomes are large, and populations are small.
• 5:01
  SPEAKER_00: Mystery solved?
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  SPEAKER_00: Many would argue no.
• 5:04
  SPEAKER_00: For one thing, most of these benefits go to the population over the long run.
• 5:09
  SPEAKER_00: But natural selection works on individuals, or even genes.
• 5:13
  SPEAKER_00: It will not promote something that helps the species and hurts its carrier,
• 5:17
  SPEAKER_00: because carriers get outcompeted by those who don't make the sacrifice.
• 5:21
  SPEAKER_00: These processes are also slow.
• 5:23
  SPEAKER_00: The twofold cost is paid right now, every generation.
• 5:26
  SPEAKER_00: So we need a reason why a sexual individual does better than an asexual one, and quickly.
• 5:32
  SPEAKER_00: Part three, the ecological perspective.
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  SPEAKER_00: This is where many biologists turn to ecology.
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  SPEAKER_00: George Williams argued that if the environment changes quickly,
• 5:41
  SPEAKER_00: What worked for the parent may not work for the child,
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  SPEAKER_00: so varied offspring are a hedge.
• 5:47
  SPEAKER_00: It's a lottery,
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  SPEAKER_00: and sex buys many different tickets, where cloning buys the same ticket many times.
• 5:53
  SPEAKER_00: Makes intuitive sense, but have faced some challenges in terms of predictive power.
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  SPEAKER_00: It could be argued that sex should then be most common where conditions swing
• 6:01
  SPEAKER_00: the most, like at high latitudes and high altitudes.
• 6:05
  SPEAKER_00: Broadly, we find the opposite.
• 6:07
  SPEAKER_00: Sex is more common in stable environments, like lakes and tropical rainforests.
• 6:12
  SPEAKER_00: A related idea is known as the Tangled Bank.
• 6:14
  SPEAKER_00: Even a small patch of nature has many slightly different niches.
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  SPEAKER_00: Clones all compete for the same one.
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  SPEAKER_00: Sexual siblings differ, so they can spread across several and compete less with each other.
• 6:26
  SPEAKER_00: It's an elegant idea.
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  SPEAKER_00: The empirical support is not super strong.
• 6:30
  SPEAKER_00: The most promising ecological explanation is the Red Queen.
• 6:33
  SPEAKER_00: She's a character in Lewis Carroll's Through the Looking Glass,
• 6:36
  SPEAKER_00: who tells Alice that it takes all the running you can do to keep in the same place.
• 6:41
  SPEAKER_00: The running here is against parasites.
• 6:43
  SPEAKER_00: Parasites evolve fast, and they evolve to crack the most common host.
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  SPEAKER_00: If you're a clone,
• 6:49
  SPEAKER_00: you're a lock that a parasite has already picked,
• 6:52
  SPEAKER_00: and so is every one of your daughters.
• 6:54
  SPEAKER_00: Sex changes the locks every generation.
• 6:57
  SPEAKER_00: The best evidence comes from a small mud snail in New Zealand,
• 7:00
  SPEAKER_00: which exists in both sexual and asexual forms, living side by side.
• 7:05
  SPEAKER_00: Where parasites are more common, the sexual form becomes more common too.
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  SPEAKER_00: But there are doubts about whether parasites alone are enough.
• 7:12
  SPEAKER_00: In computer simulations by Howard and Lively,
• 7:15
  SPEAKER_00: sex only won decisively when parasites spread very easily and were brutal,
• 7:20
  SPEAKER_00: costing the host more than 80% of its fitness.
• 7:23
  SPEAKER_00: Such parasites exist, they are not everywhere though, and sex is.
• 7:27
  SPEAKER_00: Part 4.
• 7:28
  SPEAKER_00: Synthesis.
• 7:30
  SPEAKER_00: So where does that leave us?
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  SPEAKER_00: Many promising theories, and no single one that seems strong enough to explain sex alone.
• 7:37
  SPEAKER_00: But there is no reason why one theory should have to do it alone.
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  SPEAKER_00: These ideas are not mutually exclusive.
• 7:43
  SPEAKER_00: When the mentioned Howard and Lively put Muller's Ratchet and the Red Queen in
• 7:47
  SPEAKER_00: the same model, Sex did far better than under either alone.
• 7:51
  SPEAKER_00: More recent models go further.
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  SPEAKER_00: They combine selective interference from mutations in populations
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  SPEAKER_00: with strong selection from parasites or changing environments.
• 8:00
  SPEAKER_00: Under those models,
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  SPEAKER_00: sex can benefit the individual immediately and beat cloning in a wide range
• 8:06
  SPEAKER_00: of circumstances.
• 8:07
  SPEAKER_00: To conclude, sex brings good mutations together and helps get rid of bad ones.
• 8:12
  SPEAKER_00: And it arms the organism with variability in a world that keeps changing.
• 8:17
  SPEAKER_00: Sexual reproduction undeniably comes with great costs.
• 8:21
  SPEAKER_00: But through a mixture of factors reminiscent of the process of sex itself, it prevails.
• 8:26
  SPEAKER_00: Thanks for listening.
• 8:27
  SPEAKER_00: The full paper,
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  SPEAKER_00: with figures and references,
• 8:29
  SPEAKER_00: was published in Bikuben, the biology student journal at the University of Bergen.