6 ms·

That is blatantly false. Sex speeds up evolution exponentially with population size. With sex, each beneficial mutation can spread across the species even after

That is blatantly false. Sex speeds up evolution exponentially with population size. With sex, each beneficial mutation can spread across the species even after arising only once. With asexual reproduction, beneficial mutations are confined to that particular strain. Sex is enormously beneficial for the ability to evolve and adapt quickly. This has been confirmed by mathematical models.

lukasLansky 11y ago
I downvoted you because although individual points of the article can argued, the text is certainly not "blatantly false" and the overall dismissive attitude is harmful to potential discussion about nuances of the issue. I would recommend you to read https://en.wikipedia.org/wiki/Punctuated_equilibrium https://en.wikipedia.org/wiki/Punctuated_equilibrium for a start.
- PepeGomez 11y ago
  It's premise, that the costs of sex obviously outweigh the benefits, is false. Sex is an extremely advantageous ability. There is no discussion to be had, because the assumed problem doesn't in fact exist. What does punctuated equilibrum have to do with it?
  - Confusion 11y ago
    That's not its premise. Its premise is that the benefits tend to be subtle and become evident only over many generations, [while] its costs are heavy and immediate and so sex *seems* like a waste emphasis mine. Your reading of the article is extremely uncharitable.
    - PepeGomez 11y ago
      The costs aren't heavy, the math in the article is wrong. Yes, each of your offspring will have half of your genes, but twice as many offspring counts as yours.
      - deleted 11y ago
        [deleted]
        deleted 11y ago
        [deleted]
  - lukasLansky 11y ago
    Punctuated equilibrium is a reaction to your claim that evolution speeds up exponentialy with the size of the population. In practice, it's more like you need very small, separated group to move things forward. It's explained by the fact that one gene contribution to fitness is heavily dependent on other genes, so an in-one-context-advantageous mutation has problems with crossing-over destroying some conditions of its success.
    - PepeGomez 11y ago
      Oh that, yes, but that's exactly what sex allows to happen. Imagine there are three populations of desert flowers, each with its own adaptations to drought. One population has mutations a and b, other has c and the last one has d and e. When these populations meet, their genes recombine and in a few generations there will be individuals with all five, much more resistant than any of the parent groups. This is entirely impossible with asexual species - the strains would merely compete with each other and nothing interesting would happen.
  - Double_Cast 11y ago
    Consider a startup. In the long run, it may have a large advantage over its incumbent competitors. But at inception, it often needs a capital investment to overcome a barrier of entry. Without the initial investment, the startup may never see the light of day. The article's thesis is analogous: though sexual selection may be a global maximum, evolution might have gotten stuck in a local maximum (asexual selection) if not for a little boost. Off the top of my head, I can imagine possible adaptations which humans would find strictly beneficial, but we won't likely evolve (without genetic engineering). E.g. there's no good reason why humans have a blind spot. We wouldn't have it if our optic nerves were attached behind our retina instead of in front. But we're stuck with it because evolution can't conjure a discontinuous jump in our anatomy.
jballanc 11y ago
You're misunderstanding how gene flow works. Consider one individual with one advantageous mutation that has 1000 offspring. In an asexual organism all 1000 will contain the mutation (minus any small frequency of the mutation spontaneously reverting). In a sexual organism only 500 will carry the advantage.
- Retric 11y ago
  There is no free lunch. 2 parents can have more children than 1 parent. Hermaphroditism means twice as many offspring with 1/2 as many copy's of your DNA. Even single celled organisms will exchange DNA.
- guard-of-terra 11y ago
  This individual invariably carries ten of disadvantageous mutations too. It will pass all ten to offspring when reproducing asexually, but only five of them on average to every sexually-produced offspring, and a dozen will only get 2 or 3 such bad mutations. This arithmetic becomes ugly fast for asexual reproduction.
  - PepeGomez 11y ago
    Indeed. Asexual reproduction is basically like being 100% inbred.
  - jballanc 11y ago
    Again, it's the exact opposite. In an asexual species, a disadvantageous mutation will be flushed from the population very quickly, as those without the mutation will outcompete those with. You'll never get an individual with ten disadvantages. On the other hand, in a sexually reproducing organism, a disadvantageous mutation can be masked if recessive, and selective pressure will only remove homozygotes from the population. The end result is that harmful mutations can stick around for a long time, and even accumulate in individuals.
    - guard-of-terra 11y ago
      "a disadvantageous mutation will be flushed from the population very quickly" In asexual case, an individual with one advantageous mutation and ten slightly disadvantageous will be flushed out. In sexual case, such individual will produce some survivable offspring and the advantageous gene has good chance of flourishing in the population.
kryptiskt 11y ago
Bacteria get around that by swapping genes with each other, so they don't have to reproduce to spread genes around.
dekhn 11y ago
Nothing is "confirmed" by mathematical models, ever. Things are confirmed by experiments run in the real world.
- conceit 11y ago
  ... which may then be confirmed by mathematical models, what else?
  - dekhn 11y ago
    "supported by", or "anticipated by", but not confirmed.
    - deleted 11y ago
      [deleted]
- arcanus 11y ago
  I wish it was that clear cut. This is a tricky business in the sciences. Often, the quantity you are trying to validate is unobservable. In this case, I suspect (I'm not a squishy scientist) it is because evolution has such long time scales. In other fields, it can be simply because it is prohibitively difficult or expensive to instrument the quantity of interest. This is extremely common, even in the 'hard' scientific fields and engineering. You very, very seldom have all the data you want. This is all a long winded way of saying that confirmation of a hypothesis by mathematical models certainly is a mechanism by which science creates a stronger degree of belief in said hypothesis. Would it be a full fledged validation of the model? Probably not. But it is absolutely useful evidence. Full disclosure, I am a computation scientist.
  - dekhn 11y ago
    I'm a computational scientist too, but those mathematical models don't provide confirmation. It certainly can help build evidence, and when a model starts to make predictions outside of the model, then we really can a lot of confidence in it. But a model is just a model, and unless the universe is truly mathematical in structure (not a completely crazy idea), and our models match that structure, then they can't really confirm anything.
bhickey 11y ago
Two of the models that Pepe alluded to are Mueller's Ratchet and Kondrashov's Hatchet. We know that sex is powerful because it persists despite the high cost. Mueller's model assumes that fitness decreases as a linear function of deleterious mutations. In simplified terms: Got n bad mutations? You're more likely to reproduce than someone with (n+1) bad mutations. In an asexual population, individuals will have the set of mutations plus those acquired by mutation from their time of divergence. When you add sex to the mix (ha!), your population stops looking so flat. It introduces variance and selection culls the maladapted individuals. What's really cool about this, is you can model the deleterious mutations being driven out of the population. Kondrashov's model is even more powerful. Basically, it says that there's some capacity for deleterious mutations. A couple of things wrong? Yeah, you'll reproduce. Too many? Well, you stepped off an mutational cliff and you aren't long for the world. This step function actually removes deleterious mutations from the population faster than Mueller. The advantage of sex is obvious even on an individual basis. We all should have a chromosome that's "worse than average". When you mix and match them among your gametes, you'll end up with some non-viable gametes and some superstars. By failing fast, any parental investment can go toward offspring who are more likely themselves to be viable. [For more information, check out Gillespie's Population Genetics]
- jballanc 11y ago
  1. Mueller's ratchet is still a hypothesis. A well supported hypothesis, but there are cases that it doesn't yet explain (Bdelloid rotifers, for example). 2. Even accepting the premise, it doesn't explain why sexual reproduction should be favored over asexual reproduction with occasional conjugation. 3. Even assuming that this mechanism is sufficient to explain the advantages of sexual reproduction, it still operates over a long time scale (i.e. long enough for enough deleterious mutations to accumulate in the asexual reproducing population). What this comment and the parent (and many of the others in this thread) fail to understand is that no one is debating that sexual reproduction is advantageous over long enough time scales. The question that the authors of the original paper are addressing is how sexual reproduction could be advantageous enough on a short enough time scale sufficient for those organisms to avoid being outcompeted by asexual organisms.
StefanKarpinski 11y ago
This was my immediate reaction as well, although I'm not sure I would dismiss the article as "false". Since there seems to be a significant amount of misunderstanding of what PepeGomez is saying in the rest of this subthread, I'll take a crack at an explanation. One way to understand is to look backwards at a single individual's ancestors. In an asexually reproducing species, the number of ancestors is O(n) in the number of generations you look back – and in any prior generation the individual has exactly one ancestor. In a sexually reproducing species, the number of ancestors is O(2^n) until you reach the point (surprisingly rapidly) where the whole species either is an ancestor or doesn't have any surviving descendants. Why does this matter? Because multiple beneficial mutations are far more likely to have arisen in the O(2^n) ancestors in the sexual scenario than in the O(n) ancestors in the asexual scenario. In other words, sexual reproduction allows beneficial mutations to occur independently in any pair of unrelated individuals in a population and eventually both mutations will spread to the whole population – the whole species ends up with all the "winning" mutations. With asexual reproduction, unless two mutations just happen to fall on the same lineage – which is very unlikely – they will compete instead of merging, because there is no mechanism for merging lineages. Bottom line: in asexual reproduction, independent mutations compete; in sexual reproduction, they can cooperate. Sexual species literally evolve exponentially faster than asexual ones.
- PepeGomez 11y ago
  Exactly. You described it much better than I did.
- jballanc 11y ago
  Except that's not how evolution works. You can't just randomly decide to "count backward" and you definitely can't ignore the fact that ancestors overlap. You might look up "The 7 Daughters of Eve". In short: all humans alive today descended from just 7 women, but those 7 women were not the only women alive at the time. All the other women living at the same time as the 7 had their genetic information lost to the sands of time.
  - StefanKarpinski 11y ago
ealloc 11y ago
A lot of experts disagree with you that this is an open-and shut-question, including leaders in the field of population genetics. It is by no means so clear cut. Below I've linked a nice blog post on the question, as well as a Nature review from 2002 and even a Nature article from two weeks ago (Mar 2016). http://sandwalk.blogspot.com/2014/03/everything-you-thought-you-knew-about.html http://sandwalk.blogspot.com/2014/03/everything-you-thought-... http://www.nature.com/nrg/journal/v3/n4/abs/nrg761.html http://www.nature.com/nrg/journal/v3/n4/abs/nrg761.html http://www.nature.com/nature/journal/v531/n7593/abs/nature17143.html http://www.nature.com/nature/journal/v531/n7593/abs/nature17... Let me quote from one of them: "The vast majority of species engage in some form of sex or genetic exchange. Yet the evolutionary forces that make sex widespread in nature remain incompletely understood. In principle, asexual reproduction should be more efficient: it avoids the costs of mating and allows individuals to pass all (rather than half) of their genetic material to their offspring. Extensive theoretical work has sought to understand why sex is pervasive despite these substantial costs."
- PepeGomez 11y ago
  No matter how big experts they are, they can't disagree with math. Asexual species pass on all their genetic material to half the number of offspring. In a stable population, one offspring on average survives in asexual reproduction, two in sexual reproduction. In other words, while you pass half of the genes per offspring, twice as many offspring is technically yours. The amount of genetic material passed on average remains the same.
  - jballanc 11y ago
    The amount of genetic material passed on does not remain the same. Each offspring is an independent event, and which half of the genes gets passed along is selected randomly. So if an asexual organism has genes "A B C", then both it's two offspring will get "A B C". If a sexual organism has genes "Aa Bb Cc", and its first offspring inherits "A b C" from that parent, the second offspring could inherit "a b c" in which case the "B" allele is lost.
    - 11y ago
bedhead 11y ago
"Hey baby, want to speed up evolution through beneficial mutation?" Definitely trying this one.

lukasLansky 11y ago

I downvoted you because although individual points of the article can argued, the text is certainly not "blatantly false" and the overall dismissive attitude is harmful to potential discussion about nuances of the issue. I would recommend you to read https://en.wikipedia.org/wiki/Punctuated_equilibrium https://en.wikipedia.org/wiki/Punctuated_equilibrium for a start.

PepeGomez 11y ago
It's premise, that the costs of sex obviously outweigh the benefits, is false. Sex is an extremely advantageous ability. There is no discussion to be had, because the assumed problem doesn't in fact exist. What does punctuated equilibrum have to do with it?
- Confusion 11y ago
  That's not its premise. Its premise is that the benefits tend to be subtle and become evident only over many generations, [while] its costs are heavy and immediate and so sex *seems* like a waste emphasis mine. Your reading of the article is extremely uncharitable.
  - PepeGomez 11y ago
    The costs aren't heavy, the math in the article is wrong. Yes, each of your offspring will have half of your genes, but twice as many offspring counts as yours.
    - deleted 11y ago
      [deleted]
      - deleted 11y ago
        [deleted]
- lukasLansky 11y ago
  Punctuated equilibrium is a reaction to your claim that evolution speeds up exponentialy with the size of the population. In practice, it's more like you need very small, separated group to move things forward. It's explained by the fact that one gene contribution to fitness is heavily dependent on other genes, so an in-one-context-advantageous mutation has problems with crossing-over destroying some conditions of its success.
  - PepeGomez 11y ago
    Oh that, yes, but that's exactly what sex allows to happen. Imagine there are three populations of desert flowers, each with its own adaptations to drought. One population has mutations a and b, other has c and the last one has d and e. When these populations meet, their genes recombine and in a few generations there will be individuals with all five, much more resistant than any of the parent groups. This is entirely impossible with asexual species - the strains would merely compete with each other and nothing interesting would happen.
- Double_Cast 11y ago
  Consider a startup. In the long run, it may have a large advantage over its incumbent competitors. But at inception, it often needs a capital investment to overcome a barrier of entry. Without the initial investment, the startup may never see the light of day. The article's thesis is analogous: though sexual selection may be a global maximum, evolution might have gotten stuck in a local maximum (asexual selection) if not for a little boost. Off the top of my head, I can imagine possible adaptations which humans would find strictly beneficial, but we won't likely evolve (without genetic engineering). E.g. there's no good reason why humans have a blind spot. We wouldn't have it if our optic nerves were attached behind our retina instead of in front. But we're stuck with it because evolution can't conjure a discontinuous jump in our anatomy.

jballanc 11y ago

You're misunderstanding how gene flow works. Consider one individual with one advantageous mutation that has 1000 offspring. In an asexual organism all 1000 will contain the mutation (minus any small frequency of the mutation spontaneously reverting). In a sexual organism only 500 will carry the advantage.

Retric 11y ago
There is no free lunch. 2 parents can have more children than 1 parent. Hermaphroditism means twice as many offspring with 1/2 as many copy's of your DNA. Even single celled organisms will exchange DNA.
guard-of-terra 11y ago
This individual invariably carries ten of disadvantageous mutations too. It will pass all ten to offspring when reproducing asexually, but only five of them on average to every sexually-produced offspring, and a dozen will only get 2 or 3 such bad mutations. This arithmetic becomes ugly fast for asexual reproduction.
- PepeGomez 11y ago
  Indeed. Asexual reproduction is basically like being 100% inbred.
- jballanc 11y ago
  Again, it's the exact opposite. In an asexual species, a disadvantageous mutation will be flushed from the population very quickly, as those without the mutation will outcompete those with. You'll never get an individual with ten disadvantages. On the other hand, in a sexually reproducing organism, a disadvantageous mutation can be masked if recessive, and selective pressure will only remove homozygotes from the population. The end result is that harmful mutations can stick around for a long time, and even accumulate in individuals.
  - guard-of-terra 11y ago
    "a disadvantageous mutation will be flushed from the population very quickly" In asexual case, an individual with one advantageous mutation and ten slightly disadvantageous will be flushed out. In sexual case, such individual will produce some survivable offspring and the advantageous gene has good chance of flourishing in the population.

kryptiskt 11y ago

Bacteria get around that by swapping genes with each other, so they don't have to reproduce to spread genes around.

dekhn 11y ago

Nothing is "confirmed" by mathematical models, ever. Things are confirmed by experiments run in the real world.

conceit 11y ago
... which may then be confirmed by mathematical models, what else?
- dekhn 11y ago
  "supported by", or "anticipated by", but not confirmed.
  - deleted 11y ago
    [deleted]
arcanus 11y ago
I wish it was that clear cut. This is a tricky business in the sciences. Often, the quantity you are trying to validate is unobservable. In this case, I suspect (I'm not a squishy scientist) it is because evolution has such long time scales. In other fields, it can be simply because it is prohibitively difficult or expensive to instrument the quantity of interest. This is extremely common, even in the 'hard' scientific fields and engineering. You very, very seldom have all the data you want. This is all a long winded way of saying that confirmation of a hypothesis by mathematical models certainly is a mechanism by which science creates a stronger degree of belief in said hypothesis. Would it be a full fledged validation of the model? Probably not. But it is absolutely useful evidence. Full disclosure, I am a computation scientist.
- dekhn 11y ago
  I'm a computational scientist too, but those mathematical models don't provide confirmation. It certainly can help build evidence, and when a model starts to make predictions outside of the model, then we really can a lot of confidence in it. But a model is just a model, and unless the universe is truly mathematical in structure (not a completely crazy idea), and our models match that structure, then they can't really confirm anything.

bhickey 11y ago

Two of the models that Pepe alluded to are Mueller's Ratchet and Kondrashov's Hatchet. We know that sex is powerful because it persists despite the high cost. Mueller's model assumes that fitness decreases as a linear function of deleterious mutations. In simplified terms: Got n bad mutations? You're more likely to reproduce than someone with (n+1) bad mutations. In an asexual population, individuals will have the set of mutations plus those acquired by mutation from their time of divergence. When you add sex to the mix (ha!), your population stops looking so flat. It introduces variance and selection culls the maladapted individuals. What's really cool about this, is you can model the deleterious mutations being driven out of the population. Kondrashov's model is even more powerful. Basically, it says that there's some capacity for deleterious mutations. A couple of things wrong? Yeah, you'll reproduce. Too many? Well, you stepped off an mutational cliff and you aren't long for the world. This step function actually removes deleterious mutations from the population faster than Mueller. The advantage of sex is obvious even on an individual basis. We all should have a chromosome that's "worse than average". When you mix and match them among your gametes, you'll end up with some non-viable gametes and some superstars. By failing fast, any parental investment can go toward offspring who are more likely themselves to be viable. [For more information, check out Gillespie's Population Genetics]

jballanc 11y ago
1. Mueller's ratchet is still a hypothesis. A well supported hypothesis, but there are cases that it doesn't yet explain (Bdelloid rotifers, for example). 2. Even accepting the premise, it doesn't explain why sexual reproduction should be favored over asexual reproduction with occasional conjugation. 3. Even assuming that this mechanism is sufficient to explain the advantages of sexual reproduction, it still operates over a long time scale (i.e. long enough for enough deleterious mutations to accumulate in the asexual reproducing population). What this comment and the parent (and many of the others in this thread) fail to understand is that no one is debating that sexual reproduction is advantageous over long enough time scales. The question that the authors of the original paper are addressing is how sexual reproduction could be advantageous enough on a short enough time scale sufficient for those organisms to avoid being outcompeted by asexual organisms.

StefanKarpinski 11y ago

This was my immediate reaction as well, although I'm not sure I would dismiss the article as "false". Since there seems to be a significant amount of misunderstanding of what PepeGomez is saying in the rest of this subthread, I'll take a crack at an explanation. One way to understand is to look backwards at a single individual's ancestors. In an asexually reproducing species, the number of ancestors is O(n) in the number of generations you look back – and in any prior generation the individual has exactly one ancestor. In a sexually reproducing species, the number of ancestors is O(2^n) until you reach the point (surprisingly rapidly) where the whole species either is an ancestor or doesn't have any surviving descendants. Why does this matter? Because multiple beneficial mutations are far more likely to have arisen in the O(2^n) ancestors in the sexual scenario than in the O(n) ancestors in the asexual scenario. In other words, sexual reproduction allows beneficial mutations to occur independently in any pair of unrelated individuals in a population and eventually both mutations will spread to the whole population – the whole species ends up with all the "winning" mutations. With asexual reproduction, unless two mutations just happen to fall on the same lineage – which is very unlikely – they will compete instead of merging, because there is no mechanism for merging lineages. Bottom line: in asexual reproduction, independent mutations compete; in sexual reproduction, they can cooperate. Sexual species literally evolve exponentially faster than asexual ones.

PepeGomez 11y ago
Exactly. You described it much better than I did.
jballanc 11y ago
Except that's not how evolution works. You can't just randomly decide to "count backward" and you definitely can't ignore the fact that ancestors overlap. You might look up "The 7 Daughters of Eve". In short: all humans alive today descended from just 7 women, but those 7 women were not the only women alive at the time. All the other women living at the same time as the 7 had their genetic information lost to the sands of time.
- StefanKarpinski 11y ago

ealloc 11y ago

A lot of experts disagree with you that this is an open-and shut-question, including leaders in the field of population genetics. It is by no means so clear cut. Below I've linked a nice blog post on the question, as well as a Nature review from 2002 and even a Nature article from two weeks ago (Mar 2016). http://sandwalk.blogspot.com/2014/03/everything-you-thought-you-knew-about.html http://sandwalk.blogspot.com/2014/03/everything-you-thought-... http://www.nature.com/nrg/journal/v3/n4/abs/nrg761.html http://www.nature.com/nrg/journal/v3/n4/abs/nrg761.html http://www.nature.com/nature/journal/v531/n7593/abs/nature17143.html http://www.nature.com/nature/journal/v531/n7593/abs/nature17... Let me quote from one of them: "The vast majority of species engage in some form of sex or genetic exchange. Yet the evolutionary forces that make sex widespread in nature remain incompletely understood. In principle, asexual reproduction should be more efficient: it avoids the costs of mating and allows individuals to pass all (rather than half) of their genetic material to their offspring. Extensive theoretical work has sought to understand why sex is pervasive despite these substantial costs."

PepeGomez 11y ago
No matter how big experts they are, they can't disagree with math. Asexual species pass on all their genetic material to half the number of offspring. In a stable population, one offspring on average survives in asexual reproduction, two in sexual reproduction. In other words, while you pass half of the genes per offspring, twice as many offspring is technically yours. The amount of genetic material passed on average remains the same.
- jballanc 11y ago
  The amount of genetic material passed on does not remain the same. Each offspring is an independent event, and which half of the genes gets passed along is selected randomly. So if an asexual organism has genes "A B C", then both it's two offspring will get "A B C". If a sexual organism has genes "Aa Bb Cc", and its first offspring inherits "A b C" from that parent, the second offspring could inherit "a b c" in which case the "B" allele is lost.
  - 11y ago

bedhead 11y ago

"Hey baby, want to speed up evolution through beneficial mutation?" Definitely trying this one.