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You're misunderstanding how gene flow works. Consider one individual with one advantageous mutation that has 1000 offspring. In an asexual organism all 1000 wil

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.
    - jballanc 11y ago
      Correct. Sex is advantageous over the long term. This is not up for debate. The question is how to compensate for the considerable costs in the short term so that sexual organisms are not immediately outcompeted by asexual organisms.
      - guard-of-terra 11y ago
        Asexual organisms can outcompete some part of population, but doubtfully all of it. And their competitive advantage shrinks as their numbers grow, due to clone interference. A lot of insects can reproduce asexually but they are also capable of sexual reproduction. Ones who are not eventually die off.
        jballanc 11y ago
        You're assuming that a population of asexual organisms was mixed with a population of sexual organisms. In reality, asexual reproduction came first, so the inability of asexual reproduction to overcome a large contingent of sexual organisms is irrelevant. And yes, optional sex is almost certainly how sexual reproduction arose. Even today I'd wager there are more facultatively sexual organisms than obligate sexual. That's a non-sequiter, though, as even optional sex would be bred out of a population if it could not show a benefit in a small number of generations.
        guard-of-terra 11y ago
        We do now know how first sexual organisms arose from asexual. If you'lll figure it out you'll become a big name in the field of evolutionary biology. It could have involved very unlikely events.
PepeGomez 11y ago
No, that is not correct. A hermaphrodite species will all things being the same have twice as many offspring as an asexual species, in your example 1000 from its eggs, 1000 from its sperm. In a strictly male/female species, a successful female will indeed likely spread the genes half as fast, but a succesful male can spread them much faster. But this is with only one gene. Imagine there are five a, b, c, d and e. An individual with all five will arise much sooner in a sexual species. The evolution of sex isn't a mystery any more than an eye is a mystery. It's not all or nothing. Life didn't go from bacteria to peacocks in one step. You can have something like optional hermaphrodite sex (in mollusks, I think) where you can exchange genes when the conditions arise, but it's not mandatory. The costs are virtually zero, but the benefits are still there. But then it becomes advantageous for idividuals to cheat and impregnate others, without producing eggs themselves and you get separate sexes.
- jballanc 11y ago
  Absolute numbers don't matter, only frequency, as in evolution you have to assume that individuals are reproducing fast enough to exceed the carrying capacity of the environment. And yes, once you get to more mutations then sex is more advantageous for a number of reasons. No one is claiming that sex is not better over long enough time scales. The question is how can it be advantageous enough to become fixed in a population over the span of a couple generations. And yes, sex did evolve gradually. Slime molds have optional sex (and more than 2 sexes!). But still keeping the machinery around is expensive. One would expect that individuals that ditched it would outcompete those that kept lugging it around unless there was a relatively short-term benefit to sex.
  - PepeGomez 11y ago
    Exactly, in a stable population, one offspring of each individual survives on average in an asexual species. Two with half of your genes each survive in a sexual species. It doesn't need a long timescale, asexual species get out-evolved by sexual species and go extinct eventually. Sexual species have a much higher probability of entering new niches or surviving when the conditions change. This can be seen in many species that reproduce asexually when the conditions are good, but reproduce sexually when stressed. A species that loses the ability to reproduce sexually is doomed to extinction. Asexual reproduction is essentially putting all your eggs into one basket. When you die of a disease for example, chances are that most of your offspring dies as well AND there is a much higher chance that the disease adapts to your genome in the first place. This can be seen in agriculture, where plants are reproduced asexually to a much bigger extent than what would happen naturally. Many are impossible to grow without heavy doses of pesticides, as the diseases quickly adapted to the commonly planted strains. The strains need to be replaced every few decades, before growing them becomes essentially impossible, even with pesticides. Evolution is much faster than you assume.
Lawtonfogle 11y ago
You can't look at a single individual. Even as soon as you include two individuals with advantageous mutations, the chance of them having an off spring with both is extremely higher than for an asexual organism. Scale this up to 1000 individuals, each with mutations that are advantageous or disadvantageous, which impacts their chance to have children, and run it across a few generations (with a small chance of children getting new mutations not in their parents), and the chance of a subset of the population getting many of the positive mutations is much much higher than if they were an asexual species. The asexual species will end up with more member of the final population having the single most advantageous mutation, but with none having more than that.

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.
  - jballanc 11y ago
    Correct. Sex is advantageous over the long term. This is not up for debate. The question is how to compensate for the considerable costs in the short term so that sexual organisms are not immediately outcompeted by asexual organisms.
    - guard-of-terra 11y ago
      Asexual organisms can outcompete some part of population, but doubtfully all of it. And their competitive advantage shrinks as their numbers grow, due to clone interference. A lot of insects can reproduce asexually but they are also capable of sexual reproduction. Ones who are not eventually die off.
      - jballanc 11y ago
        You're assuming that a population of asexual organisms was mixed with a population of sexual organisms. In reality, asexual reproduction came first, so the inability of asexual reproduction to overcome a large contingent of sexual organisms is irrelevant. And yes, optional sex is almost certainly how sexual reproduction arose. Even today I'd wager there are more facultatively sexual organisms than obligate sexual. That's a non-sequiter, though, as even optional sex would be bred out of a population if it could not show a benefit in a small number of generations.
        guard-of-terra 11y ago
        We do now know how first sexual organisms arose from asexual. If you'lll figure it out you'll become a big name in the field of evolutionary biology. It could have involved very unlikely events.

PepeGomez 11y ago

No, that is not correct. A hermaphrodite species will all things being the same have twice as many offspring as an asexual species, in your example 1000 from its eggs, 1000 from its sperm. In a strictly male/female species, a successful female will indeed likely spread the genes half as fast, but a succesful male can spread them much faster. But this is with only one gene. Imagine there are five a, b, c, d and e. An individual with all five will arise much sooner in a sexual species. The evolution of sex isn't a mystery any more than an eye is a mystery. It's not all or nothing. Life didn't go from bacteria to peacocks in one step. You can have something like optional hermaphrodite sex (in mollusks, I think) where you can exchange genes when the conditions arise, but it's not mandatory. The costs are virtually zero, but the benefits are still there. But then it becomes advantageous for idividuals to cheat and impregnate others, without producing eggs themselves and you get separate sexes.

jballanc 11y ago
Absolute numbers don't matter, only frequency, as in evolution you have to assume that individuals are reproducing fast enough to exceed the carrying capacity of the environment. And yes, once you get to more mutations then sex is more advantageous for a number of reasons. No one is claiming that sex is not better over long enough time scales. The question is how can it be advantageous enough to become fixed in a population over the span of a couple generations. And yes, sex did evolve gradually. Slime molds have optional sex (and more than 2 sexes!). But still keeping the machinery around is expensive. One would expect that individuals that ditched it would outcompete those that kept lugging it around unless there was a relatively short-term benefit to sex.
- PepeGomez 11y ago
  Exactly, in a stable population, one offspring of each individual survives on average in an asexual species. Two with half of your genes each survive in a sexual species. It doesn't need a long timescale, asexual species get out-evolved by sexual species and go extinct eventually. Sexual species have a much higher probability of entering new niches or surviving when the conditions change. This can be seen in many species that reproduce asexually when the conditions are good, but reproduce sexually when stressed. A species that loses the ability to reproduce sexually is doomed to extinction. Asexual reproduction is essentially putting all your eggs into one basket. When you die of a disease for example, chances are that most of your offspring dies as well AND there is a much higher chance that the disease adapts to your genome in the first place. This can be seen in agriculture, where plants are reproduced asexually to a much bigger extent than what would happen naturally. Many are impossible to grow without heavy doses of pesticides, as the diseases quickly adapted to the commonly planted strains. The strains need to be replaced every few decades, before growing them becomes essentially impossible, even with pesticides. Evolution is much faster than you assume.

Lawtonfogle 11y ago

You can't look at a single individual. Even as soon as you include two individuals with advantageous mutations, the chance of them having an off spring with both is extremely higher than for an asexual organism. Scale this up to 1000 individuals, each with mutations that are advantageous or disadvantageous, which impacts their chance to have children, and run it across a few generations (with a small chance of children getting new mutations not in their parents), and the chance of a subset of the population getting many of the positive mutations is much much higher than if they were an asexual species. The asexual species will end up with more member of the final population having the single most advantageous mutation, but with none having more than that.