Why Sex?

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Why Sex?

Postby Friend0rags » Sun May 23, 2010 11:48 am UTC

Sorry for the title, but maybe one of you brilliants could enlighten me.

I have been reading about evolution and reproduction and was wondering......why sex? Why would my genes want to mix up someone else's instead of multiplying and copying themselves into offspring, something similar to mitosis. Isn't this how bacteria or some simpler form of life propagate their genes?

I understand that complex is too simple of a word to explain ourselves, and I am wondering if any one could shed some light on the subject about organisms reproducing without the aid of sex or artificial insemination.
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Re: Why Sex?

Postby dragon » Sun May 23, 2010 1:13 pm UTC

Allowing combination of genetic material between individuals means that you don't have to wait for the (very slow) accumulation of multiple benificial mutations in the one cell line. The genetic re-shuffle can produce a setup that works great together, just by putting existing options in a new combination. The ability to put together several superior qualities into one unit in a single generation allows much faster adaptation, provided that the mutations you need are just waiting to be re-combined in existing individuals.

Bacteria are not restricted to just duplicating themselves, plus random mutations. They are also capable of horizontal gene transfer.
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Re: Why Sex?

Postby firechicago » Sun May 23, 2010 1:13 pm UTC

I sure others can give a longer and more complete answer but the short version is that sex improves the chances of an individual's genes' surviving by giving their progeny an opportunity to acquire new adaptive traits. Similarly, it improves the chances of survival for a species by allowing individuals to combine adaptive traits.

A thought experiment:

Suppose a species of asexual organisms is facing changes in its environment. A small percentage of organisms have a trait which provides resistance to a new toxin, and another small percentage have a trait which provides greater resistance to heat. This is all well and good if the environment gets hotter or more toxic, but if both happen at the same time, the species dies out. If our species reproduced sexually, on the other hand, as the individuals with neither adaptive trait died out, an increasing percentage of the survivors' progeny would have both adaptive traits and would survive.

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Re: Why Sex?

Postby Soralin » Sun May 23, 2010 1:20 pm UTC

As far as I know, it's basically because it allows for the concentration of useful mutations and traits. Say for example you have a couple of bacterium (some of those do reproduce by sexual reproduction), #1 has a gene A which is makes a good protein to do x, and gene b, which makes a not very good protein for doing y. #2 on the other hand, has gene a, which makes a not very good protein for doing x, and B, which makes a good protein for doing y.

Now, if there's no sexual reproduction, then there's no horizontal transfer of genes, these two genes will be completely separated, any chance at one of them evolving a better gene is down to just chance mutations and evolution. On the other hand, if they can reproduce, they can end up making more aB's and Ab's, but they can also end up making AB's which could do much better than either of them, and produce far more offspring, and which reaches that result far faster than just mutation working on the initial genomes would. Of course, you also would end up with some ab's, but in a competitive environment, those would be more likely to die off or not produce offspring, and in the process, make those genes less prevalent in the general population, still leading to the reproducing ones having a big advantage over multiple generations.

You can also think of it like this, that in an asexual line, a given organism only has the mutations of it's direct ancestors, you go back 100 generations, and you have only 100 individuals in which mutations could have occurred in. In a sexually reproducing species on the other hand, 100 generations means a potential 2100 ancestors (obviously not that many in practice, as that number is > 1030, so you'd run out of unique individuals after not too many generations back). What that means, is while an asexual organism can only use beneficial mutations from it's single ancestral line, a sexually reproducing organism can eventually end up with beneficial mutations that occurred in any individual in the entire population. And a greater proportion of those would be beneficial ones. Because where the good ones get concentrated, that individual is more likely to survive and reproduce, and have those genes continue on, and where bad ones get concentrated, that individual would be less likely to reproduce, and would take the bad ones out of the population with it.

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Re: Why Sex?

Postby Pez Dispens3r » Sun May 23, 2010 1:25 pm UTC

Posting because I'm interested in the answers that may appear. For genes, it is not particularly advantageous, in the short term, if you have a fifty-or-so percent chance of not being passed on to the next generation. I mean, what if you're a shit gene, and know it? May as well hang on another fifty-thousand generations until your species goes extinct, than be chopped off within fifty.
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Re: Why Sex?

Postby Pastinator » Sun May 23, 2010 2:27 pm UTC

Actually by definition sex must be advantageous for individual genes, if we accept that sex is evolutionary advantageous at all.
We used to describe altruistic behaviour as 'beneficial for the species', implying that evolution occurred on a species level, this is wrong, and we realise that we can bring the evolution of altruistic behaviour down to the level of individual genes, as these are the fundamental units of selection.
Following the argument that the fundamental unit of evolution is the gene, sex must be advantageous for individual genes. Perhaps the chance of being completely cast out of the germ line in meiosis is overwhelmed by the chance of genetic disease in clonal reproduction.
(I am not sure quite how controversial the idea of a gene as the fundamental unit of selection is in scientific circles, so feel free to correct me for stating this as fact).

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Re: Why Sex?

Postby qetzal » Sun May 23, 2010 3:40 pm UTC

Short answer: we don't know. This is still a major unanswered question in evolutionary biology.

There are a variety of different hypotheses, not necessarily mutually exclusive. I couldn't find a good free full-text review to link, but here are two links to free full text papers on particular hypotheses:

Hadany & Beker

Each has an intro with a very brief review of the current thinking and references other papers that give more detail on them.

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Re: Why Sex?

Postby Korrente » Sun May 23, 2010 8:15 pm UTC

I would think it's also a good way to keep bad genes from spreading. A cancer cell is going to keep on multiplying and multiplying happily because it thinks it's supposed to. A cancer organism (whatever that is) isn't going to get much sex, and therefore the trait won't spread. A bit semantic, it would depend on if bad mutations are more common than good ones, which they probably are.

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Re: Why Sex?

Postby qetzal » Sun May 23, 2010 10:40 pm UTC

Korrente wrote:I would think it's also a good way to keep bad genes from spreading.

But from an evolutionary perspective, anything that promotes better spreading of genes is good by definition.

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Re: Why Sex?

Postby gmalivuk » Mon May 24, 2010 5:04 am UTC

Not for other genes that it prevents from spreading.
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Re: Why Sex?

Postby D.B. » Mon May 24, 2010 8:01 am UTC

Mackay talks about sexual vs asexual reproduction in one of his books (Information Theory, Inference, and Learning Algorithms, Chapter 19, available on his website for free download here). It's more an exercise in information theory than serious biology with what he confesses to be a crude model, but intresting none the less.

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Re: Why Sex?

Postby Cobramaster » Mon May 24, 2010 10:43 am UTC

To be very blunt and to the point on answering the original question, the random recombination of genes in sexual reproduction both speeds evolution and increases the chance for the survival of a species in a relatively small number of generations. Asexual reproduction on the other hand can be faster time wise to make a significant change in the genetics of a species through random mutation alone, but this is over the course of (typically) tens of thousands of generations all of which require a survivable environment to make the typically small change to survive. A good set of examples would be rodents and bacteria, the rodents can adapt to new conditions very quickly allowing for new breeds to be developed in just a few years and a handful of generations, for the bacteria you can still get the change in a couple to few years but it required the resources of thousands of generations for a genetically similar amount of change.
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Re: Why Sex?

Postby Coffee » Mon May 24, 2010 10:52 am UTC

Why not?
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Re: Why Sex?

Postby Agent_Irons » Tue May 25, 2010 6:20 am UTC

There are a couple interesting ones. The concept of 'drag' where advantageous mutations by the single genetically-best-off bacterium are the only ones that matter. In a sense, its inevitable that the best bacterium wins(well, its progeny) but in the meantime it competes with all these well-adapted bacteria for the exact same resources: these bacteria are its species-mates. In a sexual species any advantageous mutation by any member can contribute to the survival of the species.

There are others, like the theory that sexual species can adapt more quickly to changing environments(true, the mix of alleles can be altered almost instantly evolutionarily speaking) even supposing mutation is negligible relative to environment change.

It's almost certainly a combination of all of these.

For further reading look up the Red Queen Hypothesis and hybrid vigor.

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