Asexual to sexual reproduction? How?

Your argument reminds me of a whole load of other anti-evolutionist arguments. Form a story about how an evolution might have happened, show that story is impossible, claim that evolution is flawed. The trouble is that showing one way of doing something is impossible doesn't mean that doing it at all is impossible. It's like saying I can't get to America because I'd have to walk and there's an ocean in the way, when in reality I just get in a plane.The fact is we don't know how sexual reproduction evolved, and probably will never be certain - the first fossils we have of multi-celled organisms already show signs of sexual reproduction. It's likely that it evolved among single celled organisms sometime before 600 million years ago - things like that just don't leave fossils. All we can do is construct plausible and likely scenarios as to how it happened.I'm guessing at this point you're thinking something like "so you're just making it up! I knew it!". Well, not really. We have very good evidence from many sources that evolution has happened among more recent life, so it is reasonable to suppose it happened among life all those years ago. We know that the earth didn't exist at all five billion years ago. Thus sometime between five billion and six hundred million years ago sexual reproducing life appeared. It could be that it started that way and we should be asking how asexual reproduction evolved but since sexual reproduction is a much more complex process than asexual reproduction we can reasonably assume that asexual occured first.So then, we've got as far as thinking that sexually reproducing life evolved from asexually reproducing life now we need to figure out how that happened. Ideally we'd like to do this by direct observation - looking at fossils - but we can't 'cos there's no such fossils. So we fall back to the second line of evolutionary reasoning. It's general form goes like this: in order to demonstrate that something could have evolved we need to show that a) there is a sequence of steps from the start state to the end state, b) those steps are functional, c) each step provides an evolutionary advantage (i.e. organisms possessing that trait will outcompete their fellows).In reality steps a and b are done in conjunction, because we look at existing species to establish what would or wouldn't work. If we can find an existing species that uses the technique we want to use as a step then we have proof positive that it is a functional trait (that's step b), so we aim to line up existing methods to create a potential "pathway" evolution could have taken.Let's return to reproduction. As it turns out asexuality and mammalian-style sexual reproduction (two seperate sexes with different roles) are not the only players. There's slime molds with hundreds of sexes, there's haemophrodites (sexual reproduction but without two sexes), there's bacteria that horizontally interchange genetic material (one injects DNA into the other), there's animals that change sex during their lifetime or according to environmental conditions (there's a kind of fish that starts off as male. If they find a female while being male they'll attach themselves on to her and live like a little sperm producing parasite for the rest of their lives but if they don't they'll carry on growing bigger until, eventually, they switch sexes and become female).So our next task is to think about how we could go from an asexual organism to a sexual one by moving using as small a step as possible. Crash presents such a scenario above, let's go through it.asexual organisms Our starting pointasexual organisms with a mechanism for gene transfer Almost every kind of unicellular life actually uses this. Unicellular life is actually quite weakly seperated, and it's relatively easy for one organism to push a bit of DNA into another. This benefits the pusher, because it gets to spread it's DNA (and remember, selection usually operates at the genetic level - Dawkin's whole Selfish Gene metaphor) and the receiver benefits because it can get rapid change in response to adverse conditions.asexual organisms that require gene transfer to reproduce (sexual now, but haermaphroditic) This is found among a wide variaty of bacteria. Fact is, we're not wholy clear on what the advantage of this approach is. The current fore-runners are: ability to deal with changing environments and the ability to deal with parasites. These have been emprically verified by studies of molluscs. Molluscs are interesting because closely related snails use both asexual and haermaphroditic reproduction. The study found that species of molluscs living in more changeable environments or ones with more paracites are more likely to use sexual reproduction than ones living in stabler or safer environments.sexual haermaphrodites, now evolved to multicellularity, where the offspring gestates internally The evolution of multicellularity is another topic altogether, so I'll just assume it for now - you can start another topic on it if you wish. A multi-cellular animal relies on those specialised cells to function, feed and survive properly. The advantage of producing offspring more advanced than a single cell in terms of survival rates should be fairly obvious. There is, however, a greater cost to the parent in terms of feeding, maintaining and allowing the growing offspring to live internally. The next two stages suggested by Crash are refinements on this same advancement.sexual, sexed organisms where one organism is fated, congenially, to bear offspring and the other to provide necessary genetic material. This is the big kick then, why would an organism make this kind of distinction? The answer, I suspect, comes down to game theory. In a population of haermophrodites, a rogue operator with a strategy to impregnate but not get impregnated has an advantage - the cost of impregnating is small while the cost of baring young is higher. And the difference in cost rises with the size and complexity of the organism. Even among haermophrodites, some species of snails fight more than make love, each trying to impregnate the other but not be impregnated themselves. So, our rogue haermaphrodite is going around impregnating, but never getting impregnated (probably it actively rejects impregnation) since it never gets impregnated itself it never has to bare the cost of pregnancy while it still gets to have plenty of young. Over time, this strategy comes to dominate around 50% of the population (I'll leave out the maths, but it can be demonstrated that there is no advantage to this strategy when less than 50% of the population can bare young), now the "males" since they have no need to manage pregnancy will outcompete any of the remaining haemophrodites when it comes to impregnating another thus any haemoprhodite who simply drops the biological cost of their male side (can no longer impregnate another) will gain an advantage in terms of lower build/maintainance costs and the cost of searching for a mate but lose little in terms of reproductive effectiveness. These are now our females.And now we have a chain of possible strategies (each represented in the current world by at least one extant organism) arranged stepwise with each step lending an evolutionary advantage. That's a, b & c accounted for.But there's an important point to be noted here: this isn't how it happened, or a claim that is how it happened it's a theoretical demonstration of how it could have happened.

Think? There's no thinking going on here, it's simple cause and effect. Partial DNA transfer between Bacteria in close proximity doesn't require complex machinery it just requires the DNA to be pushed through the (permeable) cell wall. It's even simpler in virii. Yes. I already told you that. Bacteria horizontally transfer DNA among themselves. I explained above why this is beneficial, and thus selected for. Again, I explained this above. The bacteria pushing DNA into another bacterium gets it's DNA reproduced, in effect, for free. The bacteria receiving the DNA gets a higher variability and thus ability to survive under adverse conditions. Understand that this isn't any deliberate kind of an act it's simply that populations in which this happens have a bigger chance of survival so, over time, those who practice gene transfer will tend to dominate. In other words, natural selection occurs. No, it doesn't. But neither I nor Crash claimed it did - which is where the intermediary stages come in. The bacteria that practice horizontal gene transfer - a strategy we know is viable because many, many bacteria practice. The single-celled organisms that have structured gene transfer (two cells meet, exchange DNA in an orderly manner and then reproduce) i.e. haermophrodies - again, we know this is a viable strategy because it is practiced by many, many single celled organisms today. You're acting as if asexual and sexual were the only options. They're not, and there's a clear, unbroken line of intermediaries between them. You go from a system in which one organism tranfers a few genes into another, to one in which these exchanges are two-way, to one in which the exchange is two way and a bit structured, to a full-fledged half-and-half exchange. I wondering whether you even read what I wrote in my first post, because you don't seem to have taken in a single thing I wrote. Where?

I was planning to ignore this on grounds of snarkiness, but thinking about I think there is an important point to make here.The scenario I presented is not "how sexual reproduction evolved" but "how sexual reproduction could evolved". I've already explained why we can't provide the first of those alternatives. Now, when we're talking about "could"s we're not to trying to prove that is how it happened (as I've said, we have no evidence to allow us to make that determination and are unlikely to get any) but instead to demonstrate that it is possible. At that level we have quite a lot of evidence: we have the extant examples of organisms successfully using the intermediary strategies; mathematical modelling and simulation (in particular, game theory) to investigate and demonstrate the advantages of certain strategies; we have laboratory experiments demonsrating the possibility of some of the suggested mutational steps, and other experiments supporting the results from our mathematical models; we have a huge basis of evidence for evolution in general in areas in which we can obtain direct evidence (e.g. horse evolution, whale evolution, genetic vs. stratiographic phylogenies, etc). That's quite a lot of evidence.

Not always. Earthworms, for example, will mate with themselves if they fail to locate a suitable mate. I believe this behaviour occurs in a number of other haermaphroditic species.

There's a species of tree frog which lives in pools of water high in the branches of the rain forest. Trouble is the frogs are carnivorous and the only food source in algae - so they create a load tadpoles which eat the algae for them, and then eat the tadpoles.