How did male and female evolve from an asexual organism?

It is not clear that they did. The two forms of reproduction could have developed separately.

Short answer is that there were steps and whole species in between. Most major milestones in evolution still have a representative or two around today. Think about plants that are neither male or female, but reproduce sexually with some help (bees or the wind). That would be an example of the go betweens from something that reproduces completely asexually to something that has a distinct male and female that are required to produce offspring.

Are you asking if a lab can create a female and male from an asexual organism or are you asking if an experiment could reproduce the forces of evolution?

The first, I’d say no, because there’s a huge number of intermediaries and dead ends that occur in the mean time. We’re talking millions of years, which unfortunately it’s difficult to get funding for. ~

If you’re asking if the forces of evolution can be mimicked in an experiment to produce similar results. Then yes, take a look at dog breeding or any modern food crop. Someone has selected traits they’d like to emphasize and bred for those traits.

Now replace “traits they’d like to emphasize” with “traits that increase the likelihood of survival” and you’ve got evolution in an extremely simplified nutshell.

Is it a theory? Yes. Anything we can’t see from start to finish is a theory and it would be impossible to see the very start of the evolution of life. So it’s a theory. A theory with an overwhelming tide of evidence that seems to be reinforced the more we’re able to observe. It’s a pretty safe bet at this point.

I’m not an expert on evolution, even if I do know a fair bit more than average regarding the topic. However, I am quite good at research, and found these links you will no doubt find helpful.

The Evolution of Mammalian Sex Chromosomes and the Origin of Sex Determining Genes

The Evolution Of Sex Chromosomes

This second one seems to be a list of about 20 PDF’s on the subject.

@squirbel, the gradual evolution of sexual evolution over millions and millions of years is not something that can be “tested” in a lab. Most science is not tested in the manner of your high school physics experiments. This is important to keep in mind if you’re interested in educating yourself further about science.

We don’t know exactly how it happened, but we have a good idea of the general steps it took. And it’s complicated! So it will take a while to explain. But: First of all, we need to step back and think about what “male and female” actually means. In animals, males create sperm that fertilizes females’ eggs.

But plants and fungi also have “male” and “female”—though it’s a little stranger (to us animals). Some individual plants have both male and female organs!

And to throw another monkey wrench into the whole concept of male and female—some plants, animals and fungi can reproduce asexually. For example, some jellyfish can “bud off” new members. This is how bacteria reproduce.

And speaking of bacteria! Most bacteria reproduce asexually (by simply dividing their cells in half). But many bacteria also reproduce by swapping genes. This isn’t the same as how plants and animals reproduce with male female parts, but it’s just another example of the diversity of ways that organisms reproduce.

But back to sexual reproduction. Because that’s what we’re really talking about when you say “male and female”— which is to say: instead of simply dividing in half (like a bacteria), the organism reproduces by

• splitting up its DNA
• putting one half in a “male” reproductive cell, which are usually small and quick
• putting another half in a “female” cell, which are usually larger and contain nutrients for growth
• recombining those two halfs to make a new organism.

Are you with me so far, @squirbel? If you have any questions, please ask them; if not let me know and I’ll go on.

I’m with you.

My question is how did this occur, this change from reproducing asexually to actually separating into two separate types of DNA delivery?

We’re getting there! Your question is one of the great mysteries in biology, you know.

Another way to think of your question, in terms of evolutionary biology, is as follows:

Why is it more advantageous for an organism to reproduce sexually, rather than asexually?

Right? Because obviously, if you are a bacteria, or a jellyfish, or whatever and you split in two, and each of those two split in two—you get a lot of offspring very fast. Whereas if you have to monkey around with sexual reproduction, you don’t get as many offspring.

One way to answer this is to simply point out that most of the time sexual reproduction isn’t advantageous. The vast, vast majority of organisms on this planet are bacteria, and they reproduce asexually. Many simple animals like jellyfish often prefer asexual reproduction and only occasionally reproduce sexually.

So, you are basically asking “what tipped the balance towards sexual reproduction for that tiny sliver of life that now reproduces that way, the eukaryotes?”

So let’s look at how sexual reproduction—or rather, something like sexual reproduction (because it would have evolved gradually) pays off. Let’s say you are a simple bacterium. You reproduce asexually and form a huge colony of clones, just like all the other bacteria.

Now let’s say a virus infects your colony. Since you’re all clones, with the same DNA, the virus can easily wipe out your colony.

On the other hand, the virus doesn’t affect other species of bacteria. Their code is too different. So if you want to survive, bacterium, your best bet is to steal some of that other bacterium’s virus-immune DNA!

With me so far?

No, I am not asking “What tipped the balance”, I am asking “why” and “how”, at a time when sexual reproduction did not exist – did it appear.

The questions are the same to an evolutionary biologist. The why and how, in biology, are answered by natural selection; i.e. why is this organism/behavior/structure/way of reproduction more “fit” than its predecessor?

But there was no predecessor… There was no natural selection in this occurrence.

Huh? Why would you think that?

I mean, natural selection always exists, everywhere. So I’m not sure what you even mean.

At a point where asexual organisms are the only ones that exist, natural selection does not apply to a nonexistent form vs an existing form.

Okay. But nobody is suggesting that sexual reproduction sprung into being fully formed. Like I said, we’re getting there. That’s why I started by focusing on bacteria and gene swapping—something, by the way, that we can observe bacteria doing now. Something like gene swapping could have been a stepping stone to more complex forms of sexual reproduction. Does that make sense?

It does.

Okay. So do you agree that, at least in certain bacteria populations, the ability to swap genes with other bacteria would come in handy? (which is to say, would be favored by natural selection)

Not only to survive viruses. When your population consists entirely of clones, sometimes “mistakes” build up in the cloned DNA (think making photocopies of photocopies). Gene swapping, in a roundabout way, helps to guard against this too.

If this makes sense, then now we have some bacteria who evolve the ability to swap genes with other bacteria. That’s certainly a major part of sexual reproduction. Not the whole story, of course.

Now let’s think about what happens over time, when you have a population of organisms that are good at gene-swapping, vs. the rest of the organisms who are just clones. Remember that only the eukaryotes—organisms that have more complex structures (including multicellular organisms like plants and animals)—are the ones that are having sex. You could rightly ask which came first, the sexual reproduction, or the complex eukaryotes. But if you think about the implications of gene-swapping, it starts to look less like a chicken-and-egg question, and more like a feedback cycle.

This is because when you have organisms that are swapping genes, it is much easier for those organisms to evolve new, complex structures than organisms that are just cloning themselves. This doesn’t mean that complex organisms are more favored by natural selection than simple ones (on the contrary—almost every organism alive is a bacterium. Complexity is rare!) But this just drives home the point that, when complexity does evolve, swapping genes makes it more likely.

Any questions?

No, and I prefer something other than the lesson format you’re carrying on. Write out your thoughts in their entirety?

I have one objection: Why would organisms decide that gene swapping is more favorable?

Organisms don’t decide anything. Gene swapping is simply more favorable for certain organisms, for the reasons I mentioned (protects against viruses and copying errors). Thus, the bacteria that gene-swap produce more offspring (which, in turn, can gene swap) than the ones that don’t.

If we’ve established that
• gene swapping can be advantageous for certain organisms, and
• gene swapping enables organisms to develop complex features much moreso than cloning

then we’ve already solved most of the puzzle of sexual selection. It’s a feedback cycle. Gene swapping enables certain organisms to survive better. Those organisms retain the ability to gene swap. They also are more likely to evolve complex features. Those complex features can then be used to make gene swapping more efficient.

As for the specific features of male and female reproduction—the way the “sex cells” swap their genes—those vary widely among eukaryotes. But a relatively common feature is the egg/seed/spore (large, nutrient-rich) combining with a smaller, more mobile sperm. Why would this feature exist, and why does it exist almost everywhere among eukaryotes?

The fact that it’s so common means it probably evolved early. But current living things provide a glimpse of how it might have transitioned. For example, some species of algae reproduce sexually, but their sex cells are very similar. This is called isogamy. In other algae, the sex cells are very different—there are large, non-mobile “female” cells and small, mobile “male” cells. This is called oogamy. How did oogamy arise from isogamy? Well, in certain algae, researchers have actually tracked down the genes responsible for the transition.

We don’t know if the first case of oogamy’s mechanism was the same as it was for these algae; we don’t know how many times oogamy has independently evolved. And I’m not sure how much we know about why oogamy persists, though I remember reading something about it a while ago.

But, there you have it. I hope that isn’t too lesson-y for your tastes—but I’m always happy to answer any more questions you have.

I know that organisms in your model do not “decide” anything.

But what exposed the organisms to gene-swapping?
How did gene swapping become an option?

Like I said, most organisms on Earth don’t reproduce this way. Almost every living thing is an asexually-reproducing bacteria. So you have to understand that we are talking about a rare sliver of life here.

I think I’ve covered how gene swapping would be advantageous in certain circumstances. But as for how the actual “machinery” of gene-swapping came about: it’s impossible to say for sure. We don’t know when it happened, with what kind of organism it happened with, or how many times it evolved.

What we do know is that bacteria today do it. And we can look at all the kinds of bacteria that gene swap and see a kind of transition, from bacteria who occasionally take in some bits of DNA from other organisms they touch (or eat) to bacteria who specialize in full-on gene-transfer.

If you want to know more details, we’d have to get into cellular biology and biochemistry, which I am frankly not very knowledgeable about. But DNA is just a molecule. All cells (bacteria are cells) have DNA floating around in their cytoplasm (and bacteria don’t even sequester their DNA inside a specialized nucleus compartment). When cells eat other cells they break down their food’s DNA into nucleotides. The idea that some of these nucleotides become attached to the host bacteria’s DNA shouldn’t seem too out-there. Viruses are also specialists at attaching themselves to other organisms’ DNA.