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Evolution of Sex

Dustin Kesselberg

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Nov 30, 2004
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I was wondering if anyone could provide any sites that detail the evolution of sex.

From A-Sexuality to Sexuality and detailing the between stages.

Also I wanted to know,Are there any fossils of animals that are in a between stage of sexuality? Asexual/Sexual? I know worms are animals and are Asexual.


How did our evolutionary tree evolve sexualy? Asexuality-Sexuality? Which were the first sexual organisms?
 
We do not know for sure, since sexuality evolved too early to be seen in the fossil record, but we can infer quite something from extant species, since they cover the full range from asexuality to sepearate sexes.

Let's look at the animal kingdom (a number of parallels exist in the plant kingdom):

Simple single-celled creatures reproduce by dividing. They occasionally swap genes by a process called zyzygy, where two individuals couple up and exchange genetic material. You could define this as asexual reproduction, although it might also be seen as a form of hermafrodism.

Polyps generally reproduce by budding. They exchange genetic material by several processes, sepending on species, from a zyzygy-like coupling, to a full-fledged sexual connection. They are either hermafrodites or separate sexes.

A number of non-vertebrates (snails, worms), are full-fledged hermafrodites. They mate with another individual, and both produce offspring. Some species are have a seasonal sexual specification, that is, they are males at some times, females at others, some alternating, others going through a life-cycle where they are first one gender, then the other. Insects and aracnids mostly have specific genders.

Finally, moving to the vertebrates, these have nearly invariably separate genders, but there are exceptions. Certain species of fish can change gender at least once in their lifetime, being fully functional as both genders.

If we try to infer the evolution of sexual reproduction from this, it seems to be like this:

Single-celled organism, asexual reproduction. Genetic material exchanged by zyzygy or similar process.

Multi-celled, but primitive organism, specific sexual glands develope, full hermafrodites.

More complex organisms, seasonal or life-cycle dependent sexual specialisation.

Fully separated gender specialisation.

Hans
 
I don't have much to add to Hans' description but I thought I'd add a little something that I find absolutely fascinating.

It appears that some Aphids actually produce genetic clones of themselves. It's only ever females and they can produce varying body types, despite being genetically identical. Why they do this I'm not sure but it would appear that this happens when there is no male around to reproduce with. As an evolutionary trait this would seem to make sense as you get another chance to pass on your genes if partners are in short supply, you just pop out a few clones to take over after you expire.

Yes I know it's not strictly speaking on topic but, I thought you might find it interesting.

Kaydens.
 
I don't know if I can answer how, but I've always wondered why. One of the most confusing aspects of evolution is why anything would have evolved from asexual to sexual. It's not nearly as effecient at face value: asexual organisms can replace themselves, with their genome (generally) intact without too much effort. Sexual organisms run around looking for suitable mates, they risk mating with an infertile individual, and only pass on half their genome per offpsring. They risk breaking up successful genetic combinations.

The Red Queen hypothosis states that sexual organisms are better at adapting to areas that have more parasites. The genetic diversity offered by sexual reproduction allows individuals a sporting chance in a parasitic world. Asexuality is more often found in cold, dry climates; sexualilty is found in hot, wet places like the tropics...in other words places that are teeming with life (read: parasites, predators etc).
Concerning organisms that can reproduce sexaully and asexually: the seasonality is rooted in enviornmental conditions. If the enviornment is harsh, one mode of reproduction is used, if it is favorable, they switch.
I'd give you some examples, but Hans and Kaydens already listed all the awesome examples I know about. ;P
 
Kaydens said:
I don't have much to add to Hans' description but I thought I'd add a little something that I find absolutely fascinating.

It appears that some Aphids actually produce genetic clones of themselves. It's only ever females and they can produce varying body types, despite being genetically identical. Why they do this I'm not sure but it would appear that this happens when there is no male around to reproduce with. As an evolutionary trait this would seem to make sense as you get another chance to pass on your genes if partners are in short supply, you just pop out a few clones to take over after you expire.

Yes I know it's not strictly speaking on topic but, I thought you might find it interesting.

Kaydens.
Click to expand...
I think it is on topic, because while I don't know Dustin's reason for asking the question, it is often encountered as one of creationst's attempts to discredit evolution: "How did a male and female of anything happen to come into existence at the same time?"

So the fact sexual specialisation is far from the strict boundary we usually think of is important in such a discussion. It is very likely that the first sexual separation were creatures that could manifest as both genders.

Yes, it is very fascinating that aphids are capable of cloning, or parthenogenesis when needed. And they are capable of producing winged versions when food gets scarce in one location.

One single female can then fly out and colonize a new location. Actually, this is one reason you often see them in clumps on plants.

Hans
 
Dagny said:
I don't know if I can answer how, but I've always wondered why. One of the most confusing aspects of evolution is why anything would have evolved from asexual to sexual.
Click to expand...

You're right in that asexual reproduction would seem to be more stable in terms of genetics propogation, but this really isn't the advantage you appear to think it is.

Remember that evolution isn't about the individual, but about the species as a whole.

Other than by mutation, if an offspring is drawing from the genome of two parents, you can expect some genes to combine to produce some degree of natural advantage. Some may combine to produce some degree of natural disadvantage. Some combinations may be relatively neutral.

Advantaged offspring are statistically more likely to survive to reproduce, passing on the better genes. Disadvantaged offspring are statistically more likely to die before reproducing, thus tending to eliminate bad genes in the long run.

It seems to me that sexual reproduction produces smoother and more frequent opportunities for natural selection, generation to generation, than does asexual reproduction.

[Edit: slight rephrase for clarity.]
 
Sexual reproduction is actually a rather evolved from of gene swapping, although they shouldn't be seen as exactly the same things. Most organisms that reproduce asexually have the means to pick up and exchange novel genes, and this might have begun less as a way of rearranging genotypes and more as a way of competing for phosphate.

Early in earth's geological history, oxygen was rare. Nucleic acid requires phosphate as part of its structure, and the chemical was in short supply due to limited free oxygen to combine with phosphorus. Nucleic acid from other broken down organisms was a resource; those who had the means of obtaining it easily were fitter. This seems to be the original driving force behind collecting novel genes.

Athon
 
Moose,

That's a fairly well argued point but I have to pick you up on this:

Moose said:

Remember that evolution isn't about the individual, but about the species as a whole.
Click to expand...

That would very much depend on who you asked. AFAIK a lot of biologists would decry this term as it implies species selectionism which is not usually considered to be a tenable position. At the risk of sounding like a Dawkins poster boy I'll mention that he would tell you that evolution is about neither the individual nor the species but about the genes. I had planned to contrast this with other current theories but I've rapidly become aware that just about all my knowledge on evolutionary theory comes from Dawkins, does anyone know of any differing points of view? I'll need to fix my lack of alternatives but I've got Chaos Theory to cover first.

I've just spotted my copy of The Selfish Gene which I know has a chapter on sexual evolution in it. It'll be theory but I'll see if I can grap a few choice sections for my next post.

Kaydens.

Edited for ease of reading
 
Unfortunately it was as I thought. The Selfish Gene has an area about the development of sexual dimorphism in redproductive cells but not on the genesis of sexual reproduction.

For what it's worth though, I did find a reference to this book in the endnotes: http://www.amazon.co.uk/exec/obidos...8-3/ref=sr_8_xs_ap_i3_xgl/026-2437055-8801253 The title is The Redundant Male and the authors are John Gribbin and Jeremy Cherfas. From what I can see (which is very little, the book appears to have last been published in 1985) it doesn't just deal with this theory but doubtless it is a part of the book.

On another note, if you accept the selfish gene theory then you could form an argument that, since genes are competing with each other for survival, it would benefit them to try and recombine often. This is on the grounds that a gene's closest neighbour (and therefore closest rival) will be its allele, which is naturally different with most reproductive recombinations. This would have the effect of allowing differential rates of transmission down the generations for different genes (which would be the only way to adjudicate such competition in my opinion).

Kaydens.

Edited for a typo
 
Kaydens said:
At the risk of sounding like a Dawkins poster boy I'll mention that he would tell you that evolution is about neither the individual nor the species but about the genes.
Click to expand...

A subtle point, but a good one. Cheerfully ceded to the good Dr. Dawkins.
 
Glad to see that I'm not completely behind the times on this! :D

Kaydens.
 
MRC_Hans said:
...Simple single-celled creatures reproduce by dividing. They occasionally swap genes by a process called zyzygy, where two individuals couple up and exchange genetic material...
Click to expand...

I've always seen it spelled "syzygy". My dictionairies don't list your variant.
 
Just as a matter of interest, where would isogamy fit into your timeline Hans?

For those not aware, Isogamy is (as far as I understand it at least) a form of sexual reproduction where the gametes lack a sexual dimorphism and are thus able to fuse with any other gamete. In essence there is no male or female, just reproducers.

Kaydens.
 
quote:
--------------------------------------------------------------------------------
Originally posted by Moose
You're right in that asexual reproduction would seem to be more stable in terms of genetics propogation, but this really isn't the advantage you appear to think it is.

Remember that evolution isn't about the individual, but about the species as a whole.

Other than by mutation, if an offspring is drawing from the genome of two parents, you can expect some genes to combine to produce some degree of natural advantage. Some may combine to produce some degree of natural disadvantage. Some combinations may be relatively neutral.

Advantaged offspring are statistically more likely to survive to reproduce, passing on the better genes. Disadvantaged offspring are statistically more likely to die before reproducing, thus tending to eliminate bad genes in the long run.

It seems to me that sexual reproduction produces smoother and more frequent opportunities for natural selection, generation to generation, than does asexual reproduction.

[Edit: slight rephrase for clarity.]
--------------------------------------------------------------------------------



First off, I wasn't arguing that asexual reproduction is in ACTUALITY better than sexual reproduction. The fact that sexuality evolved in the first place, and is more prevelant than asexuality means that it's is a viable way of passing on genes. But just saying "sexuality exists, and there must be a good reason" isn't really an adequate answer.

And when did I make any illusions to group selection?

Here's a clarification:

Asexual organisms, if they have survived and reproduced are necessarily (barring the occasional mutation) passing on their successful genome to their offspring. So if the enviornment remains the same, the offspring will survive as well as their parent did.
Sexual organisms, although they are reproducing (and thefore successful darwinistically), will NOT pass on their genonmes intact. Recombination occurs. The offspring are not replicas of the parents, they are just similar. Now, I know you're with me here; there are three possible outcomes to this recombination: good, bad, and neutral.

Assuming that enviornemt is relitivley static:

If an asexual individual reproduces 10 times 10 offspring will survive (barring mutations, which are rare).

A sexual idividual has 10 offspring who do not necessarily have the same advantages as their parents. Maybe they do, maybe they have something even more advantages. But odds are, they aren't all going to be good.

Who wins? Who has passed on more copies of their selfish genes? Asexuals. The odds are reversed in a chaotic/changing enviornemnt full of parasites, when recobination is a good thing.

And I don't think the point of survival is to make natural selection smoother.
The point is to survive in order to pass on your (selfish) genes.
So why risk such a high rate of recombination? My whole point is that recombination is advantageous in environments that change.

It really isn't about "what's better, asexual or sexual?" it's about "When is (a)sexuality an advantage? When is it a disadvanatge?"

The Red Queen Hypothesis states that in hot/wet places (teeming with life) sexuality wins; and in cold dry (fairly static) environments asexuality wins.

(edited. Excitment leads to way too many typos)
 
Hans,

Do you have a link that describes zyzygy?

Thanks,

CBL
 
A sexual idividual has 10 offspring who do not necessarily have the same advantages as their parents. Maybe they do, maybe they have something even more advantages. But odds are, they aren't all going to be good.
Click to expand...
They will vary in reproductive goodness. The fitter ones will reproduce and the lesser ones will not. The children of the fitter ones will on average be fitter than the original parents. (Or from a Dawkin's POV, the fitter genes will reproduce and accumulate.)

Sexual organism will steadily improve. Asexual organism are static barring mutation. This implies that sexual reproduction will better regardless of the stability of the environment.

The only exception that I can see is when there is difficulty in getting a mate. In this case, I can see asexuality being better. I imagine cold, dry environments are sparsely populated which would explain asexuality's advantage in them.

CBL
 
CBL4 said:
They will vary in reproductive goodness. The fitter ones will reproduce and the lesser ones will not. The children of the fitter ones will on average be fitter than the original parents. (Or from a Dawkin's POV, the fitter genes will reproduce and accumulate.)

Sexual organism will steadily improve. Asexual organism are static barring mutation. This implies that sexual reproduction will better regardless of the stability of the environment.

The only exception that I can see is when there is difficulty in getting a mate. In this case, I can see asexuality being better. I imagine cold, dry environments are sparsely populated which would explain asexuality's advantage in them.

CBL
Click to expand...

I agree the positive mutations and recombinations will accumulate. And that finding a mate is an important factor.

Here’s the thing: sexuality is inherently more costly than asexuality. Regardless of environment. (This is a list from my intro behavior class.)
a) The cost of making males (the cost of meiosis). Here’s a link for it. You could Google it, and probably get something similar. http://sunflower.bio.indiana.edu/~clively/Research/cost of males.html
b) The cost of sexually transmitted diseases (the more times a female sierra-dome spider mates, the more likely she is to contract a venereal disease which kills eggs)
c) The cost of breaking up co-adapted gene complexes.
d) The cost of mate finding and mating.

The reason asexuals are found in cold/dry places is not because sexuals can’t find mates. It is because the direct cloning of a successful genome works in a static environment.

The experiments that lead to the Red Queen Hypothesis as an explanation for the evolution of sex were based on organisms that can reproduce sexually or asexually.
Examples: New Zealand Water Snails and Mexican Topminnows
 
CBL4 said:
Hans,

Do you have a link that describes zyzygy?

Thanks,

CBL
Click to expand...
Mmm, that was a taller order than I thought. First of all, I misspelled it, sorry. It is syzygy (a rather absurd word, really, I wonder what its origin is). Secondly, it has several other meanings as well. Finally, I can't seem to find something that goes out and explains it directly, but instead, loads of papers and articles that simply refer to it as something we are expected to know about, like this one :(.

Edited to add: This is a little more informative..

Two (sometimes more) mature trophozoites join in a non-sexual union known as syzygy and together form a spherical gametocyst enclosed in a single shared cyst envelope (Fig 6E). The two cells remain independent and do not unite. The former trophozoites are now called gamonts (or gametocytes) and they are of opposite mating types, for convenience referred to as male and female although they are not visibly different and do not have different reproductive roles.
Click to expand...

From here

Hans
 
Dagny,

I had forgotten the cost of sex and attracting mates. I had never heard of the cost of males. Interesting.

I would think that STDs would not become prevalent until sexual reproduction had gone on for a while. By this time, the sexual organisms would be much fitter.

Co-adapted genes are important but it depends on their prevalence. If they are universal, it would not matter. I would think that natural selection would cause them to re-accumulate even if they are only in part of the population. I guess it depends on the frequency, the extra fitness and the dominant/recessive nature of the gene.

It is because the direct cloning of a successful genome works in a static environment.
Click to expand...
If it is successful, it will work but without sexual reproduction it is more difficult to become successful. Mutation takes much longer than sexual reproduction. If the cost of sexual reproduction are low enough that it can get started, I would think it would spread. But with the costs you mention, sexual reproduction might be too expensive for a poorly adapted organism to survive.

Thanks for the info about the cost of reproduction. I have read a fair amount about genetics and it is great to learn more.

CBL
 
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