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Author Topic:   Asexual to sexual reproduction? How?
Eledhan
Inactive Member


Message 1 of 78 (245145)
09-20-2005 11:04 AM


I'm just wondering how it's possible for any organism to make the change from asexual reproduction into bisexual reproduction if evolution is true? Can any evolutionist explain this to me? Heck, I'll even take a Creationists theory on how it could have happened if evolution is true. But first I'll show all the problems that must be overcome:

1) there must be a genetic mutation in the gene pool. (rare)

2) this mutation must be beneficial. (very rare. As a matter of fact, we've never seen it happen, and if we have, then why don't evolutionists show us beneficial mutations instead of nonbeneficial ones?)

3) This mutation would have to happen to more than one organism and the organisms would have to be the same species (or at least EXTREMELY similar). (this would be more rare than both of the first two put together and multiplied by two)

4) Both the organisms would have to have the same mutation. (also extremely rare, as a matter of fact, it probably never happens)

5) They would have to have this mutation at the same time. (extremely rare, as a matter of fact, it probably never happens)

6) They would need to have this mutation in the same place. Remember, we're talking about asexual reproductive organisms. These critters are usually one-celled little beasties. (extremely rare, as a matter of fact, it probably never happens)

7) These two organisms of the same species, with the same mutation occuring at the same time, same place, and the same mutation (which, by the way, you have to have two of the same mutations, and yet allow for male and female) all need to occur, and then these previously asexual organisms need a reason to mate with their only partner on the face of the planet (sex drive). (obsolutely impossible)

8) Then, this jury-rigged system must work the first time, because these little organisms lifespans are so short that they only have enough energy to mate once in their lives (I made this up, but it makes a little sense). So, if it doesn't work the first time, then all of these things must happen all over again. (Also absolutely impossible. Evolutionists will say, "Well, if there was enough time, then theoretically anything is possible." Folks, there will never be enough time for this sort of thing to happen. It is IMPOSSIBLE.)

There, I've said it. Now I would like to hear if my example is flawed in any way.

Have fun!

Edited by Adminnemooseus, : Changed topic title from "Asexual to bisexual reproduction? How?" to "Asexual to sexual reproduction? How?".


Replies to this message:
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 Message 5 by RAZD, posted 09-20-2005 11:19 PM Eledhan has replied
 Message 6 by Dr Jack, posted 09-21-2005 7:35 AM Eledhan has replied
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AdminNosy
Administrator
Posts: 4754
From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 2 of 78 (245343)
09-20-2005 8:22 PM


Thread moved here from the Proposed New Topics forum.

  
Graculus
Inactive Member


Message 3 of 78 (245362)
09-20-2005 9:29 PM
Reply to: Message 1 by Eledhan
09-20-2005 11:04 AM


I can see a couple of problems with your basic assumptions.

1) there must be a genetic mutation in the gene pool. (rare)

Mutation rate is one mutation per 50 million bp/organism. Each human is carrying over 100 mutations. I'm not sure how that counts as "rare"

2) this mutation must be beneficial. (very rare. As a matter of fact, we've never seen it happen, and if we have, then why don't evolutionists show us beneficial mutations instead of nonbeneficial ones?)Beneficial mutations are well known, if you haven't heard of any then you haven't looked. Apo-A1 Milano made quite a bit of news. Nor does a mutation have to be immediately benefical, neutral mutations do quite well.

A recent study showed that only 36% of point mutations rendered a protein non-functional, so that means that over 60% of mutations are non-deleterious

Every other point beyond this is a strawman.

Reproduction is a continuum, not a boundary.

The earliest organisms capable of sexual reproduction would also be capable of asexual reproduction, such as is seen today amongst the algaes. Nor would "male" and "female" be a requirement... just two haploid cells.

The cells of most sexually reproducing animals contain two sets of genes, asexually reproducing creatures carry only one set. That means that the proto-sexual reproduction was a matter of two organisms combining, or a polyploidy. But these early fusions didn't have to be sexually reproducing, they could go along merrily asexually reproducing their diploid selves forever. With diploid genes in place, the only thing you need is meiosis, an error in cell division. If this is an environmental trigger (as it is in certain algaes) then many of these creatures would produce haploid cells at the same time, thus increasing the chances of finding another haploid. If they didn't? Then they are still capable of reproducing asexually.

Being as we have such organisms with us today, why would the earliest sexual reproduction have to be more complicated ans developed?


This message is a reply to:
 Message 1 by Eledhan, posted 09-20-2005 11:04 AM Eledhan has not replied

  
crashfrog
Member (Idle past 778 days)
Posts: 19762
From: Silver Spring, MD
Joined: 03-20-2003


Message 4 of 78 (245368)
09-20-2005 9:45 PM
Reply to: Message 1 by Eledhan
09-20-2005 11:04 AM


I'm just wondering how it's possible for any organism to make the change from asexual reproduction into bisexual reproduction if evolution is true?

It's just "sexual" reproduction, actually.

Here's one idea, based on penis-fencing snails - asexual organisms -> asexual organisms with a mechanism for gene transfer (the vast majority of today's unicellular life) -> asexual organisms that require gene transfer to reproduce (sexual now, but hermaphroditic) -> sexual hermaphrodites, now evolved to multicellularity, where the offspring gestates internally, incurring a biological cost of resources and vulnerability -> sexual hermaphrodites with a biological mechanism to determine which gets to gestate the offfspring -> sexual hermaphrodites where some individuals have phenotypical structures to aid internal gestation -> sexual, sexed organisms where one organism is fated, congenially, to bear offspring and the other to provide necessary genetic material.

It's sketchy, but based on organisms that exist to this day, and doesn't require going from genderless organisms to full-on sexual reproduction in one single mutation.

Now I would like to hear if my example is flawed in any way.

Well, the part where you make it up that an organism can only reproduce once is a little sketchy. Most organisms are capable of the production of many, many offspring over the course of their lives. An organism that could only reproduce one offspring once would not be able to build a population because it can only replace itself.


This message is a reply to:
 Message 1 by Eledhan, posted 09-20-2005 11:04 AM Eledhan has not replied

  
RAZD
Member (Idle past 716 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 5 of 78 (245384)
09-20-2005 11:19 PM
Reply to: Message 1 by Eledhan
09-20-2005 11:04 AM


Eledhan writes:

... rare ...

It does not matter how rare an event was when it has happened. A meteor hit the Yucatan, a highly rare event, but it happened. Adding {numbers\qualifiers} to make the rarity event seem even more highly improbable is just the argument from incredulity, and is nothing less than the inability of the {incredulous person} to be as ingenious as nature {is\was}.

We surmise from mitochondria that some early bacterial organisms injected themselves into, or we engulfed by, others.

We also observe viruses injecting segments of {DNA\RNA} into genomes as well as coopting cells to reproduce the virus.

We see snails, as Crashfrog mentions, that are {a\bi}sexual and can inject {sperm} into another making it pregnant or vice-versa: either can be "male" or "female" -- and it is highly likely that this is the form of {early sex} because it is not that different from a single organism making a {clone\child}.

But. I will agree that this is hypothesis with very little observational data to base it on from the deep past when this would have occurred.

On the other hand, if sex is a "designed feature" ... then it is surely a candidate for Silly Design ...

ps welcome to the fray.


we are limited in our ability to understand
by our ability to understand

RebelAAmerican.Zen[Deist
... to learn ... to think ... to live ... to laugh ...
to share.


This message is a reply to:
 Message 1 by Eledhan, posted 09-20-2005 11:04 AM Eledhan has replied

Replies to this message:
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Dr Jack
Member (Idle past 1416 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 6 of 78 (245412)
09-21-2005 7:35 AM
Reply to: Message 1 by Eledhan
09-20-2005 11:04 AM


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 point

asexual 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.


This message is a reply to:
 Message 1 by Eledhan, posted 09-20-2005 11:04 AM Eledhan has replied

Replies to this message:
 Message 7 by RAZD, posted 09-21-2005 7:54 AM Dr Jack has not replied
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RAZD
Member (Idle past 716 days)
Posts: 20714
From: the other end of the sidewalk
Joined: 03-14-2004


Message 7 of 78 (245421)
09-21-2005 7:54 AM
Reply to: Message 6 by Dr Jack
09-21-2005 7:35 AM


nice

I was also going to bring up mosses
http://www.hiddenforest.co.nz/bryophytes/mosses/reproduction.htm

that use both mechanism for reproduction, showing another group with dual capabilities.

I also thought about the transition from distributing spores to distributing pollen. Not that difficult a transition (and mollusks use similar in the water): one starts new {individual} the other impacts and existing individual and modifys the {spores} that it ejects.


we are limited in our ability to understand
by our ability to understand

RebelAAmerican.Zen[Deist
... to learn ... to think ... to live ... to laugh ...
to share.


This message is a reply to:
 Message 6 by Dr Jack, posted 09-21-2005 7:35 AM Dr Jack has not replied

  
Eledhan
Inactive Member


Message 8 of 78 (245696)
09-22-2005 9:37 AM
Reply to: Message 6 by Dr Jack
09-21-2005 7:35 AM


What?
Okay, I appreciate your whole page-long essay on something that even you admit has no proof and is simply an imaginative idea.

Here's my point: why, pray tell, would an organism somehow think that it could "overtake" the population by empregnating others of its own species? Do we see bacteria and such attempting to dominate other bacteria with its DNA? I don't know, that's why I'm asking. Also, how does this fit with Natural Selection? If sexual reproduction is "better", then why is the most effecient reproducing organism bacterium? Don't they reproduce asexually? Why then, would you claim that this hybrid asexual-to-sexual organism survive in a world full of organisms who only asexually reproduce? And just because an organism (one which doesn't have any code for sexual reproduction) infuses another (also one without genetic info for sexual reproduction) with part of its genetic code, that doesn't mean that you have added any new genetic material to create an entire sexual reproduction system. See, the only way for a sexual reproductive system to work is for it to be completely developed in order to compete with asexual reproductive organisms. Otherwise evolution's hero (Natural Selection) will destroy any organism that is "stupid" enough to waste energy on a sexually reproductive system. As I stated in my first post, this organism(s) must survive long enough to get it right, if not, then all of this must happen several times.

So, my question still remains: How can you claim that it is a gradual process, when the parts that are added (even though I highly doubt anything could be added) are going to make the organism less efficient than the asexual organisms? Don't you think it's kind of odd that an organism wasting energy on supporting a sexual reproductive system would be able to survive? And even if it could, how would it find a mate? If it simply asexually reproduces, and then mates with "itself", then you will have serious problems with the offspring, and they will not be able to sexually reproduce.

Therefore, I do not feel that you have answered the question properly. You still must show how Natural Selection suddenly stops selecting, and then, continues selecting organisms further down the road. In my opinion, you have violated two very important parts of evolution, and those are: Natural Selection and Uniformitarianism.


This message is a reply to:
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Eledhan
Inactive Member


Message 9 of 78 (245698)
09-22-2005 9:45 AM
Reply to: Message 5 by RAZD
09-20-2005 11:19 PM


Silly Design...
I hope you don't think that sex was a silly thing to design, because if I'm right, then God must really have known what He was doing when He created sex. I just think it's funny how wars have been faught over something as simple as our reproductive system (sex...duh). If sex is silly design, and I'm right, then I can't wait to see what He's got in store in Heaven.

P.S. I realize that this is not really related to the topic, but I figured I would just try to rebutt the comment RAZD made about Silly Design relating to this current topic.

P.P.S. Thanks for the welcome. I've been in here before, but it seems as if I'm one of the few Christians on this forum who actually try to defend their beliefs and show how they feel evolution is wrong, so I got tired of being the only one responding to all you well educated evolutionists. I couldn't keep up with all of the posts, and it was interfering with work.


This message is a reply to:
 Message 5 by RAZD, posted 09-20-2005 11:19 PM RAZD has replied

Replies to this message:
 Message 21 by RAZD, posted 09-22-2005 6:48 PM Eledhan has not replied

  
nwr
Member
Posts: 6035
From: Geneva, Illinois
Joined: 08-08-2005
Member Rating: 3.8


Message 10 of 78 (245700)
09-22-2005 10:03 AM
Reply to: Message 8 by Eledhan
09-22-2005 9:37 AM


Re: What?
why, pray tell, would an organism somehow think that it could "overtake" the population by empregnating others of its own species?

There isn't any evidence that bacteria, and other simple organisms, are able to think.

Do we see bacteria and such attempting to dominate other bacteria with its DNA?

The theory of evolution is not about "attempting to dominate".

If sexual reproduction is "better", then why is the most effecient reproducing organism bacterium? Don't they reproduce asexually?

Take a look at this description of bacterial conjugation.

How can you claim that it is a gradual process, when the parts that are added (even though I highly doubt anything could be added) are going to make the organism less efficient than the asexual organisms?

Less efficient at what?

The evidence shows that sexually reproducing species are quite effective at reproducing. Otherwise there wouldn't be any of them around.

Don't you think it's kind of odd that an organism wasting energy on supporting a sexual reproductive system would be able to survive?

No, it doesn't seem odd. It's clear that sexual reproduction provides a survival advantage in dealing with a changing environment.

And even if it could, how would it find a mate?

Did you miss the part about hermaphroditic species in the earlier posts?

If it simply asexually reproduces, and then mates with "itself", then you will have serious problems with the offspring, and they will not be able to sexually reproduce.

Do you have any evidence for that assertion?

You still must show how Natural Selection suddenly stops selecting, and then, continues selecting organisms further down the road.

And why would that ever be necessary?

This message is a reply to:
 Message 8 by Eledhan, posted 09-22-2005 9:37 AM Eledhan has not replied

  
Dr Jack
Member (Idle past 1416 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 11 of 78 (245701)
09-22-2005 10:04 AM
Reply to: Message 8 by Eledhan
09-22-2005 9:37 AM


Re: What?
why, pray tell, would an organism somehow think that it could "overtake" the population by empregnating others of its own species?

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.

Do we see bacteria and such attempting to dominate other bacteria with its DNA?

Yes. I already told you that. Bacteria horizontally transfer DNA among themselves. I explained above why this is beneficial, and thus selected for.

Also, how does this fit with Natural Selection? If sexual reproduction is "better", then why is the most effecient reproducing organism bacterium? Don't they reproduce asexually? Why then, would you claim that this hybrid asexual-to-sexual organism survive in a world full of organisms who only asexually reproduce?

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.

And just because an organism (one which doesn't have any code for sexual reproduction) infuses another (also one without genetic info for sexual reproduction) with part of its genetic code, that doesn't mean that you have added any new genetic material to create an entire sexual reproduction system.

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.

See, the only way for a sexual reproductive system to work is for it to be completely developed in order to compete with asexual reproductive organisms.

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.

Therefore, I do not feel that you have answered the question properly. You still must show how Natural Selection suddenly stops selecting, and then, continues selecting organisms further down the road. In my opinion, you have violated two very important parts of evolution, and those are: Natural Selection and Uniformitarianism.

Where?


This message is a reply to:
 Message 8 by Eledhan, posted 09-22-2005 9:37 AM Eledhan has not replied

  
Cal
Inactive Member


Message 12 of 78 (245722)
09-22-2005 11:16 AM
Reply to: Message 8 by Eledhan
09-22-2005 9:37 AM


I made this up
I appreciate your whole page-long essay on something that even you admit has no proof and is simply an imaginative idea.

This complaint seems unfair when brought against a response to an opening post containing the statement: "I made this up, but it makes a little sense".

There is room in science for 'imaginative ideas'. Another way to refer to these is: 'logical propositions'. Your opening post was itself essentially a series of such logical propositions, each presented as a rather loosely bounded (and somewhat familiar) thought experiment, in the form of a request to imagine a set of circumstances, followed by the pointing out of an apparent logical difficulty encountered as a result.

When Mr. Jack (quite generously, I thought) offers to play along, you reject his contribution as lacking in empirical support. Even more so than the complete failure of their arguments, it is disingenuous tactics like this that earned creationists such a poor reputation in the scientific community.


This message is a reply to:
 Message 8 by Eledhan, posted 09-22-2005 9:37 AM Eledhan has not replied

  
Dr Jack
Member (Idle past 1416 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 13 of 78 (245725)
09-22-2005 11:31 AM
Reply to: Message 8 by Eledhan
09-22-2005 9:37 AM


Re: What?
Okay, I appreciate your whole page-long essay on something that even you admit has no proof and is simply an imaginative idea.

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.


This message is a reply to:
 Message 8 by Eledhan, posted 09-22-2005 9:37 AM Eledhan has not replied

  
EZscience
Member (Idle past 4465 days)
Posts: 961
From: A wheatfield in Kansas
Joined: 04-14-2005


Message 14 of 78 (245744)
09-22-2005 1:48 PM


Evolution of sexuality
There have been a number of major books written on the evolution of sexuality that anyone truly interested in understanding the problem should read. You might begin with GC Williams "Sex and Evolution". Another, more advanced and formal treatment is Graham Bell's "The Masterpiece of Nature". Essentially, sexuality must provide a fitness advantage to individuals over asexuality to evolve and become fixed within a population. We can ignore the mechanics for a moment, some primitive forms of which have already been discussed below. It is much more important to understand the 'why' part of the question. Many hypothetical advantages have been proposed for evolution of sexuality and they are not mutually exclusive. We might touch on them one at a time, but there are far too many to deal with in a single post.

Let's review some facts first so there is stable ground for a discussion.

Asexuality is the primitive condition.

However, the vast majority of higher organisms are obligately sexual.

Sexual reproduction leads to more genetic variation in progeny than does asexual reproduction.

Sexual reproduction exacts a significant fitness cost for an individual compared to asexual reproduction, known as the 'cost of meiosis' (Maynard Smith). This arises because females only obtain 50% genetic representation in each offspring instead of the 100 % they would obtain if they produced asexually (all daughters the same as themselves). This means we must show how a sexual female can gain more fitness (reproductive success) than an asexual female.

Thus, the advantages of genetic diversity in one's progeny must somehow more than compensate for this cost in order for sexuallity to evolve.

I think there is great insight to be gleaned from organisms such as aphids (that I happen to study for a living, among others) that employ BOTH modes of reproduction. We can see that when environment is favorable (and food supply abundant and predictable) all reproduction is asexual. Males and sexual females are only produced at the end of the season when unpredictable conditions are faced (overwintering etc.) and a diverse range of progeny increases ones chances that some will survive. Williams calls this the 'lottery strategy'. Would you want to hold a thousand lottery tickets all with the same number, or a thousand with different numbers? However, this works only for organisms producing large numbers of offspring and investing very little in each.

There are many other scenarios where the cost of meiosis can be overcome. Perhaps I will have time to cover some others later.

Is anyone familiar with the 'Red Queen' theory? Named by Bell for the red queen in Alice in Wonderland who stated: "sometimes it takes all the running you can do to stay in the same place". It hinges on frequency dependent selection operating on populations inhabiting unpredictable or capricious environments.

Sex seems to pay off whenever uncertainty prevails for the next generation.

This message has been edited by EZscience, 09-22-2005 12:55 PM


Replies to this message:
 Message 19 by Cal, posted 09-22-2005 3:37 PM EZscience has replied

  
clpMINI
Member (Idle past 4476 days)
Posts: 116
From: Richmond, VA, USA
Joined: 03-22-2005


Message 15 of 78 (245748)
09-22-2005 2:18 PM
Reply to: Message 1 by Eledhan
09-20-2005 11:04 AM


Present day critters
Zebra mussels are hermaphrodites...only takes one to tango. I think the same is true for asian clams (Corbicula fluminea) That's why they are so invasive.

Lots of freshwater snails are hermaphrodites.

Aphids and water fleas (Daphnia magna) spend most of their lives as asexual female critters by producing clones of themselves, until they come under environmental stress, at which point they crank out a male instead of another clone.

Certain sea slugs are both male and female, and when they meet up, they have this neat little battle to decide who gets to be the male and who has to be the female. The male being the one who jabs his business into the other one first.

These are just a few examples and variations where asexual and sexual reproduction are seen within single creatures that are alive today, so extrapolation to how it all started can begin with what we can observe today.

clpMINI


This message is a reply to:
 Message 1 by Eledhan, posted 09-20-2005 11:04 AM Eledhan has not replied

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