Can't say how sex evolved, therefore Adam and Eve
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24-05-2015, 07:45 PM (This post was last modified: 24-05-2015 07:57 PM by Reltzik.)
Can't say how sex evolved, therefore Adam and Eve
A forum user sent me the following PM (I've edited it to remove identifying information), and with his/her permission I'm reposting it with my reply. People who are more competent in the subject than me, feel free to tear this to shreds. This might go better in the science or creationism forums, but it came in the context of debating with a Christian, so here's where I'm posting it.

Quote: From reading your posts, it seems like you have a pretty good grasp of evolution. I have one question that I am hoping you can answer for me--how did the male/female sex evolve. Why not just asexual etc. I was explaining evolution to my christian friend and they kind of stumped me with that one. So of course, their answer is Adam and Eve as God created them. *eye roll*

I don't really have a great grasp of evolution. My area of expertise is math, which means I have a good grasp of patterns, probabilities, fractals, logic, and several other concepts useful in evolutionary theory. However, much of my knowledge of evolution has been gleaned from high school science, the odd science podcast, and counter-apologetics. Oddly, in arguing against evolution, Christian apologists have provided our side with discussion points for covering evolution in more detail. Big Grin In any case, I am not an expert and my grasp on evolution is moderate at best. You might wish to peruse Wikipedia on the subject (I've only briefly skimmed this page).

The short answer, to my very limited understanding, is that sexual reproduction would have been a relatively easy trait to evolve, would have been almost necessary around the time that organisms went multicellular (and thus be linked to all the advantages that multicellular organisms have), and would have provided a huge boost to variation. More variation -- especially variation of a sort that tends to be benign rather than deleterious -- translates into species that evolve faster and adapt to new conditions more quickly, which is itself a survival trait.

It should also be noted that evidence from this era of evolution is slight, and we would not expect it to be otherwise. Organisms by this point were not large enough to leave behind fossils, and we are so many generations back that genetic evidence is arcane and obscure... and given how the advent of sex would have completely altered how that genetic evidence was created, it's questionable whether we could interpret it even if we could find it. Very little is actually proven on the subject, and most of it may be unprovable at this late date. But what's at question here is whether it is plausible and fits in with the pieces of the puzzle that we do have put together. To show that it is plausible, we simply have to produce models of what MIGHT reasonably have happened, and to show that those models would have produced evolutionary advantage. If we can show it is plausible, then it is not the Achilles Heel that your creationist friend would wish it to be.

Recall that evolution requires three ingredients: Reproduction, Variation, and Selection. To put it another way, organisms create copies of themselves which largely have the same traits but may differ in small ways, and are subject to survival pressures which tend to favor (on average, even if the advantage is very slight) some combinations of traits and cull others.

Variation in single-celled organisms occurs through multiple mechanisms, and studying that will inform both how plausible and advantageous sexual reproduction's evolution might be. The mechanism most people think of is single-point mutation. Some particle of radiation penetrates the cell, strikes the DNA, knocks a base pair out of place, and they get replaced with something else. Or maybe there's a copy error for that pair. This is equivalent to an entire book with a single character randomly changed into another. It's pretty rare and is prone to produce no changes (because much of the book doesn't get read, it doesn't matter if there's a typo in some parts) or debilitating changes (because you're replacing something proven with something random). Sometimes it just produces changes by altering WHICH part of the book gets read, reactivating old traits and putting the current ones on the shelf. And occasionally it will produce an advantageous trait through sheer dumb luck. These bad odds still provide a survival advantage, because if only one percent of organisms are affected like this, and of those only one percent of mutations are beneficial, then selection guarantees that those beneficial mutants will still thrive and soon outnumber all others. However, in the race to evolve, this is still a very poor strategy. It is entirely outside of the organism's control, and there's no real way to copy another organism's successes. All advantageous mutations of this type -- if it is the only type of mutation to be had -- will propagate only down to the organism's progeny. There's no way to spread to other branches of the family tree.

Another common mechanism for mutation is genetic crossover. Two strands of DNA get torn in half, and each strand gets glued on to the end of the other one. This is a bit like taking two books, cutting each and half, and swapping their endings. Similar mechanisms (not crossover, but similar in effect, I just forget their names) can essentially glue one book on at the end of the other, or cause a chapter to be repeated twice, or so on. While this mutation can also be quite deleterious, it isn't as bad. The bulk of "new" information -- and there's quite a bit of it -- is good DNA, already tested for fitness in its previous location. Its context, not its content, has been changed. In essence, we're getting far more variation for only a little more risk of harmful effects, which means faster evolution, quicker adaptation to new or changed environments, and a win in the war of fitness. Crossover most often occurs in chromosome pairs (which are currently linked to sexual reproduction, but may have evolved before it) but can also occur with RNA within the cell. It is also much more under the control of the organism (if an unthinking automaton organism can be said to control anything). Reliability of mitosis is itself a genetically determined trait. Those organisms that do nothing but foul their reproduction up with genetic crossovers will quickly die out, and those which are so perfect in their reproduction as to never mutate in this manner will fail to adapt to their environments and be crowded out by those species that do. Between the two extremes lies a sweet spot that species will evolve towards, and that is what I mean when I say that the organism can control genetic crossover. In essence, variation itself evolves, and the mode or combination of modes of variation that is fittest survives. Like point mutation, by itself genetic crossover will not propagate to other branches of the tree of descent. It should also be noted that crossover has a synergistic effect with point mutation. Existing chromosome DNA can cross over into RNA as a sort of backup copy, which might cross back generations later, restoring old traits that were momentarily disadvantageous and providing redundancy against damage caused by point mutation and a sort of storehouse of deactivated traits that might be useful again in the organism's evolutionary future. Point mutation can happen in RNA and be crossed over into different parts of chromosomes of the descendents of the mutated individual, some places advantageous and some disadvantageous, with the net effect of more variation for the same point mutation, with the advantage of redundancy. Genetic crossover blunts some of the problems of point mutation while heightening some of the advantages.

A third mechanism is the incorporation of foreign DNA into the organism. Sometimes this happens because the organism consumes another one but doesn't entirely digest it, and this appears to be how we got mitochondria. (Incidentally, mitochondria provide an even better reserve for genetic crossover than does RNA, to the point where our matrilineal descent can be traced back through our mitochondrial DNA tens or hundreds of thousands of years to a "genetic Eve", and crossover mutation can still be patching DNA from her back into our modern chromosomes even to this day.) Stray RNA can also be absorbed into a single-celled organism. Viruses also love to replace bits of our DNA with their own -- it's how they reproduce -- and often bits of our DNA get fused into theirs in the process... and then they go on and repeat this with another victim. In this manner viruses can spread DNA originating in one member of a species to another member, or even between different species. Often, damaged or starving bacteria will combine with others of the similar type in a sort of reversed-mitosis, causing them to share their DNA. Again, most of this DNA is tested DNA, its advantageous elements already proven through natural selection (albeit in a different context), and is thus less likely to be deleterious than point mutation. And again, this mechanism of variation has a synergistic effect with the others. These mechanisms of incorporating foreign DNA allow mutations to skip from one branch of the tree of descent to another, which increases variation, speeds evolution, and disadvantages those organisms which fail to employ this mechanism. These unfit organisms die out, and we're left with the organisms that do.

All of these mechanisms can and do happen in single-celled, asexual organisms as well as multicelled sexual organisms, and I'm reviewing them primarily to establish their plausibility (they happen often, after all) and also what sort of variation natural selection seems to favor.

Enter the first multicellular organisms. We don't really know what they were, but for purposes of a conversation about plausibility we can postulate that they were originally colonies of single-celled organisms that grouped together for mutual advantage. Perhaps they were all like descendents of a single ancestor -- siblings and cousins, so to speak, with a mutation causing them to stick together -- or perhaps they were relatively unrelated symbiotes. In the first case, successive generations of cells would have mutated and enjoyed distinct survival advantages dependent on WHERE they were in the colony. Cells near the exterior would have needed to be most focused on defense from other organisms, perhaps secreting mucus or minerals to wall themselves off from the world. If the colony were shaped so that water and nutrients could be flooded into the interior, the organisms in that part of the colony could have ignored defense in favor of focusing on food collection and digestion, to the point where they may have had such plenty that they could afford to be wasteful in their digestion, leaving some of their food only partially digested because they'd already had their fill. That food could then drift through the colony to the defensive cells, who would therefore not need to focus as much energy on digestion and could focus more on defense. In a model of symbiotic combination rather than common descent of the colony, organisms more suited for one role than the other could have moved within the colony to their location of maximum advantage. In both cases, the mechanisms for incorporating foreign DNA would have been at work, causing DNA from one type of cell to be shared with the other types. Again, we don't know how this played out, but these scenarios are pretty plausible, and this is an argument about what's plausible.

Enter into this a new type of cell: The egg. The role of an egg cell would be to reproduce the colony in full, ideally far afield from the existing colony. To do this it would need to have DNA for reproducing all the other types of cell in the colony. This could initially happen because of how foreign-body DNA from the rest of the colony was incorporated into it, which makes the siblings-and-cousins model a bit more plausible to my mind than the symbiotic-coming-together model. At this point, all the fancy new specialized variation organisms within this colony are in a tenuous position, because their lives or at least their comfort (perhaps... likely... they could have survived being broken off, but would not have done particularly well on their own) is dependent upon the colony. If the rest of the colony dies, they suffer, and they lose the contest for survival. However, colonies could have reproduced by splitting apart and then "repairing" themselves into full colonies, a dangerous process but one that would likely have been forced upon them through injury. Eggs would allow their genetic information to be seeded afar from the colony in a new colony, WITHOUT requiring injury to the original colony, permitting their genes to survive even when they do not, and causing the multicellular colony model to propagate, prosper, and come to dominate. Once an egg cell evolves, an egg cell that can produce all the other cells needed for the colony, it becomes the center-point of reproduction. It doesn't matter so much what mutations happen elsewhere in the colony because the egg carries the blueprints for new colonies (though, still, the mechanisms for sharing DNA between cells would be in play), and variation from the egg's blueprint would be much more limited. At this point, we plausibly have the first multicellular organism -- not just a colony of symbiotes, but a single entity reproducing as a whole individual.

None of this is implausible. All of it relies on demonstrable mechanisms for genetic variation and all of it would confer survival advantage at each step. It's still asexual -- obviously multicellularity would have to come before we can have multiple types of dedicated sex cells -- but we're almost there. Sperm cells are little more than mobile viruses, and could have evolved in that manner to introduce genetic information into egg cells. After all, colonies that could send out free-ranging viruses to infect egg cells with their own DNA, "hijacking" it, would survive in the genetic sense much better than those that didn't. Alternatively, egg cells could have had the ability to combine like bacteria do, and may even have acquired their wealth of genetic information about the entire colony in exactly that manner. I'll talk about the advantages of this in a moment; for now I just want to note the plausibility of this ability developing. From that point, there could have been a divergence of two types of specialized cells: rare, expensive egg cells that contained the large stores of nutrients to get an entire colony started, and cheap sperm cells that were little more than short-lived, mobile DNA packets hunting for eggs to infect. At that point you have a sort of ur-sexual reproduction... a creature that can reproduce equally well asexually (through an un-fertilized egg) or sexually (with a fertilized egg). We have the two traits in competition in the same environment for the same resources and natural selection comes into play. From here it is not implausible to imagine that eggs could lose the ability to spawn new individuals without fertilization, if sexual reproduction provided a great evolutionary advantage over asexual reproduction. If it provides a huge advantage, it could even make sense for the egg to evolve the ability to remain dormant until fertilized, rather than just spawn a new organism with its existing DNA as soon as possible. If one form of behavior by organisms favored seeding sperm while others favored producing eggs, sexual dimorphism (dedicated sexes of different physical characteristics within the same species) would also become an advantageous trait.

So now that we've established that it was plausible for sexual reproduction and sexual dimorphism to come about, the only question that remains is whether this would have provided an evolutionary advantage. It definitely would have, and done so along the same principles as those governing variation in single-celled organisms. By having two sets of chromosomes, rather than just one, variation skyrockets. With increased variation comes increased speed of evolution, faster adaptation to one's environment, and greater survival advantage. Furthermore, DNA inherited from both parents is not damaged the way that mutation from crossover, point mutation, and virus alteration is, nor is it alien to the organism (and thus potentially discordant) the way that absorbed RNA or half-digested prey is. The risks involved in variation drop... again, providing an evolutionary advantage. This is also the best and fastest type of variation, the sort that can share useful traits with other branches of the same tree (by having the branches combine). The branches can crib each others' notes, so to speak. Having a second set of chromosomes also greatly increases redundancy. If one chromosome is damaged, the other can often be used in its stead; this is something we see a lot even in humans, and provides a huge survival advantage. Useful traits can spread to and even take over other branches of the tree, greatly speeding the process of survival of the fittest genes and salvaging good genes that are linked through misfortune to bad genes (by allowing for offspring which have the good genes but not the bad). Finally, sexual reproduction's advantages in variation is synergistic with all the other methods of variation. Genetic crossover and foreign-body DNA both produce more variation with less risk of deleterious effect in sexually-reproducing creatures, and point mutation causes fewer casualties (due to increased redundancy) and can propagate further in its advantageous cases. In short, sexual reproduction dumps nitro-oxide into the engine of evolution, making it run faster and leaving most of its competition in the dust. Anyone not engaged in it adapts far more slowly, losing the race to control resources and evolutionary niches, and thus dies out.
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25-05-2015, 02:21 PM
RE: Can't say how sex evolved, therefore Adam and Eve
(24-05-2015 07:45 PM)Reltzik Wrote:  [SNIP] to save bandwidth and therefore not actually a quote...

Excellent post. Like you my background is stats/probability/maths. But you expressed yourself much better than I could have.
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25-05-2015, 02:58 PM
RE: Can't say how sex evolved, therefore Adam and Eve
(24-05-2015 07:45 PM)Reltzik Wrote:  
Quote: From reading your posts, it seems like you have a pretty good grasp of evolution. I have one question that I am hoping you can answer for me--how did the male/female sex evolve. Why not just asexual etc. I was explaining evolution to my christian friend and they kind of stumped me with that one. So of course, their answer is Adam and Eve as God created them. *eye roll*

Just thinking more about the original question...why do people think this is such a gotcha question? It's just a god of the gaps argument. In what way does this "prove" god? And why does it prove "your" god? Do they know that plants also sexually reproduce? And why are there two sexes (in the traditional approach)? Why not 3 or 4?
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25-05-2015, 08:56 PM
RE: Can't say how sex evolved, therefore Adam and Eve
(25-05-2015 02:58 PM)jockmcdock Wrote:  
(24-05-2015 07:45 PM)Reltzik Wrote:  

Just thinking more about the original question...why do people think this is such a gotcha question? It's just a god of the gaps argument. In what way does this "prove" god? And why does it prove "your" god? Do they know that plants also sexually reproduce? And why are there two sexes (in the traditional approach)? Why not 3 or 4?

Cuz goddunnit.

That's not the sort of question that's really going on in the heads of people who think this way.

I think they're engaged in a game of a sort of mental king-of-the-hill. Their beliefs are currently on top of the hill. How did they get on top? Doesn't matter! They're there! And they'll stay there until some punk pushes them off! Oh, wait, here comes a challenger trying to climb up? Knock that challenger down! Once the challenger's knocked down, your position is once again secure!

Knocking one's own beliefs down isn't how you win that game.

... nor is realizing that it's a pretty stupid game to play and you should be trying to find the truth instead.

"If I ignore the alternatives, the only option is God; I ignore them; therefore God." -- The Syllogism of Fail
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25-05-2015, 10:08 PM
RE: Can't say how sex evolved, therefore Adam and Eve
(25-05-2015 08:56 PM)Reltzik Wrote:  
(25-05-2015 02:58 PM)jockmcdock Wrote:  Just thinking more about the original question...why do people think this is such a gotcha question? It's just a god of the gaps argument. In what way does this "prove" god? And why does it prove "your" god? Do they know that plants also sexually reproduce? And why are there two sexes (in the traditional approach)? Why not 3 or 4?

Cuz goddunnit.

That's not the sort of question that's really going on in the heads of people who think this way.

I think they're engaged in a game of a sort of mental king-of-the-hill. Their beliefs are currently on top of the hill. How did they get on top? Doesn't matter! They're there! And they'll stay there until some punk pushes them off! Oh, wait, here comes a challenger trying to climb up? Knock that challenger down! Once the challenger's knocked down, your position is once again secure!

Knocking one's own beliefs down isn't how you win that game.

... nor is realizing that it's a pretty stupid game to play and you should be trying to find the truth instead.

The problem is young earthers believe that they have the truth given to them by God in the Bible. When the Bible explains how males and females came to be, well, that's the way males and females came to be--no further explanation needed. In their minds trying to find another explanation that contradicts the Bible is heresy.

"Let the waters settle and you will see the moon and stars mirrored in your own being." -Rumi
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26-05-2015, 12:13 AM
RE: Can't say how sex evolved, therefore Adam and Eve
Their bullshit is easy to disprove cause its all silly or irrational so they need to keep the focus on the aspects of the other side where there aren't areas of 100% certainty.

They can't prove they are right so they gotta play games. And in some aspects they're just to stupid to grasp the knowledge provided by science.
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26-05-2015, 07:11 AM
RE: Can't say how sex evolved, therefore Adam and Eve
I like to think about yeasts whenever someone asks about sexes. Yeast have something like 35+ different sexes. Could you imagine that for humans? Sexism would be a lot different, that's for sure Consider

Being nice is something stupid people do to hedge their bets
-Rick
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26-05-2015, 07:17 AM
RE: Can't say how sex evolved, therefore Adam and Eve
(26-05-2015 07:11 AM)TheBeardedDude Wrote:  I like to think about yeasts whenever someone asks about sexes. Yeast have something like 35+ different sexes. Could you imagine that for humans? Sexism would be a lot different, that's for sure Consider

Then that would mean that too many yeast chose to be gay.

"If we are honest—and scientists have to be—we must admit that religion is a jumble of false assertions, with no basis in reality.
The very idea of God is a product of the human imagination."
- Paul Dirac
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26-05-2015, 07:18 AM
RE: Can't say how sex evolved, therefore Adam and Eve
(26-05-2015 07:17 AM)The Organic Chemist Wrote:  
(26-05-2015 07:11 AM)TheBeardedDude Wrote:  I like to think about yeasts whenever someone asks about sexes. Yeast have something like 35+ different sexes. Could you imagine that for humans? Sexism would be a lot different, that's for sure Consider

Then that would mean that too many yeast chose to be gay.

Which sex is the gay one? Consider

Being nice is something stupid people do to hedge their bets
-Rick
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26-05-2015, 12:32 PM
RE: Can't say how sex evolved, therefore Adam and Eve
(26-05-2015 07:18 AM)TheBeardedDude Wrote:  
(26-05-2015 07:17 AM)The Organic Chemist Wrote:  Then that would mean that too many yeast chose to be gay.

Which sex is the gay one? Consider

I don't know. Therefore god and the sinning yeast. Angel

"If we are honest—and scientists have to be—we must admit that religion is a jumble of false assertions, with no basis in reality.
The very idea of God is a product of the human imagination."
- Paul Dirac
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