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Author Topic: How did the two sexes evolve?  (Read 15636 times)

Caroline Sant

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How did the two sexes evolve?
« on: 20/05/2008 23:53:38 »
Caroline Sant  asked the Naked Scientists:

Hi Naked Scientists!
  
I am a keen listener, although through the podcasts as I live in Melbourne, Australia.
  
I love science - I think about it most of the time, I like to know why and how things happen - I studied biochemistry and microbiology at uni. I think your show is the best science show on the planet!

Thank you for making science approachable, and for answering many questions that I have without me having to ask any.

I love to listen to your show while I am at work (as a data analyst for an environmental company) or in the car on the way to and from work. I have downloaded all of your podcasts through itunes, and am working through them slowly.
  
Oh and I have a question about evolution, my most favourite science topic....

I can understand the concept of how life may have come about ie from single cells, then mulitcellular organisms etc, and how species evolve into other species - common ancestors etc, but where in evolution did male and female develop? That is something that puzzles me!
  
 Thank you :) :)
  
 Caroline


What do you think?
« Last Edit: 21/05/2008 00:10:52 by chris »


 

Offline bryan

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How did the two sexes evolve?
« Reply #1 on: 04/08/2008 14:28:37 »
There are lots of advantages to sexual reproduction but how it originated is, like the origin of life itself, unknown. Although, it probably first occurred in a single celled eukaryote. It may have orginated from a mechanism to exchange genetic material between cells, bacteria do something similar. Another theory is that one cell ate another but incorparated some of its DNA into its own insted of digesting it all.
 

Offline sarah cp

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How did the two sexes evolve?
« Reply #2 on: 04/08/2008 14:53:55 »
The story of why two sexes evolved really goes back to the advantages of having sexual reproduction. Asexual reproduction - by budding or fission of cells into identical daughter cells - is how organisms such as bacteria and yeast reproduce. However, the problem with asexual reproduction is that over time (we're talking hundreds or thousands of generations here), bad mutations will build up in the genetic code. This can cuase developmental problems. The way to overcome this is to mix your genes with someone else's to produce offspring. Even asexual organisms have some way of doing this - bacteria transfer genes between each other using pilli, and only one group of organisms is known to be 100% truly asexual - the bdelloid rotifers. These are tiny aquatic organisms that have not had sex for over 80 million years!

But back to the sexes...at first, the way for individuals to combine their genes to create offspring was to have 'mating types' - like A and B rather than male or female. Each produced gametes (like eggs or sperm), which then fused with the gamete of the other mating type. Originally, gamete size would have been equal, but through a process known as disruptive selection, larger and smaller were favoured over equal size ones. Males of species are thought of as the ones that produce many small gametes and the females produce fewer large gametes. This difference in investment in gametes is a major component of 'intersex conflict' and one possible explanation for the 'choosy females, ardent males' observation, with females investing more in their gametes, so making them more choosy over which male gets to fertilise them, leading males to have showy feathers, or mating displays, or weapons for fighting other males.

A particularly good explanation of this can be found in 'Behavioural Ecology: An Introduction' by Krebs and Davies
 

Offline Asyncritus

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How did the two sexes evolve?
« Reply #3 on: 11/08/2008 14:30:55 »
There are lots of advantages to sexual reproduction but how it originated is, like the origin of life itself, unknown. Although, it probably first occurred in a single celled eukaryote. It may have orginated from a mechanism to exchange genetic material between cells, bacteria do something similar. Another theory is that one cell ate another but incorparated some of its DNA into its own insted of digesting it all.

This question is one that bedevils evolution. Sure, we all know the advantages of sexual reproduction, but that explains nothing about HOW it came about.

I know the advantages of having a million bucks - but does it happen? Like hell it does!

So, what does evolution say about this titanically important question? Well, nothing really.

Some prokaryotes ate some eukaryotes or was it the other way round - n billion years ago, and lo and behold, we now have the incredible complexity of sexual reproduction.

The whole thing shouts Intelligent Design, Purposeful Design, Teleology to use the technical term.

Why should a sperm evolve if there wasn't an ovum to fertilise? And how is it that meiosis in the cells produces exactly half the normal number of chromosomes SO THAT when the 2 gametes combine the normal number will be produced once again? That's very purposeful in my opinion, and purpose demands Intelligence at the back of it - especially if the plan works as well as it does everywhere.

So evolutionist friends - tell me how it could have happened by chance.

PS Why don't we get Dawkins on here to answer a few questions?
 

Offline BenV

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How did the two sexes evolve?
« Reply #4 on: 11/08/2008 15:18:33 »
Asyncritus - this is not a question that bedevils evolution.  It's easily understood in the way that Sarah has stated above and taught in a-level biology classes here in the UK.  It's certainly taught on biological science degrees.  SO actually, evolution answers this completely, rather than the 'saying nothing' you imagine.

This question was not about whether the sexes evolved, but how.  Sarah has given a perfectly good answer.

Why does meiosis produce half the number of chromosomes?  Because if it produced the whole number it would be mitosis, and if it produced a different amount it wouldn't work.  All of the genetic pathways that led to a meiotic division producing more or less than half the chromosomes did not survive.

Once again, this is how evolution works, and you don't seem to understand.

Why should a sperm evolve?  In Sarah's post above she clearly explains that there was disruptive selection, as having a large gamete fuse with a small gamete was efficient, effective and offered selective advantage.  (We already know that having gametes offered selective advantage, unless you would like to suggest that there is no advantage of sexual reproduction over asexual reproduction?) Over time, mutations that made these gametes more specialised (such as developing motility) also offered selective advantage, and eggs and sperm evolved.

No intelligence required.

Please return to the 'Do intelligent design ideas make you wince?" thread and explain to us your understanding of evolution, so that we may find the source of our disagreements.

Edit - I suspect that Professor Dawkins has these conversations enough already, and would find it very frustrating to argue with yet more people who will not put aside their preconceived ideas of god and design for long enough to understand evolution.
« Last Edit: 11/08/2008 15:21:20 by BenV »
 

Offline Asyncritus

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How did the two sexes evolve?
« Reply #5 on: 11/08/2008 16:10:58 »
Sarah has indulged in the characteristic question begging technique that so characterises evolutionary arguments. Thus:

But back to the sexes...at first, the way for individuals to combine their genes to create offspring was to have 'mating types' [and where, and why did these arise?] - like A and B rather than male or female. [and HEY PRESTO...] Each produced gametes (like eggs or sperm), which then fused with the gamete of the other mating type [HEY PRESTO!]. Originally, gamete size would have been equal, but through a process known as disruptive selection, larger and smaller were favoured over equal size ones. [again HEY PRESTO!] Males of species are thought of as the ones that produce many small gametes and the females produce fewer large gametes.[This is pure nonsense. We KNOW that this is NOW the case - but when, and more important WHY did such a thing take place?] This difference in investment in gametes is a major component of 'intersex conflict' and one possible explanation for the 'choosy females, ardent males' observation, [but this ASSUMES THE EXISTENCE of males and females. Sexuality has already appeared - but this is what the original question was asking, and this is begging the question furiously, not addressing it] with females investing more in their gametes, so making them more choosy over which male gets to fertilise them, leading males to have showy feathers, or mating displays, or weapons for fighting other males.

If this is the quality of material presented at degree level, which is not severely questioned, then it's hardly surprising that evolution manages to stagger on, concocting more and more outrageous hypotheses to support its existence. As Professor W R Thompson described it so hurtfully as:

"..fragile towers of hypotheses piled upon hypotheses" and worse, presented as fact.

 
 

Offline BenV

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How did the two sexes evolve?
« Reply #6 on: 11/08/2008 16:42:32 »
You're not reading the replies are you?

I have asked you a number of times to explain your understanding of evolution.  You have not done so at any point.

I'll have another go at this, but I really think that the problem is that you simply don't understand, rather than see any genuine problem.  Bear in mind that if a change is advantageous, it will be selected for - this is the core of what you need to understand.

So - mating types arose because they were advantageous.  Sharing DNA gives a better chance for the next generation to be better suited to the environment.  Mating types is a way of doing this, so why wouldn't this arise?

Gametes - a way of sharing DNA - an advantageous adaptation.  Via meiosis, you get a full complement of DNA in the offspring, with the greatest chance of survival.  This means that meiotic division into gametes is a process that will be selected for.

Where is the 'Hey Presto' in disruptive selection?  It's advantageous to have a large gamete stocked with resources that combines with a smaller gamete.

Males/Females - Sarah said
Quote
Males of species are thought of as the ones that produce many small gametes and the females produce fewer large gametes.
You think this is nonsense - this leads me to suspect you're simply trolling.  There is nothing nonsensical in the fact that we call one group male and another group female.

Intersex conflict - of course this assumes the existence of males and females - intersex conflict couldn't exist without it - are you reading these, or just randomly inserting comments in bold?

Evolution has been witnessed in the wild and in labs, and predictions made by evolution play out.  I don't think there's any evidence of intelligent design, or any evidence that stands against evolution.  Now, for the fourth time, please explain to us your understanding of evolution, so we can see why we disagree.  Also, please explain your idea of how the sexes developed.  I suspect we will find many many more holes in it than in the evolutionary explanation.
 

Offline atrox

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How did the two sexes evolve?
« Reply #7 on: 12/08/2008 00:37:47 »
You're not reading the replies are you?

Apperently not...
 

Offline bryan

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How did the two sexes evolve?
« Reply #8 on: 12/08/2008 13:52:09 »
Asyncritus, your tone comes off as quite angry. Why does the lack of a full and comprehensive explanation of the evolution of the sexes bother you so much? Do you have an explanation you feel is superior? If so, please detail it in full.
 

Offline Asyncritus

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How did the two sexes evolve?
« Reply #9 on: 17/08/2008 11:13:25 »
Asyncritus, your tone comes off as quite angry. Why does the lack of a full and comprehensive explanation of the evolution of the sexes bother you so much? Do you have an explanation you feel is superior? If so, please detail it in full.

I'm sorry if it does come across as angry. I apologise for that,  but the subject does get up my nose quite sharply because of the unscientific speculation that goes by unchecked and uncritically.
 

Offline BenV

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How did the two sexes evolve?
« Reply #10 on: 17/08/2008 13:06:11 »
I think Bryan's question still stands, though, kindly supply your explanation for how the sexes arose.  There may be an element of speculation in the above, all within the biological rules of evolution, but I expect that your explanation will involve speculation outside the rules of reality, i.e. a divine intervention of sorts.
 

Offline Asyncritus

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How did the two sexes evolve?
« Reply #11 on: 18/08/2008 00:18:04 »
I think Bryan's question still stands, though, kindly supply your explanation for how the sexes arose.  There may be an element of speculation in the above, all within the biological rules of evolution, but I expect that your explanation will involve speculation outside the rules of reality, i.e. a divine intervention of sorts.
You're absolutely right. There is no other possible explanation or accounting for the phenomenon.

"He made them male and female". Special creations for a very special purpose.
 

Offline Asyncritus

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How did the two sexes evolve?
« Reply #12 on: 18/08/2008 00:28:05 »
I don't quite know what you mean by 'the biological rules of evolution'. If by that you mean speculation rampaging unchecked, then I'm afraid I don't abide by those rules.

It is completely obvious to me that the amazing complementarity existing in the sexual differences had to be overseen somehow.

It is particularly obvious at the genetic level.

The process of meiosis DIVIDES THE CHROMOSOME COMPLEMENT by exactly half.

Why should it do so?

Because when the two gametes combine, the chromosome complement returns to the normal diploid number.

So in our case, the 46 chromosomes in the normal cell is divided by meiosis into 23 chromosomes in the male gamete, and 23 in the female gamete.

23 chromosomes from the male combine with the 23 chromosomes from the female, and the full number of 46 has reformed.

If it didn't reform properly and correctly, then abormalities would result, and the species would perish or deteriorate severely.

Now, did that happen by chance, or by design? It looks very much like counting, doesn't it? Then who did the counting?
« Last Edit: 18/08/2008 00:30:51 by Asyncritus »
 

Offline BenV

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How did the two sexes evolve?
« Reply #13 on: 18/08/2008 09:22:16 »
Strangely, you've answered your own question.

Quote
If it didn't reform properly and correctly, then abormalities would result, and the species would perish or deteriorate severely.

So natural selection keeps it in check, and meiosis divides to the haploid number.  No need to count chromosomes, you see? Cell division is genetically controlled - so accurate meiosis has been selected for.
 

Offline Carol-A

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How did the two sexes evolve?
« Reply #14 on: 19/08/2008 07:49:17 »
Phew, I nearly got involved in this discussion, having been involved in research into the evolution of two sexes, but I can't compete with divine intervention. That allows you to ignore all rules and logic to conclude anything. Glad I waited to see where the thread was going!
 

Offline Asyncritus

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How did the two sexes evolve?
« Reply #15 on: 19/08/2008 09:20:55 »
Nobody is asking you to compete with anything Carol, merely to recognise the facts.

We have a phenomenon that is nearly universal in the living world, and no amount of rationality is going to explain its origin.

It is so obvious that TELEOLOGY RULES in the sexes.

Take mammalian physiology for example. If a biologist cannot see that the male organs and female organs are totally complementary to one another, and one without the other is entirely useless, and the species will thereby become defunct - if indeed it could arise at all - then I don't hold out much hope for the future of Biology.

I mentioned the phenomenon of meiosis above . Is it not obvious that this process had to overseen? That Somebody, somewhere, could count very exactly? And knew that if 46 chromosomes joined up with another 46, then that would be the end? And therefore HALVING the number in the gametes was the logical and simplest thing to do?

If that isn't intelligent designing, I don't know what is.

The whole phenomenon of the attraction and mating processes in the higher animals screams design. Why should males have an organ of intromission, and a female a receptor mechanism? If both had the same, then bye bye species. So when did they separate? And how? And why?

And I imagine you know something about the physiology and biochemistry of the reproductive processes. When I was studying them, I was stunned at the sheer complexity of the hormonal guidance and direction of the processes in both male and female.

There is absolutely NOTHING in the whole process that could be described as 'simple'. The production of ova by the ovaries is governed ruthlessly by biochemical substances of extraordinary complexity.The generation of spermatozoa is an extraordinarily complex process too - not as complex as the female, but decidedly not simple.

Then there's the phenomenon of pregnancy with all its attendant problems that needed solving. At birth the pelvic bones need softening, and there's a hormone to do it. The uterus must contract to expel the foetus - again hormonally controlled. And in the meantime, the foetus has to be fed via the placenta, and the extraordinary phenomenon occurs where the foetus is protected BY the placenta from the mother's ill health.

Did that happen by chance?

At birth the mother produces the colostrum in her milk, which contain antibodies to confer immunity on the child, and the hormones like oxytocin have to cause lactation.

Those are only a few of the phenomena I learned about, with my mouth agape at the complexity of what was happening.

Evolution? Chance? Random mutations causing all that? A total impossibility. If we ever manage to produce self-replicating computers, say, then you can be absolutely certain that they will be extraordinarily complex beasts - AND COULD NOT HAVE BEEN PRODUCED BY CHANCE.

But by Intelligent Design.
 

Offline bryan

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How did the two sexes evolve?
« Reply #16 on: 28/08/2008 17:49:08 »
So if something is too complicated for you to understand then it must be the result of a magic man in the sky?
 

Offline Asyncritus

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How did the two sexes evolve?
« Reply #17 on: 28/08/2008 21:56:41 »
I don't understand a computer's guts. Let's say I found one on the sea shore.

I don't assume that it came about as the result of waves pounding on the shore for the last 3 billion years. I'm sure you wouldn't either. Or would you?

There's brain power involved in its construction. Intelligent brain, I mean.

So there's an intelligent magic man somewhere.

Now here's a fish on the same sea shore. One billion times more complicated than the computer.

There must be a magic man somewhere. But where?

In the sky, sez I. What do you say?
 

Offline Don_1

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How did the two sexes evolve?
« Reply #18 on: 29/08/2008 08:58:30 »
These are tiny aquatic organisms that have not had sex for over 80 million years!


I can beat that!!!  [:-'(] [:-'(] [:-'(]

 

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How did the two sexes evolve?
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