How Did Sexual Reproduction Evolve?

EXCREATIONIST
Hi,
On another messageboard there is a creationist who wants to know how sexual
reproduction first evolved. I could find links about why it would evolve -
because of its survival advantages - but I couldn't find anything about how.

I have only studied some biology in the earlier grades in high school so I
wouldn't be able to make a very educated guess about this. I guess this
subject will be extremely technical. People can just offer links to sites
that explain how the DNA copying system evolved. Creationists might like to
comment on the explanations.

[Administrator: Excreationist has done a good job of bringing up what could be a difficult thread. If it can be kept understandable and not offensive both then we can see where we get with it.]

 

RUFUSATTICUS

Excreationist,

The evolution and maintenance of sexual reproduction is one of the largest
topics in evolutionary biology. In fact, John Maynard Smith wrote an entire
book on the subject, The Evolution of Sex. There are many different
reasons postulated for sex and many of them are probably correct.

Here are some links discussing the various models.

http://www.zoo.ufl.edu/bolker/eep/notes/week1b.html

http://www.pbs.org/wgbh/evolution/sex/advantage/index.html

http://gales.ecs.soton.ac.uk/~gep98r/third.html

http://www.sonoma.edu/users/r/rank/bio301s01/301_lec_13.html

-RvFvS

 

EXCREATIONIST

RUFUSATTICUS,
Those links seem to mainly be about *why* sexual reproduction would evolve.
I was wondering if you could explain in a few paragraphs *how* it probably
could have evolved. And what group of animals did it first start with?
Molluscs? It seems quite complicated how a creature could go from having a
single sex where its DNA is just copied - cloning itself - to having two
sexes where the two sets of DNA are randomly combined (using fertilization).
Could this happen in a step-by-step way? Please explain this so that those
that don't know much about biology can understand it. Thanks.

 

PAUL OF EUGENE

The links in the previous post are excellent but they don't provide any speculation, as requested, for how sexual reproduction could have started; they speak to why it is useful.

The most primitive form of sexual reproduction takes place with bacteria that get together and swap genes.

I can speculate that this behavior started as an attempt by two ancestral bacteria to eat each other. The attempt failed but some of their innards got switched including the genes. This conferred some advantage - to at least one of them - and the process of evolution refined the practice into the cooperative gene exchanging we see today.

Then as bacteria began to find advantage in grouping together, then specializing, those that found innovative ways to retain the benefits of gene exchanging were positively selected for, and true sex was on its way.

That's just a bare outline but the details are, of course, lost in the mists of time. Such things don't leave a fossil record!

[ March 14, 2002, 02:39 PM: Message edited by: Barnabas ]

 

HELEN

RufusAtticus, I agree with the comments that these links show WHY, or at
least some of the why’s as to why sexual reproduction may have developed
in the evolutionary view of things, but the HOW is not nearly as easy.

Paul of Eugene speculates it came from a sort of cannibalistic
exercise. But this does not get anywhere near explaining multicellular
sexual differences, haploid cells, or mating cues. In addition, the
idea of simply exchanging bits of genome, while lovely for bacteria,
again does not even remotely approach the concept of not only sexual
reproduction, but sexual interactions and the incredible differences
between male and female in so many kinds of animals and plants.

In a purely logical approach, sexual reproduction makes no sense at all
for evolution. Mutations happen more quickly with single sex organisms,
such as bacteria and the full genome can then be passed on to progeny.
If Dawkins’ point about selfish genes holds, then sexual reproduction is
out. Sexual reproduction REDUCES the number of mutations passed on to
progeny, thus DECREASING potential evolution. Nor does sexual
reproduction itself particularly ‘care’ or differentiate whether a
mutation is beneficial or not when it occurs. Mixing is mixing and
natural selection takes second place as far as heritability is
concerned.

Sexual reproduction, however, is the evidence of the sort of creation we
read about in Genesis 1, where each is created according to kind and
created in such a way as to perpetuate the kind itself as a distinct
kind despite future variations and speciations. It makes sense ONLY in
light of the plan to keep all mutations, and most especially damaging
mutations, to a minimum in a population for as long as possible.

 

RUFUSATTICUS

Excreationist,
The problem with answering questions with respect to sex is that most people
are hardly clear as to what they are asking.
Are you asking about the evolution of
</font>

• sex in general? </font>
• males and females? </font>
• sexual organs? </font>
• internal fertilization? </font>
• meiosis and recombination? </font>
• gene exchange? </font>
• human sexuality?
  </font>

This is important because you seem to have some misconceptions about the
biology of sex. Animals are not the only organisms that have sex: plants
have sex; protists have sex; parents have sex ; etc. The process of
combining two gametes to form a new cell is a characteristic of eukaryotes.
It arose in unicellular life and involves isogametes, i.e. still one sex.
Some unicellular organisms, like yeast, have mating types but no differences
in the structure of the cells. Only later, in multicellular organisms, do
you have the evolution of large, rich eggs and small, motile sperm; still
even then you have hermaphroditic organisms. Dioecy (having distinct males
and females) is a later trait.
Here's a link that might tide you over:
http://www.bio.purdue.edu/courses/BIOL597/lecturenotes.html

Helen,
The evolution of sex is a complex topic. I hardly fell qualified enough to
do it justice. Would you please refrain from making unsubstantiated and
unscientific assertions on what evolution could or couldn't do with respect
to sex? I would be happy to try to answer any specific and honest questions
you might have with respect to this topic. I would like to avoid
creationist propaganda in this thread. Thank you.

His comments were only intended to help explain the evolution of meiotic
fusion. Multicellular differences and mating cues came later; haploids came
earlier. If gene exchange is good for bacteria, why wouldn't it be good for
all life?

Did you even bother to pay attention to the webpages I've linked to? If you
had, I don't see how you can make this statement.

Mutation rates in cellular division do not depend on the reproductive
strategy. They are the result of the molecular machinery used to copy DNA.
One possible reason for sex is to actually dampen the deleterious effects of
mutation rates (Muller's ratchet). It allows cells to purge themselves of
bad genes and collect good ones. In asexual organisms, if two adaptive
mutations occur in separate individuals, there is no way for them to get
collected in the same individual except for the mutations to occur again.
In sexual populations, which regularly mix genes from different organisms,
these mutations have a higher chance of being collected in the same
organism. Do you see the evolutionary advantage? Of course, the full
formation of this explanation is much more complex, taking into
consideration such things as linkage disequilbrium and Mendel's rules.

I don't see your point. Sex is controlled by selfish genes that persist
because sex is advantageous. In other words, sex maintains their long-term
persistence.
I thought you argue that mutations are destructive for organisms. Thus
wouldn't there be good reasons, in your view, to reduce the number of
mutations passed to offspring. You should also note that offspring inherit
mutations from both parents.

I think you mean inheritance not heritability. Nevertheless, this comment
still makes no sense. I don't think you fully understand the models
discussed in the links I provided. Please read them again. I will answer
any questions that you have.

-RvFvS

 

HELEN
RufusAtticus: That’s a bit of a side trip you took there with
Excreationist! He asked HOW sexual reproduction first evolved. You put
up links about WHY it MIGHT have evolved and when he called you on that,
you then asked him what he meant! Sexual reproduction refers to the
fusion of two haploid cells from different parents, one being male and
the other being female, of ANY species, and the resultant diploid cell
which then matures into an adult of that species. Organs and types of
fertilization can be different. Simple bacterial (or otherwise) gene
exchange is NOT the same thing. I doubt very much is Excreationist had
any confusion at all about what ‘sexual reproduction’ means, even though
you tried to derail the argument by wandering off into those woods.

And yes, I know the evolution of sex is a “complex topic.” That is
because there is no way the evolution of sex has been able to be
explained. The farthest anyone can get is to try to point to different
kinds of reproduction and state, as you did, “See, one kind led to
another.” That is pure presupposition based on the idea that sexual
reproduction evolved in the first place!

You would like me to avoid “creationist propaganda” in this thread?
Well, I most certainly would like you to avoid evolutionist propaganda
and fantasies dressed up as fact in this thread. But I know for sure
that ain’t gonna happen, so I guess we are stuck with each other, eh?

You said, If gene exchange is good for bacteria, why wouldn't it be
good for all life?

Your statement, not mine. If it were good for all life, simple gene
exchange is all that would have evolved (we won’t discuss how on earth
genes developed in the first place…). The development, or advent, of
complementary haploid cells is a WHOLE different matter!

Your webpages admitted that sexual reproduction reduces the number of
mutations in a population. I stated that it does not matter to sexual
reproduction whether or not those mutations are putatively beneficial or
not. And it doesn’t. So-called beneficial mutations can be wiped out
by sexual recombinations as quickly as deleterious ones can, as that
sort of combining has nothing to do with natural selection at all. So I
repeat, that from a purely logical approach (the need to maintain a
certain set of mutations for the sake of evolution), sexual reproduction
makes no sense at all for evolution.

And I am quite aware that “mutation rates in cellular division do not
depend on the reproductive strategy.” I never said they did. Mutation
rates are higher in unicellular organisms because that is the primary
way asexually reproducing organisms can produce enough variation to
survive in changing environments. Because we can see and identify
unicellular organisms from deep in the fossil record, we know that,
given the evolutionary timescale, these strategies work extremely well.
So well, in fact, that changing reproductive strategies to sexual
reproduction would have been nonsense for evolution to do. The whole
idea of natural selection is “if it is working well, keep it.”

In the meantime, I absolutely DO agree with you that sex dampens the
deleterious effects of mutation rates. In keeping the numbers of
heritable mutations (and yes, I do know how to use that word) to a
minimum, the very process by which evolution is said to produce novel
forms is also controlled and actually brought to a screeching halt.
(And yes, I also know that offspring inherit SOME mutations from both
parents.)

Sort of in line with “created by kind” and all that that implies.

In the meantime, the entire concept of ‘selfish genes’ is ludicrous.
The genes do not have wills of their own. They cannot forsee the future
to know what might be needed evolutionarily. It is the survival of the
organism AS A WHOLE which determines which genes get through and which
don’t.

In the meantime, here is a quote from a genetics text:

"MEIOSIS Gamete formation presents an entirely new engineering
problem to be solved. To form gametes in animals (and, for the
most part, to form spores in plants), a diploid organism with two
copies of each chromosome must form daughter cells that have only
one copy of each chromosome. In other words, the genetic material
must be reduced by half so that when gametes recombine to form
zygotes, the original number of chromosomes is restored, not
doubled. If we were to try to engineer this task, we would first need
to be able to recognize homologous chromosomes We could then
push one member of each pair into one daughter cell and the other
into the other daughter cell. If we were unable to recognize
homologues, we would not he able to ensure that each daughter cell
received one and only one member of each pair The cell solves this
problem by pairing up homologous chromosomes during an
extended prophase. The spindle apparatus then separates members
of the homologous chromosome pairs, just as it separates sister
chromatids during mitosis. But there is one complication. As in
mitosis, cells entering meiosis have already replicated their
chromosomes. Therefore, two nuclear divisions without an
intervening chromosome replication are necessary to produce
haploid gametes or spores. Meiosis is, then, a two-division process
that produces four cells from each original parent cell. The two
divisions are known as meiosis I and meiosis II." (Tamarin R.H.,
"Principles of Genetics," International Edition, [1996], McGraw-
Hill: New York NY, Seventh Edition, 2002, pp.55-56)

That is the work of an Intelligent Creator, Rufus, not of undirected,
unintelligent, purposeless evolution!

 

EXCREATIONIST

RufusAtticus,
I'm talking just about organisms that are our ancestors. I was wondering how
a single-celled organism that reproduces by cloning itself could have
evolved into a creature (or single celled organism?) that produced eggs and
sperm. I would just like a step-by-step list of how this could have
happened, where each step seems to give the species a survival advantage.

An example is this 15 step guess about how bombardier beetles could have
evolved:
http://www.talkorigins.org/faqs/bombardier.html

 

THE BARBARIAN

It needs to be constantly kept in mind that evolution under naturual
selection is not "undirected".
I would be very interested in the evidence showing that meiosis and mitosis
have to be "designed".

Could you show us?

How do you suppose Pelomyxa palustris fits into this. It's a eukaryote, but
it doesn't do mitosis. Instead, the nucleii pinch apart to form two new
ones. This follows by division of the cell. Often this organism has
several to many nucleii, and some go with each part. This transitional
between fission and mitosis indicates that mitosis is explainable by
evolution.

It has no mitochondria, but apparently has similar endosymbiotes.

There are also a number of eukaryotes with simplified forms of mitiosis, in
which the centers are differently arranged. Wouldn't P. palustris be
slightly improved with the evolution of even simple microtubles that could
hold the fissioned nucleii?

 

RUFUSATTICUS

Excreationist,

I provided a list of phenotypes, although it was in paragraph form. Here is
one again.

</font>

• asexually reproducing unicellular organisms </font>
• unicellular organisms, that reproduce both asexually through mitosis and
  sexually through meiosis and isogametes </font>
• multicellular organisms </font>
• differentiated multicellular organisms with germ cells for mitosis
  (possible loss of asexual reproduction) </font>
• hermaphrodites with the gradual move from isogametes to motile sperm and
  rich eggs </font>
• dioecy: the specialization of individuals to produce either sperm or eggs
  </font>

Do you want me to explain any parts of this list in better detail? Or do
you have any other questions? If you are looking for specific genes or DNA
sequences, I will not be able to provide them to you in a timely fashion.
It would probably take over a year for me to comb the literature to find
some concrete and inherently satisfying examples. You might try contacting
Mark Kirkpatrick at The University of Texas at Austin. One of his
specialties is the evolution of sex, and he knows the empirical data better
than I do.
Helen,

I'm not quite sure what your problem is. Yes, I initially didn't response
correctly to Excreationist's questions. When he pointed that out, I asked
him some specific clarifying questions so that I might not do it again.
This is always a good policy, especially because I might respond to "how did
sex evolve" by explaining the genetics of recombination when all he wants is
a list of transitional phenotypes. I have a much more complex understanding
of sex, that is why I asked him clarifying questions. I'm sorry if you felt
like I did him a disservice. I was only trying to figure out the type of
answer he was looking for. Asking an evolutionary biologist simply, "How
did sex evolve," is like asking him/her to write a thousand dissertations
and on top of that in a style the average person could understand.

Wrong. It doesn't have to be a male and a female. Unicellular organisms
can have sex. I'd like you to identify a male and female yeast. Also sex
is not restricted to diploids/haploids. Polyploids are well known. I even
read a recent paper about a triploid toad, where the ova are diploid, the
sperm are haploid, and every adult is triploid.

I'm sorry, but one cannot consider the evolution of sex with out
investigating other forms of reproduction.

Please point out a single thing I have said in this thread, pertaining to
the evolution of sex, and demonstrate that is "evolutionist propaganda" or a
"fantasy" without resulting to unverifiable assertions or an argument from
disbelief.

What leads you to this conclusion? Simple gene exchange is good. But sex
is a better method of exchange despite its costs.

I don't see what your problem is. No one has claimed that bacterial gene
exchange demonstrates the evolution of egg and sperm, which is what I think
you mean by "complementary haploid cells." The dichotomy of eggs and sperm
are the result of gradual coevolution of two types of strategies: invest a
little in a lot and invest a lot in a few. At one point in our evolutionary
history, there was just one strategy: isogametes. However, there was
variation in the size of the isogametes and the number produced. Cheaters
evolved who exploited this variation by specializing in slightly smaller
gametes but produced more. This generated selection for other individuals
to specialize in larger, but less gametes. Although, at first there were
only minor differences, selection over countless generations has produced
the dimorphism we see in many species' reproductive strategies. I have
simplified the full and complex explanation in order to keep it short and so
that you and others might at least attempt to understand the evolutionary
forces leading to anisogametes.

Maybe you should try again. The process of sexual reproduction does not
eliminate maladaptive genes from the population. Natural selection does.
However, natural selection does a more effective job of eliminating such
genes and producing novel combinations in a sexual population than in an
asexual one. The rest of your comment is likewise ineffective, since they
neglect this fact.

The mistake you are making is that plenty of unicellular organisms reproduce
sexually and plenty of multicellular organisms reproduce asexually. In
fact, many (both multicellular and unicellular) do both. Also, what data do
you have on mutation rates to back up your claims that asexually are
inherently faster?

Wrong. Natural selection is not about maintaining the status quo.
It's about preserving that which works well and promoting that which works
better. In some organisms, sex is a better strategy than no sex and natural
selection has favored it. In others, this is not the case and natural
selection has favored asexual reproduction. Still, in some organisms,
natural selection has favored a combination of both. If you are asking why
did it take so long for meiosis to evolve, it's a complex process and as
such it isn't expected to occur in one brief evolutionary jump. Also
meiosis couldn't evolve before eukaryotes did; although, it seems to be a
very early trait of them. Prokaryotes have different forms of sexual
reproduction, which are much older that meiosis and eukaryotes.

Umm, "selfish" is a metaphorical term and doesn't imply that genes have any
sort of cognition. Do you also complain about the phrase "the engine is
running" because engines don't have legs? "Selfish" applies to, among other
things, the fact that a body, which is the product of genetic instructions,
is just a mechanism for DNA to maintain itself for another generation. Your
germ cells are the only part of your body with a chance at immortality.
Not to mention the fact that so much DNA does nothing other then be
replicated and passed on to the next generation. I suggest that you look
into meiotic drive if you think that individual genes can't be "selfish."

Nice emotional assertion based upon incredulity. Now, do you happen to have
any sort of argument to present to me? Not to mention the fact that you are
still a long why from showing me that this Intelligent Creator is the God of
Jews that you pray to at night and that Genesis is an accurate explanation
for the diversity of life on this planet.

Helen, do you actually happen to have any honest questions about the
evolution of sex? I will try to answer them if you do.

-RvFvS

 

HELEN

Barbarian, suffice it to say I simply do not have the faith in natural
processes that you do. Where you see possibilities of things becoming
interrelated in complex and specified ways through time, I see what
happens through time as I have experienced it: things do not become
more specified and complex – they become more disorganized and
disordered. Since that is something that we see not only in terms of
bicycles, but in terms of cells, too, it seems to me that the burden of
proof, or at least of a reasonable indication, regarding the ‘evolution’
of something like meioses happening by chance is on the evolutionist.
Imagination can be matched with imagination, by the way, so the best bet
is to present something that shows the kind of change which transfers it
from a less organized to a more organized state without that change
having been built into it in the first place (such as one finds with the
development of a fertilized egg into a baby).

When I see something as complex as a typewriter, I assume design.
Meiosis is vastly more complex and yet you want me to abandon the idea
of design and assume it just happened via time, chance, and whatever
else you may choose to involve that was not intelligent and not design.

That does not fly with me.

So I, in my turn, would be very interested if you could show us how
meiosis could evolve without design.

To RufusAtticus,
My problem, as you refer to it, is that you seem to want to lose the
problem by complicating the response. You can’t lose it that way. It
really does not matter how complex you consider your
understanding of sex, the so-called evolution of the production of
haploid cells which would then match up with other haploid cells to
result in an embryo plant or animal of the same KIND as the parent
organisms is being questioned here. The fact that certain anatomical
accommodations are necessary is, of course, a part of it, but the entire
area of sexual reproduction simply is a mystery biologically in terms of
evolution. One does not have to be a creationist to know that.

By the way, when I gave you the definition of sexual reproduction as it
is commonly worked with, you simply said “wrong.” No, it was not
wrong. I was not talking about unicellular organisms and I made that
very clear. Nor was I talking about your poor anomalous triploid toad.
I was talking about something I am very sure you are aware of: what is
normally considered sexual reproduction. When one hears the word “sex”
one tends to think of male and female. So think in those terms for the
purposes of this conversation. Those familiar with the terms think in
terms of two haploids forming a diploid cell. Think in those terms.

Then explain the evolution of this commonly referred to sex in some way
not depending upon imagination. If you are very serious about it, you
can try to explain it without assuming from the start that evolution
happened…

Feel free to “investigate” any other form of reproduction you would like
to. Then explain normal sexual reproduction.

You then stated:
Please point out a single thing I have said in this thread,
pertaining to the evolution of sex, and demonstrate that is
"evolutionist propaganda" or a "fantasy" without resulting to
unverifiable assertions or an argument from disbelief.

You haven’t said enough to believe or disbelieve yet, as you have not
touched the actual subject of the thread. That aside, are you trying to
tell me that if I don’t go for your imaginary ideas of how sexual
reproduction evolved that I am disqualified in your book as arguing from
disbelief?

In that case, you bet I am. Imagination does not take the place of
reality in the science I know. The question was asked at the beginning
of this thread to please explain how sexual reproduction has evolved.
We have gotten precisely nowhere with that as of yet.

You yourself argued for the investigation of other forms of reproduction
and then turned around in the same post and said to me
No one has claimed that bacterial gene exchange demonstrates the
evolution of egg and sperm,…

after which you proceeded to the imaginary scenario involving “cheaters”
etc., which has no basis in empirical science at all in terms of changes
from one sort of a thing to another. It is pure imagination and I
definitely reserve the right to be highly skeptical of that sort of
explanation.

You later said something that I enjoyed reading:
The process of sexual reproduction does not eliminate maladaptive
genes from the population. Natural selection does. However, natural
selection does a more effective job of eliminating such genes and
producing novel combinations in a sexual population than in an asexual
one.

First of all, sexual reproduction does a very good job of eliminating
all manner of mutations (maladaptive or not) from a population. That’s
one of the things it does best, aside from producing babies. This is
what produces stasis in populations.

Secondly, natural selection can only delete (when it selects some, it
deletes others – this is the cost that is involved in basic population
genetics), it cannot produce any novel combination in and of itself.
Sexual reproduction does that. That is what sexual reproduction is also
good for. Natural selection only eliminates some of what doesn’t work
as well as some of what does which was just in the wrong place at the
wrong time!

In the meantime, no one has stated that sex is not a better strategy in
some organisms. It is. The question has been, “How did it evolve?”,
not “What good is it?”

You asked me if I had “any honest questions about the evolution of sex.”

Sure. Look at the question at the beginning of this thread. How did
sex – as we all understand sex to mean, please – evolve?

You have danced around the moon and back on this one and have not
answered that first question.