How did we get males and females?
Joshua Doe
to, is how the sexes evolved. How did we get sexual organs as they are
(in that work together in a specific way), and how did one sex get the
ability to bear offspring? I also wonder about not only the
development of the two sexes, but how they then got together to
produce offspring.
Dana Tweedy
"Joshua Doe" <jos...@yahoo.com> wrote in message
news:57dae333.04020...@posting.google.com...
> One question I've always had, and never been able to find an answer
> to, is how the sexes evolved.
Sexual reproduction existed long before there were multicelluar bodies to
need sex organs.
http://en.wikipedia.org/wiki/Reproduction
http://www.infoplease.com/ce6/sci/A0860695.html
>How did we get sexual organs as they are
> (in that work together in a specific way),
They evolved with the population.of organsims. Sexual organs didn't evolve
in isolation, they evolved with the rest of the body.
Systems that worked were retained, those that didn't fell by the wayside.
>and how did one sex get the
> ability to bear offspring?
By evolving the ability. We observe today a range of variation in bearing
young, from the organisms that just leave a pile of eggs to be fertilized
externally, to those who gestate internally and bear live young.
> I also wonder about not only the
> development of the two sexes, but how they then got together to
> produce offspring.
It wasn't a matter of one individual male evolving a penis, and one isolated
female evolving a vagina. Sexual reproductive organs evolved in tandem in
populations. There would have been no problem with members of the opposite
sex finding each other.
DJT
DJT
>
r norman
Sexual reproduction, technically speaking, is a pattern of
reproduction where the life cycle includes both haploid and diploid
stages. The way to go from diploid to haploid involves the special
cell division called meiosis. The way to go from haploid to diploid
involves the union of two cells in fertilization. The process of
fertilization of two gametes to produce a diploid zygote results in
the production of new combinations of genes, hence increased
variability in the population.
Sexual reproduction in this sense is widespread in the eukaryotic
organisms: animals, plants, fungi, and even some single celled forms.
It does not require two distinct sexes. It does not even require that
the gametes be different as in sperm vs. egg.
At least one of the gametes must be motile in order for the two of
them to get together. It seems to be advantageous for one of the two
be very small and motile (sperm) and the other to be very large,
filled with nutrients to help ensure the viability of the zygote, and
non-motile (egg). That is, sperm and eggs (male and female) are
widespread, found in both plants and animals.
It seems also to be advantageous for the organism that produces eggs
to retain the eggs, providing an environment that protects and
possibly nourishes the embryo to help unsure its success. Again, this
situation is widespread in both plants and animals.
The specific penis-vagina thing is mainly an animal adaptation to
terrestrial life. The sperm cannot survive exposed to air so the only
good way for the sperm to get to the retained egg is to be deposited
into an internal cavity of the female. Plants solve the equivalent
problem with pollen and the pollen tube, a strategem to carry the
sperm to the egg in a protected environment.
Abner Mintz
I'm not an expert, but the most likely route (IMO) is
hermaphrodites followed by specialization (instead of
being both sexes, they split into being one or the other
sex as being more efficient than being both for a complex
organism).
Uncle Davey
Użytkownik "Abner Mintz" <abner...@earthlink.net> napisał w wiadomości
news:1g8styw.rkxdlmzdo19cN%abner...@earthlink.net...
It doesn't make any sense, does it?
How could having only half the stock able to bear young make something more
fit to survive than hermaphrodites?
Uncle Davey
Grinder
"Uncle Davey" <no...@jose.com> wrote in message
news:c03v32$g63$0...@pita.alt.net...
Just speculation on my part:
For an organism undergoing a long, disabling gestation period, it would be
beneficial to be mated with one that is not. Perhaps you will see equating
a mating pair with a single hermaphrodite as cheating the argument, but
hopefully you can recognize that serperation of function can be more
efficient.
"Rev Dr" Lenny Flank
Uncle Davey wrote:
>
> How could having only half the stock able to bear young make something more
> fit to survive than hermaphrodites?
>
It helps prevent genetic parasites like virii.
===============================================
Lenny Flank
"There are no loose threads in the web of life"
Creation "Science" Debunked:
http://www.geocities.com/lflank
DebunkCreation Email list:
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mel turner
[Joshua Doe] wrote...
>
>One question I've always had, and never been able to find an answer
>to, is how the sexes evolved.
Sexual life cycles evolved in single-celled organisms, long before
there were males or females, or sperms and egg cells, or
muticellularity. Although all animals have differentiated sperm cells
and egg cells [oogamy], there are still many multicellular algae and
fungi that have gametes that aren't differentiated as sperms and eggs.
The evolution of differentiated gametes makes sense as a "division of
labor"; a specialized large non-moving egg cell can store lots of
extra food for the future embryo, whereas a smaller, actively motile
sperm takes on the whole job of finding the other gamete.
How did we get sexual organs as they are
>(in that work together in a specific way), and how did one sex get the
>ability to bear offspring?
All of these things are modifications of systems that went before.
Before there were animals with internal fertilization involving those
interlocking sexual organs, there were animals that simply released
sperms and eggs into the water, like most fish and many amphibians
still do today. There are animals without specialized copulatory
organs that just bring their body openings together to effect internal
fertilization.
As for the evolution individuals of different sexes, of course many
animals today are hermaphroditic. Even a few groups of vertebrates.
In plants for example, the evolution of separate male and female
individuals [dioecy] is something that has independently arisen many
times from bisexual ancestors. Again, it's probably a matter of
specialization and "division of labor". A specialized full-time male
might be better at it and better at fertilizing many more eggs than a
hermaphrodite that has to be able to play the egg-producing role as
well. And with competition from full-time males, any that are still
hermaprhodites might as well concentrate on just the female side of
things. Separate specialized males and females seems to often be a
selectively stable system.
>I also wonder about not only the
>development of the two sexes, but how they then got together to
>produce offspring.
They were already getting together to produce offspring. That's
pretty much the whole point of evolution; there obviously wasn't
any ancestral generation that was incapable of reproduction.
cheers
mel turner
What's with the retitling of the thread? And all the 4-letter
"subject" lines, for that matter?
>Użytkownik "Abner Mintz" <abner...@earthlink.net> napisał w wiadomości
>news:1g8styw.rkxdlmzdo19cN%abner...@earthlink.net...
>> Joshua Doe <jos...@yahoo.com> wrote:
>> > One question I've always had, and never been able to find an answer
>> > to, is how the sexes evolved. How did we get sexual organs as they are
>> > (in that work together in a specific way), and how did one sex get the
>> > ability to bear offspring? I also wonder about not only the
>> > development of the two sexes, but how they then got together to
>> > produce offspring.
>>
>> I'm not an expert, but the most likely route (IMO) is
>> hermaphrodites followed by specialization (instead of
>> being both sexes, they split into being one or the other
>> sex as being more efficient than being both for a complex
>> organism).
>
>It doesn't make any sense, does it?
Sure it does, why wouldn't it?
>How could having only half the stock able to bear young make something more
>fit to survive than hermaphrodites?
Because full-time males can have lots and lots of offspring,
compared to hermaphrodites who must "waste" energy making eggs as
well? Why is dioecy so common in both animals and plants? It certainly
seems to work well in the species that are dioecious. Specialized
males can have lots of offspring, and the specialized females will
have sons who can have lots of offspring. And even though half the
population can't bear young [or produce eggs], every individual
still has both a male and a female parent [apart from Hymenoptera,
etc.].
To creationists: if dioecious species are really less fit to
survive than hermaphroditic ones, why then would the "Intelligent
Designer" ever design them that way? Or is his intelligence
overrated?
This whole problem best fits a Darwinian selectionist explanation
[specialized males and females successfully competing with
hermaphroditic individuals within a transitional population] vs an ID
perspective.
Clearly, a cross-fertilizing hermaphroditic species in which every
individual could bear young or lay eggs would seem to allow for a much
higher overall reproductive rate than a similar but dioecious species.
It all shows that selection works at the competition-among-individuals-
within-a-species level, not at the for-the-optimal-good-of-the-whole-
species level.
The male peacocks and elephant seals, etc. show us that this is
indeed the case.
cheers
Dale
news:c0469p$cis$2...@gargoyle.oit.duke.edu...
Males can have lots of offspring only if there are lots of pregnable
females. If most of the females are pregnant most of the time, it's no
advantage to the males to be able to sire lots of offspring. So at some
level it seems it would be more efficient to have lots of females who can
have lots of females, needing only a few males to impregnate them all. Why,
then, are there so many species that have near 1:1 sex ratios? Or are there
many species which don't?
Steven J.
"Dale" <dmg...@nspm.airmail.net> wrote in message
news:c04ea0$d...@library2.airnews.net...
> "mel turner" <mtu...@snipthis.acpub.duke.edu> wrote in message
> news:c0469p$cis$2...@gargoyle.oit.duke.edu...
>
>
> > Because full-time males can have lots and lots of offspring,
> > compared to hermaphrodites who must "waste" energy making eggs as
> > well? Why is dioecy so common in both animals and plants? It certainly
> > seems to work well in the species that are dioecious. Specialized
> > males can have lots of offspring, and the specialized females will
> > have sons who can have lots of offspring. And even though half the
> > population can't bear young [or produce eggs], every individual
> > still has both a male and a female parent [apart from Hymenoptera,
> > etc.].
>
> Males can have lots of offspring only if there are lots of pregnable
> females. If most of the females are pregnant most of the time, it's no
> advantage to the males to be able to sire lots of offspring. So at some
> level it seems it would be more efficient to have lots of females who can
> have lots of females, needing only a few males to impregnate them all.
Why,
> then, are there so many species that have near 1:1 sex ratios? Or are
there
> many species which don't?
>
males, then one's odds of leaving descendants are vastly greater if one is a
male (with so many females, even a marginally fit male will likely find
someone, and leave as many descendants as the average female, and a highly
fit male may have many mates, with many times the offspring of the average
female). But this, in turn, means that leaving more than the average number
of male offspring increases the expected number of one's descendants. Thus
individuals with a tendency to produce more male offspring will be favored,
until the ratio of males to females rises to a point where male descendants
are no more likely (on average) than females to produce lots of
grandoffspring. Of course, a similar argument applies in reverse if the
ratio of males to females rises above 1:1 -- then any tendency to produce
more female offspring will be favored. For most species, this tends to
result in an equal ratio of the sexes; it isn't the maximally beneficial
result for the species, but it is for individuals in the species.
>
-- Steven J.
mel turner
wrote...
>"mel turner" <mtu...@snipthis.acpub.duke.edu> wrote in message
>news:c0469p$cis$2...@gargoyle.oit.duke.edu...
>> In article <c03v32$g63$0...@pita.alt.net>, no...@jose.com [Uncle Davey]
>wrote...
[snip]
>>>How could having only half the stock able to bear young make something
>>>more fit to survive than hermaphrodites?
>>
>> Because full-time males can have lots and lots of offspring,
>> compared to hermaphrodites who must "waste" energy making eggs as
>> well? Why is dioecy so common in both animals and plants? It certainly
>> seems to work well in the species that are dioecious. Specialized
>> males can have lots of offspring, and the specialized females will
>> have sons who can have lots of offspring. And even though half the
>> population can't bear young [or produce eggs], every individual
>> still has both a male and a female parent [apart from Hymenoptera,
>> etc.].
>
>Males can have lots of offspring only if there are lots of pregnable
>females. If most of the females are pregnant most of the time, it's no
>advantage to the males to be able to sire lots of offspring.
But some males are siring all the offspring that are being sired. It
may be that only a few of the males are siring the entire next
generation [such as in elephant seals with large harems controlled by
the few dominant bulls], or each male may have only a few offspring,
but both males and females make equal genetic contributions to the
next generation. [this leads to the 1:1 ratios]
So at some
>level it seems it would be more efficient to have lots of females who can
>have lots of females, needing only a few males to impregnate them all.
Right, the 1:1 ratio can seem remarkably wasteful, but evolutionary
theory explains why it would be expected to exist.
Why,
>then, are there so many species that have near 1:1 sex ratios? Or are there
>many species which don't?
The evolution of sex ratios has long been a hot topic for evolutionary
theorists and population geneticists. There seem to be good
mathematical/theoretical explanations for why 1:1 sex ratios are
common [the classic explanation was by R. A. Fisher in 1930, and
there are explanations for the examples of species that do diverge
from such ratios:
http://www.philippwesche.org/sr.html
http://www.zoo.ufl.edu/bolker/eep-2000/sexratio.html
http://www.zoology.ubc.ca/~whitlock/bio434/LectureNotes/11.SexRatio/SexRatio.ht
ml
http://www.sysc.pdx.edu/~jeff/group_sel_workshop/aviles.pdf
http://zygote.swarthmore.edu/sex7.html
http://www.u.arizona.edu/~jons/sexratio.html
http://www.scieng.flinders.edu.au/biology/people/schwarz_m/mps/kranz_et_al._200
0.pdf
http://westgroup.icapb.ed.ac.uk/pdf/WestGodfray_97.pdf
http://www.zoology.ubc.ca/~otto/PopGen500/Discussion2/Overheads.html :
"Fisher (1930) developed a compelling argument for why sex ratios
should evolve to 1/2:
The total reproductive value of the males [in a population] is
exactly equal to the total value of all the females, because
each sex must supply half the ancestry of all future generations
of the species...The sex ratio will so adjust itself, under the
influence of Natural Selection, that the total parental
expenditure incurred in respect of children of each sex, shall
be equal.
Fisher 1930.
If this were not so, parents that produced the rarer sex would gain
a larger proportion of that sex'sreproductive value and hence a
greater genetic representation among grand-offspring."
cheers
Louann Miller
<twe...@cvn.net> wrote:
>
>"Joshua Doe" <jos...@yahoo.com> wrote in message
>news:57dae333.04020...@posting.google.com...
>> One question I've always had, and never been able to find an answer
>> to, is how the sexes evolved.
>
>Sexual reproduction existed long before there were multicelluar bodies to
>need sex organs.
>http://en.wikipedia.org/wiki/Reproduction
>http://www.infoplease.com/ce6/sci/A0860695.html
Or for a book-length answer to the question, you want "The Red Queen"
by Matt Ridley.
--
The full text of the 2003 evolution debate between Nowhere Man and Lilith
can be found at http://www.geocities.com/chastity403/debate/nm-lilith-1/
Abner Mintz
> It doesn't make any sense, does it?
It does to me ... and human have based our societies around the
idea that specialization is more efficient than trying to have
everyone able to do everything. Are you trained at everything?
Or is it more efficient to train you to do a few things well
than to do everything a bit?
> How could having only half the stock able to bear young make something more
> fit to survive than hermaphrodites?
It depends on the organism's survival strategy. If the creature
survives by having as many offspring as possible and not caring
for them, it doesn't make much sense ... and in fact, we don't
see distinct sexes in bacteria, viruses, fungi, and (if I recall
correctly) many plants. Other creatures have a different strategy:
instead of having as many as possible, they have just a few,
sometimes just one at a time, and take better care of them. For
such creatures, with a much higher investment cost in their offspring,
a specialization strategy makes sense: one of them pays the
investment costs, the other has the risks of not being able to
find a partner, so each specializes in doing *one* of them better
rather than trying to be good at both of them. It's a lot more
efficient for each creature to be an efficient bearer *or* an
efficient seducer (for lack of a better term) than to be both.
Later on in such an evolutionary pathway, the creature that is
doing the bearing may be helped out by the creature that isn't
during the investment period, which leads to pairing up for longer
periods than just the period of seduction ...
Prudish people or politically correct people should not read the
following example:
As an example of a creature which is hermaphroditic but is beginning
to show signs of such a strategy, there are hermaphroditic
planaria that (ahem) duel with their penises. Each of them bobs
and weaves, trying to stab the other with the penis. The one that
loses (gets stabbed) bears the offspring. Some of the planaria
are (by genetic luck of the draw) better at stabbing, others are
better at bearing the offspring. Is it really too hard to see ahead
to the time where the best stabbers lose their ability to bear and
the best bearers lose their ability to stab, and each just specializes
entirely in one role or the other? Or to behavior where the stabber
protects the stabbee from other stabbers while the stabbee is bearing
the young?
eyelessgame
Dawkins wrote about this at some length. It's been a while since I
read the book and don't recall the reasoning -- but he cited one
counterexample, a particular species of mite where each female
gestates some hundreds of daughters and one son. The male actually
dies before birth, impregnating all of his sisters while still in the
womb. (I remember thinking 'hey, those are tribbles, only uglier').
I don't recall his reasoning well enough to discuss *why* this
counterexample fit with a thesis of most creatures being stable only
at a 1:1 gender ratio... if I can find the book and the essay, I'll
follow up later, but I'm sure there are other t.o's who have read it.
eyelessgame
Tim Tyler
> Uncle Davey wrote:
> > How could having only half the stock able to bear young make something more
> > fit to survive than hermaphrodites?
>
> It helps prevent genetic parasites like virii.
Sexual dimorphism does? How?
--
__________
|im |yler http://timtyler.org/ t...@tt1lock.org Remove lock to reply.
Tim Tyler
> > > Joshua Doe <jos...@yahoo.com> wrote:
> > > I'm not an expert, but the most likely route (IMO) is
> > > hermaphrodites followed by specialization (instead of
> > > being both sexes, they split into being one or the other
> > > sex as being more efficient than being both for a complex
> > > organism).
> > >
> >
> > It doesn't make any sense, does it?
> >
> > How could having only half the stock able to bear young make something
> > more fit to survive than hermaphrodites?
>
> Just speculation on my part:
>
> For an organism undergoing a long, disabling gestation period, it would be
> beneficial to be mated with one that is not. [...]
Yes - but males of most species bugger off after impregnating their mate -
in search of more females.
Consequently, this can hardly explain the origin of the phenomenon.
Tim Tyler
> > > One question I've always had, and never been able to find an answer
> > > to, is how the sexes evolved. How did we get sexual organs as they are
> > > (in that work together in a specific way), and how did one sex get the
> > > ability to bear offspring? [...]
> >
> > I'm not an expert, but the most likely route (IMO) is
> > hermaphrodites followed by specialization (instead of
> > being both sexes, they split into being one or the other
> > sex as being more efficient than being both for a complex
> > organism).
>
> It doesn't make any sense, does it?
>
> How could having only half the stock able to bear young make something more
> fit to survive than hermaphrodites?
There are at least three theories about this:
* Male and Female are specialised roles with specialised adaptations -
so it often makes sense to house them in separate bodies.
* The "mitochondria" theory - that says that inheritance of mitochondria
through one parent forced the division (i.e. ch.4 of The Red Queen);
* Wirt Atmar's theory that powerful selection among the male cleans
the gene pool of deleterious mutitions -
see: http://aics-research.com/research/males1.html
Resource limitation means that the 2 x expansion rate advantage of
having an all-childbearing population is only rarely much of an
advantage in practice.