Evolutionary disadvantages of Hermaphrodism in animalia?
single_digit
have had lately.... Why don't imperfect hermaphrodites (those that must
exchange DNA with other hermaphrodites) show up more in the animal
groups. Its a common way of business in the plants and it seems to have
all of the advantages at minimal cost in Darwinian terms. You still get
just as much genetic variation because both individuals undergo meiosis
(presumably), yet everyone is a potential mate and everyone can bear
offspring (a huge Darwinian advantage). I could see one argument being
sexual selection tends to maintain gender identities, but this ignores
that dioecious species are the default we started from. Since animals
emerged from asexual ancestors that eventually evolved a sexual process
(meiosis or similar) and then (likely later) evolved independent
genders, it seems there must have been some huge advantage of gender
over hermaphrodites to skew things so far in favor of gendered species.
I have been doing the usual google research the last couple of days,
but haven't found much on the topic (surprisingly). I also am curious
how often documented dually fertile mutants arise in any particular
species. By dually fertile I mean being capable of generating viable
eggs and sperm in the species' usual sense.
Tim Tyler
> This is my first post here, but I was brought by a nagging
> question I have had lately.... Why don't imperfect
> hermaphrodites (those that must exchange DNA with other
> hermaphrodites) show up more in the animal groups. Its a
> common way of business in the plants and it seems to have
> all of the advantages at minimal cost in Darwinian terms.
> You still get just as much genetic variation because both
> individuals undergo meiosis (presumably), yet everyone is a
> potential mate and everyone can bear offspring (a huge
> Darwinian advantage).
Didn't Fisher suggest investment in male and female
offspring would be expected to be equal? I think that holds
even for male and female parts of hermaphrodites. If
everyone put their resources into having babies, selection
would favour the male parts who were fertilizing them to the
extent that they would increase in size or number - and the
population would then be spending more of its resources on
male parts.
I.e. there /is/ no huge Darwinian advantage. There's a huge
Darwinian advantage to asexual reproduction - but once you
have sex, it doesn't much matter if the genders are divided.
> I could see one argument being sexual selection tends to
> maintain gender identities, but this ignores that dioecious
> species are the default we started from. Since animals
> emerged from asexual ancestors that eventually evolved a
> sexual process (meiosis or similar) and then (likely later)
> evolved independent genders, it seems there must have been
> some huge advantage of gender over hermaphrodites to skew
> things so far in favor of gendered species.
Two books which answer this question in the negative - and
propose non-adaptive solutions:
The Red Queen - Matt Ridley.
Mendel's Demon - Mark Ridley.
However, there /are/ advantages to having divided genders:
for example, sexual dimorphism allows male and female organisms
to specialize more.
Sometimes this means a big fat female egg factory, and tiny,
numerous, mobile, disposable males.
Other times, huge armoured males are produced, which fight
for nondescript females.
If the two sexes are combined in one organism, you can't do
that sort of thing to anything like the same extent. You
can't have millions of tiny mobile males. You must have the
same number of males and females - and they can't differ in
mobility.
Essentially, male and female are quite different jobs.
Putting them in different bodies avoids the 'jack of
all trades' syndrome.
Plus, you have the whole problem of avoiding selfing (most of
the time) to deal with.
--
__________
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drose...@yahoo.com
single_digit wrote:
> This is my first post here, but I was brought by a nagging question I
> have had lately.... Why don't imperfect hermaphrodites (those that must
> exchange DNA with other hermaphrodites) show up more in the animal
> groups.
almost all the flatworms (planaria, tapeworm, etc.). There is usually a
barrier against an individual mating with itself. Some fish separate
the two genders in time (e.g., a female metamorphizes into a male). I
am not sure which came first, hermaphrodism or unisexualism. Since some
families of animals and plants have both hermaphroditic and unisexual
species. On the basis of that fact, I believe that both strategies are
common through evolutionary convergence.
On the assumption that hermaphrodism came first, one
hypothesis would be that unisexual reproduction started as a means of
preventing self-fertilization. There are other solutions to the
self-fertilization problem, but unisexual reproduction turned out to be
the winning strategy for amniotes like ourselves.
A self-fertilization often causes recessive genes to express, which
is sometimes lethal. So self fertilizers are usually selected out of
the population. In contrast, asexual reproduction will not force a
recessive gene to express itself. So asexual reproduction is common,
compared to self-crossing hermaphrodism.
I read recently that a plant was recently discovered which
naturally mates with itself, and this was described as extremely
unusual.
>Its a common way of business in the plants and it seems to have
> all of the advantages at minimal cost in Darwinian terms.
Charles Darwin, again in "Origin of the Species," has a section
on "Intercrossing of Individuals." I quote, "Turning for a brief space
to animals, various terrestrial species are hermaphrodites, such as
land-mollusca and earthworms, but they all pair. As of yet I have not
found a single terrestrial animal that can fertilize itself. This
remarkable fact, which offers so strong a contrast with terrestrial
plants, ... for owing to the nature of the ferilising element there are
no means, analogous to the action of insects and the wind with plants,
by which an occassional cross could be affected with terrestrial
animals without the concurrence of two individuals. Of aquatic animals,
there are many self-fertilising hermaphrodites, but here the currents
of water offer an obvious means for the occassional cross."
"Origin of Species" has other tidbits. I recommend it.
Perplexed in Peoria
"Tim Tyler" <seem...@cyberspace.org> wrote in message news:el7dm0$2u2s$1...@darwin.ediacara.org...
> single_digit wrote:
>
> > This is my first post here, but I was brought by a nagging
> > question I have had lately.... Why don't imperfect
> > hermaphrodites (those that must exchange DNA with other
> > hermaphrodites) show up more in the animal groups. Its a
> > common way of business in the plants and it seems to have
> > all of the advantages at minimal cost in Darwinian terms.
> > You still get just as much genetic variation because both
> > individuals undergo meiosis (presumably), yet everyone is a
> > potential mate and everyone can bear offspring (a huge
> > Darwinian advantage).
>
> Didn't Fisher suggest investment in male and female
> offspring would be expected to be equal? I think that holds
> even for male and female parts of hermaphrodites.
As I understand Fisher's logic, the equal investment applies at
the species level. Individuals need not invest equally. Instead,
they should seek the greatest marginal return on investment -
which, for some individuals, will mean specializing on one sex
or the other. But for the species as a whole, specializing in
one sex over the other just doesn't make sense.
[snip remainder]
Guy A Hoelzer
jimme...@sbcglobal.net wrote on 12/7/06 11:05 AM:
This was Bob Trivers extension of Fisher's highly simplistic model.
Guy
Tim Tyler
> "Tim Tyler" wrote:
>> Didn't Fisher suggest investment in male and female
>> offspring would be expected to be equal? I think that holds
>> even for male and female parts of hermaphrodites.
>
> As I understand Fisher's logic, the equal investment applies at
> the species level. Individuals need not invest equally. Instead,
> they should seek the greatest marginal return on investment -
> which, for some individuals, will mean specializing on one sex
> or the other. [...]
E.g. this 'sex ratio biasing' article:
http://www.nsf.gov/news/frontiers_archive/1-96/1ecology.jsp
I was talking about the population, though - as my next
sentence indicated:
``If everyone put their resources into having babies, selection
would favour the male parts who were fertilizing them to the
extent that they would increase in size or number - and the
population would then be spending more of its resources on
male parts.''
seem...@cyberspace.org
[Why separate genders?]
> The Red Queen - Matt Ridley.
> Mendel's Demon - Mark Ridley.
I've looked back over this material. IMO, it is a complete mess :-(
Matt Ridley gives the wrong answer to the question of why
there are genders - and then goes on to give the wrong answer
to the question of why they are divided in separate bodies -
in Chapter 4 of "The Red Queen".
Mark Ridley muddles the two questions together, gives the
same wrong answer as Matt Ridley to the first question, and
then passes it off as the answer to the second question -
around about page 151 of "Mendel's Demon".
I can see I'll have to write an essay to straighten things out...
William Morse
@darwin.ediacara.org:
Excellent question. It appears that at least part of the answer has to do
with avoiding self fertilization. For instance, the marine branch of the
annelids, which release egg and sperm into the water, have separate
sexes, while earthworms are hermaphroditic. The pattern is repeated in
gastropods, where those branches that can control how egg and sperm come
together tend to be hermaphroditic, whereas those that release them into
the water have separate sexes. The bryozoa (moss animals), with internal
fertilization, are hermaphroditic; the molluscs , which generally release
sperm and eggs, for the most part have separate sexes.
The pattern breaks down, however, when one gets to arthropods and
chordates. Here there is almost an inverse correlation, with species
that have clear control over how egg and sperm come together often being
the least hermaphroditic.
Wirt Atmar in a long ago post on this newsgroup gave an excellent
discussion of the reasons for having differentiated sexes. A possible
thought is that these reasons can only arise once there is the ability
for female choice. This requires motility - if you are a plant, even a
highly evolved one, you can't easily choose your mate (I know little
about plants, so I may well be wrong on this point). But if you are a
motile animal with a reasonably complex sensory system, you can start to
exercise mate choice and so there are more opportunities for sexual
selection, and hence for differentiation between the sexes.
Yours,
Bill Morse
Note Ridley Red Queen, unsatisfactory answer. Focus on plants vs. animals
- what is difference, why does it matter.
Tim Tyler
> Tim Tyler wrote:
> [Why separate genders?]
>
>> The Red Queen - Matt Ridley.
>> Mendel's Demon - Mark Ridley.
>
> I've looked back over this material. IMO, it is a complete mess :-(
>
> Matt Ridley gives the wrong answer to the question of why
> there are genders - and then goes on to give the wrong answer
> to the question of why they are divided in separate bodies -
> in Chapter 4 of "The Red Queen".
>
> Mark Ridley muddles the two questions together, gives the
> same wrong answer as Matt Ridley to the first question, and
> then passes it off as the answer to the second question -
> around about page 151 of "Mendel's Demon".
>
> I can see I'll have to write an essay to straighten things out...
An essay on why there are genders at all:
http://alife.co.uk/essays/gender_origin/
An essay on why genders are often separated into different
bodies - rather than everyone being hermaphrodites:
http://alife.co.uk/essays/gender_division/