Why are We Not All Hermaphrodites?

[Luminal]

Truly hermaphroditic species (two fully functioning sets of reproductive organs) would have an immense advantage in virtually all aspects of prosperity in nature and survival. Major examples:

 

- During any calamity that drastically reduces the population (be it a natural disaster, climate shift, starvation, drought, over-predation, plague, etc.), when only a handful of individuals are left, hermaphroditism would be greatly more beneficial than two sexes. Let's say that in a given area, only 3 males and 1 female survived the catastrophe. One of the males mate with the last female. However, during or shortly after pregnancy the female dies, and her young obviously with her. The species goes extinct.

 

On the other hand, let's replace 3 males and 1 females with 4 hermaphrodites. Four potential mothers and four potential fathers. Let's say two become pregnant while the other two perhaps help protect the pack/herd/whatever, yet both mothers ended up dying from the aftermath of whatever caused the bottleneck in the first place. Now there are still two potential mothers and two potential fathers. The advantages of this are abundantly evident.

 

- Much more rapid allele frequency changes in a population, thus much faster adaption to new threats and circumstances. In hermaphrodites, if a favorable mutation occurs in an individual, it can both carry young itself with those genes and spread their seed to other members of its species.

 

- In human males, we only have one Y-chromosome and one X-chromosome. Thus, a genetic disorder in one cannot be overridden by the other chromosome, as in all other pairs. Females have two X-chromosomes, thus they do not have this problem. In hermaphrodites, every member of the species would have the advantage which only females currently possess.

 

- There would be no extreme sexual dimorphism, which often leads to one sex becoming rather weak relative to the other dominant sex. I believe the emergence of a dominant sex is generally disadvantageous to a species as a whole, even though it may be favored to exist. For example, weak females can be more easily taken by males, so weakness in females may become common, but after thousands of generations females are less capable of defending themselves against predators and less capable of hunting and catching prey.

 

It's one of those rare examples in evolution that sexual selection runs contrary to natural selection, and gets away with it if the species as a whole manages to survive for some alternative reason.

 

 

 

Now, the great question... why are hermaphroditic species so uncommon in nature even though it is a superior system? There must be some "cache" that overrules everything else mentioned in their favor. What is it, though?

Edited June 3, 2008 by Luminal

[Gilded]

I wouldn't say hermaphroditic species are uncommon in nature in general, but they are indeed uncommon in vertebrates. One reason why hermaphroditism is favorable with organisms such as gastropods is that it takes them quite a deal of effort to seek out any partners. Imagine a snail traveling for days, reaching another snail only to discover that they are unable to mate. And obviously sequential hermaphroditism would be quite a bit more difficult for a more complex vertebrate than it is for, let's say, a clownfish.

 

As to why vertebrates don't usually have both sets of simultaneously functional reproductive organs I'm not sure if there's a widely accepted, definitive answer among evolutionary biologists. One possible reason has to do with the mate-searching efficiency I mentioned earlier; when partners are easy to come by it seems to be ultimately favorable that a part of the species uses minimal resources searching and can actually just wait for suitable partners to find them. It's all about survival and efficiency.

[Luminal]

I wouldn't say hermaphroditic species are uncommon in nature in general, but they are indeed uncommon in vertebrates. One reason why hermaphroditism is favorable with organisms such as gastropods is that it takes them quite a deal of effort to seek out any partners. Imagine a snail traveling for days, reaching another snail only to discover that they are unable to mate. And obviously sequential hermaphroditism would be quite a bit more difficult for a more complex vertebrate than it is for, let's say, a clownfish.

 

As to why vertebrates don't usually have both sets of simultaneously functional reproductive organs I'm not sure if there's a widely accepted, definitive answer among evolutionary biologists. One possible reason has to do with the mate-searching efficiency I mentioned earlier; when partners are easy to come by it seems to be ultimately favorable that a part of the species uses minimal resources searching and can actually just wait for suitable partners to find them. It's all about survival and efficiency.

 

I understand, yet the benefits of hermaphroditism are tremendous. Basically, hermaphrodites have the reproductive speed of asexual organisms whilst possessing the benefits of recombinant genes that comes with sexually reproducing organisms.

 

The best of both worlds.

 

Also, in terms of efficiency, it seems hermaphrodites have the advantage as well. On average, it cuts the time spent searching for a mate in half, leaving more time for other activities essential to survival.

[ecoli]

I understand, yet the benefits of hermaphroditism are tremendous. Basically, hermaphrodites have the reproductive speed of asexual organisms whilst possessing the benefits of recombinant genes that comes with sexually reproducing organisms.

 

The best of both worlds.

 

Also, in terms of efficiency, it seems hermaphrodites have the advantage as well. On average, it cuts the time spent searching for a mate in half, leaving more time for other activities essential to survival.

it greatly reduces genetic diversity, which in terms of darwinian fitness, is extremely disadvantageous, especially in dynamic environments.

[CDarwin]

it greatly reduces genetic diversity, which in terms of darwinian fitness, is extremely disadvantageous, especially in dynamic environments.

 

It would seem as though reproducing clones of yourself, though, would always be more evolutionarily advantageous than failing to find a mate and not reproducing at all. Hermaphroditism leaves that option open.

 

This is something that has puzzled me, too (maybe for the same reason; did you read the New Scientist article, Luminal?).

Edited June 3, 2008 by CDarwin

[Daecon]

Humans just happened to evolve from a species that had already developed into two genders.

 

Luck of the draw.

[PhDP]

Eric Charnov worked on this question, I'll post a reference a little later.

 

It's one of those rare examples in evolution that sexual selection runs contrary to natural selection, and gets away with it if the species as a whole manages to survive for some alternative reason.

 

Actually, sexual selection often runs against ecological selection. Why do you think we have a 50:50 sex ratio ?

[Luminal]

it greatly reduces genetic diversity, which in terms of darwinian fitness, is extremely disadvantageous, especially in dynamic environments.

 

How does it reduce genetic diversity? If anything, it would seem to increase it. If every organism is a potential mother and father, new mutations have multiple unique paths to take to enter the gene pool at large.

 

It would seem as though reproducing clones of yourself, though, would always be more evolutionarily advantageous than failing to find a mate and not reproducing at all. Hermaphroditism leaves that option open.

 

I also thought of this, but did not mention it.

 

Let's suppose that during the majority of such a hermaphrodite's life, it would pursue sexual partners as normal. However, if it could not find a mate, it would resort to self-fertilization. It's not an ideal choice, but far better than its genes dying off with it.

 

Also, an important note: self-fertilization wouldn't produce a genetically identical offspring such as cloning would. The self-fertilized egg produced would have a different proportion of chromosomes than its parent. The original organism would have half its chromosomes from each parent, while the child could have possibly 3/4ths its chromosomes from its one of its grandparents (i.e. there would be several almost identical chromosomes that would pair up; I say almost because of possible mutations and recombinations). Not to mention, recombination would occur again. The end result would simply be an organism with less unique genetic information, thus having more genes in common with one of its two grandparents than its own parent.

 

Still, greatly preferable to not passing on one's genes at all, and, if it happens to be the last remaining member of its species, greatly preferable to one's species going instinct.

 

This is something that has puzzled me, too (maybe for the same reason; did you read the New Scientist article, Luminal?).

 

I was not aware of the article. I'd love to read it, though. Link?

 

Humans just happened to evolve from a species that had already developed into two genders.

 

Luck of the draw.

 

By "we" I meant all sexually reproducing species.

[CDarwin]

Also, an important note: self-fertilization wouldn't produce a genetically identical offspring such as cloning would. The self-fertilized egg produced would have a different proportion of chromosomes than its parent. The original organism would have half its chromosomes from each parent, while the child could have possibly 3/4ths its chromosomes from its one of its grandparents (i.e. there would be several almost identical chromosomes that would pair up; I say almost because of possible mutations and recombinations). Not to mention, recombination would occur again. The end result would simply be an organism with less unique genetic information, thus having more genes in common with one of its two grandparents than its own parent.

 

Good point, good point.

 

I was not aware of the article. I'd love to read it, though. Link?

 

My memory seemed to imagine it as longer, but it was just a little news update: Slow Movers Opt for a Single Sex. It apparently spurred me to some reverie on the selective value of permanent sexes, especially as there would seem to be one considering how widespread the phenomenon is in highly mobile taxa.

[ecoli]

How does it reduce genetic diversity? If anything, it would seem to increase it. If every organism is a potential mother and father, new mutations have multiple unique paths to take to enter the gene pool at large.

it increases genetic diversity, in comparison to not reproducing at all. But, decreases it when comparing to 'heterosexual' reproduction.

[Luminal]

it increases genetic diversity, in comparison to not reproducing at all. But, decreases it when comparing to 'heterosexual' reproduction.

 

Yet I don't see how a two-sex system would produce more genetic diversity than hermaphroditism. Recombinations and mutations would occur no more or less often in either system.

 

The only extra diversity in a two-sex system would come from a sex chromosome only one side possessed, such as the Y-chromosome in human males. Yet this seems to do more harm than it does good, as genetic diseases/conditions are more common without a second X-chromosome to step in. I know this well, considering I'm color blind.

[Edtharan]

Actually, sexual selection often runs against ecological selection.

I think this could be close to the mark.

 

In Hermaphrodites, the pressure of sexual selection is reduced. If you can mate with any other of your species, the the drive that pushes one gender to compete against others of the same gender is greatly reduced.

 

Sexual selection has many secondary effects that determine (or at least advertise) the fitness of the organism. For instance brightly coloured species are more easily spotted by predators, so only the most fit individuals would have the energy reserves to deal with this increased pressure. There is even a pressure on those that carry the children to not be brightly coloured so that they can more easily avoid predators.

 

Since there is not as much information to determine which members are more fit, and any member can mate with another, this would in effect reduce the ability of the species to select the most fit individuals.

 

So it would produce more diversity in the "gene pool" of the species, but the selection of the good alleles will be reduced.

[ecoli]

Yet I don't see how a two-sex system would produce more genetic diversity than hermaphroditism. Recombinations and mutations would occur no more or less often in either system.

But it doesn't introduce new alleles into a population (through migration, for example.)

 

When you're hermaphroditic, you only have the genetic material from 4 grandparents. But when you heterosexual, you have access to genetic information from 8 grandparents. That's a 'diversity potential' that's two times greater.

[Luminal]

I think this could be close to the mark.

 

In Hermaphrodites, the pressure of sexual selection is reduced. If you can mate with any other of your species, the the drive that pushes one gender to compete against others of the same gender is greatly reduced.

 

Sexual selection has many secondary effects that determine (or at least advertise) the fitness of the organism. For instance brightly coloured species are more easily spotted by predators, so only the most fit individuals would have the energy reserves to deal with this increased pressure. There is even a pressure on those that carry the children to not be brightly coloured so that they can more easily avoid predators.

 

Since there is not as much information to determine which members are more fit, and any member can mate with another, this would in effect reduce the ability of the species to select the most fit individuals.

 

So it would produce more diversity in the "gene pool" of the species, but the selection of the good alleles will be reduced.

 

Actually... since every hermaphroditic organism would be a potential mother, each would be able to sexually select who "she" allowed to mate with her, as do the females in many species currently. Then, when "she" decided to assume the role of a male and seek a mate, "he" would have to be fit in the eyes of a potential mother.

 

The way I see it, sexual selection is still quite alive in hermaphrodites.

 

However, it would require more intelligence to be able to switch between two roles, which may be the true reason it has not evolved. The simplest sexually reproducing multicellular species would have a M/F system by default because it only requires the ability to assume one role rather than two. And once you have large, complex multicellular organisms, evolving true hermaphroditism would quite a challenge, as the mature male and mature female anatomical features stem from the same embryonic source.

 

But it doesn't introduce new alleles into a population (through migration, for example.)

 

When you're hermaphroditic, you only have the genetic material from 4 grandparents. But when you heterosexual, you have access to genetic information from 8 grandparents. That's a 'diversity potential' that's two times greater.

 

What? 8 grandparents?

Edited June 4, 2008 by Luminal

[ecoli]

 

What? 8 grandparents?

oops... typed too fast. Make that 2 vs. 4, not 4 vs 8. Point is the same though.

[Luminal]

oops... typed too fast. Make that 2 vs. 4, not 4 vs 8. Point is the same though.

 

A hermaphrodite would have the same number of parents and grandparents as we do (except in rare cases of self-fertilization or incest).

 

How are you arriving at the number of 2 grandparents?

[ecoli]

A hermaphrodite would have the same number of parents and grandparents as we do (except in rare cases of self-fertilization or incest).

 

How are you arriving at the number of 2 grandparents?

ooo... I just realized we're not talking about the same thing. I was assuming self-fertilization this whole time. It's obvious now, that you talking about mating with another hermaphrodite. Sorry about the confusion, we're in agreement now.

[Edtharan]

Actually... since every hermaphroditic organism would be a potential mother, each would be able to sexually select who "she" allowed to mate with her, as do the females in many species currently. Then, when "she" decided to assume the role of a male and seek a mate, "he" would have to be fit in the eyes of a potential mother.

The problem with this is that speciation should occur faster, or the "less fit" organisms will die off quicker and the population size will crash.

 

Think of it like this:

 

Let's say that green colouring is better for hiding from predators, but a red colouring is better at attracting a mate (as an individual that could stand out to predators and still alive must be better at surviving, getting food, etc).

 

But, each organism could produce offspring (and with a developing embryo the parent needs more food and is heavier so can't run as fast).

 

There is now a strong evolutionary drive to develop the green colouring due to the fact that if a creature stands out they will not live long enough to give birth, or their offspring will not get enough nutrients to develop into healthy individuals.

 

So standing out will not be strongly selected for, even if it is a sexual selection signal (that is the creatures prefer to mate with red individuals). It will be those that have to settle for the undemonstrative individuals that will end up with the best chance of producing healthy offspring. Sexual selection will then likely switch from the Red to the green as that is what gives the best chance of reproducing (or another aspect will become the sexual selection characteristic).

 

Now, because the sexual selection will likely switch to the same as the environmental selection, this reduces the effect of sexual selection (it is the same, or similar to the environmental selection). Without that clear signal for sexual selection, the ability of one member to determine the fitness of another is therefore reduced too and beneficial alleles spread is slowed down.

 

I am not saying that sexual selection is completely removed, just that it's effects will be damped down as the need for safe gestation/nesting/etc will over ride demonstrative sexual selection systems. Also there might be some sexual selection systems that could survive (displays that can be hidden, or through behaviours and so forth).

[Luminal]

The problem with this is that speciation should occur faster, or the "less fit" organisms will die off quicker and the population size will crash.

 

Think of it like this:

 

Let's say that green colouring is better for hiding from predators, but a red colouring is better at attracting a mate (as an individual that could stand out to predators and still alive must be better at surviving, getting food, etc).

 

But, each organism could produce offspring (and with a developing embryo the parent needs more food and is heavier so can't run as fast).

 

There is now a strong evolutionary drive to develop the green colouring due to the fact that if a creature stands out they will not live long enough to give birth, or their offspring will not get enough nutrients to develop into healthy individuals.

 

So standing out will not be strongly selected for, even if it is a sexual selection signal (that is the creatures prefer to mate with red individuals). It will be those that have to settle for the undemonstrative individuals that will end up with the best chance of producing healthy offspring. Sexual selection will then likely switch from the Red to the green as that is what gives the best chance of reproducing (or another aspect will become the sexual selection characteristic).

 

Now, because the sexual selection will likely switch to the same as the environmental selection, this reduces the effect of sexual selection (it is the same, or similar to the environmental selection). Without that clear signal for sexual selection, the ability of one member to determine the fitness of another is therefore reduced too and beneficial alleles spread is slowed down.

 

I am not saying that sexual selection is completely removed, just that it's effects will be damped down as the need for safe gestation/nesting/etc will over ride demonstrative sexual selection systems. Also there might be some sexual selection systems that could survive (displays that can be hidden, or through behaviours and so forth).

 

You are primarily talking about the benefits of sexual dimorphism, not sexual selection. Yes, dimorphism is the result of sexual selection, but only one of many results.

 

Sexual selection, in its broadest sense, is an accelerant to natural selection. While a potential mate may have survived long enough to mate, it may not mean it is the best choice out of all that are available. The female is usually the selector because she can only propagate her genes to offspring once every X number of weeks or months (years if you include time to raise young), while the male can do so several times a day.

 

All of the advantages of sexual selection (with the sole exception of sexual dimorphism) would still exist in hermaphrodites.

 

The hermaphrodite would take care in selecting who it allowed to impregnate "her" as the option only came around every so many weeks, months, or years (depending on species). Likewise, the same organism would have be strongly favored to go out and augment the perpetuation of its genes by impregnating someone else, in which case it's fitness would be judged by the potential mate.

 

Regardless, the other advantages of being a hermaphroditic species still far outweigh something such as sexual selection. Simply the fact that they could bounce back from almost any near-extinction event with incredible speed would more than justify the existence of this system.

 

 

I believe the true reason hermaphroditism does not exist in most animal species is a cache-22 of anatomy, intelligence, and lack of transitional stages. Since the two-sex system only requires an organism to execute only one role each, it was easier for primitive multicellular sexually-reproducing creatures to evolve this capacity. Once this system got a foothold in the earliest days of the animal kingdom, animal evolution greatly advanced and streamlined the two-sex system, namely highly specialized anatomical structures for both sexes. Tens of millions of years later, when animals possessed enough intelligence to switch between two distinct roles in sex, the current system was too specialized to suddenly make the switch to hermaphroditism.

 

There are no transitional states from two-sex to hermaphroditism (no hill to climb). And with the extremely specific and complex sexual organs for both males and females in animals, it is difficult to see a mutation, or even several mutations, bridging that huge divide. One of the reasons it may have evolved in gastropods is that it occurred relatively early in their evolution when their sexual features were still primitive.

Edited June 5, 2008 by Luminal