No I am not. I am arguing that any organism that is capable of giving birth to three types of individuals genetically related to it with all three serving distinct and required reproductive functions would be considered to have three sexes. Honeybees almost fit, however strictly speaking worker bees are not required for reproduction. Personally I would consider honeybees to have three sexes – drones, queens and workers.
There doesn’t have to be a direct advantage. Just insufficient disadvantages to lose any particular mutation. Life has no goals, and evolution has no design guidelines. Most of the processes that go on in your body are not there because they are currently the most efficient solutions to some well defined problems – they are there because they mutated at some point and were not selected against sufficiently to be eliminated.
I was addressing the OPs problem with conceptualizing a third gender as well as the poster above that mentioned problems/solutions using specific sex chromosomes. I am aware that there is a plethora of species without such differentiation.
Just off the top of my head I can imagine a mutation on a Y chromosome that creates some bizarre protein that
[ul]
[li]Makes a normal offspring of a male carrying mutation A not viable (or infertile)[/li][li]Creates a much greater likelihood of a viable chimera offspring between an egg with mutation A and an egg fertilized by a male that does not carry A[/li][li]Gives some advantageous immunity to an individual who carries A [/li][/ul]
That sort of thing, however unlikely, would begin one type of split into three genetic contributors. Males carrying A have advantages but need non-A’s to mate with the same female to pass on that advantage.
Sexual reproduction is disadvantageous to the individual’s contribution in general. That does not mean it could not evolve, as it clearly did. If the advantages of every mutation on the way to having three genetic parents outweigh the disadvantages it can evolve – even if none of the advantages have anything to do with having three genetic parents.
Honeybee queens give birth to lots and lots of children that will not get the chance to pass on the queens genes. Certainly this ability evolved not through a descent line of a sterile worker bee but rather through her sibling’s comparative advantages.
I know that in haploid-diploid insects, the haploid individuals are usually males. However, imagine a species where the haploid larva is female and carries an egg, when the egg is fertilized, the individual enters the pupal stage, the haploid parts are destroyed and the egg develops into a diploid adult. Diploid females always lay unfertilized eggs that develop into haploid female larva, and the diploid males only mate with the larva. There you go, three sexes, stupid and unnecessarily complex life cycle (like most of insect life cycles), and a good foundation for a really creepy social insect.
No, that is not what you are arguing . You specifically said "I don’t understand why many posters are insisting on some sort of genetic contribution by the third individual. Ii even quoted that for you. So don’t now claim that you were referring to a situation where all three individuals make a genetic contribution. You very specifically said that was not the case.
Well this is GQ, so can we have a reference for this belief of yours?
As we have pointed out ad nauseum there will alwyas be a direct disadvantage: a loss of 17% of genetic input. that’s a massive evolutionary disadvantage.
So we can scrap that idea.
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Then you should also be aware that they are never considered to be different sexes.
Nope, doesn’t address the problem at all. Whatever this mutation confers immunity to must be lethal to over 50% of offspring just to make it evolutionarily viable. But only one of these mutations exists in the whole world when the catastrophe strikes. With 50% mortality the survivors are going to be selecting for their own survival genes far faster than you mutation can disseminate in the population.
Not only that but any female that actively resists mating with such a male (or simply refuses to mate more than once) will be giving here male offspring as big an advantage as the mutation gives them. Your mutation increases their survival by 50%, but it reduces the number of offspring they can have by an equal amount.
So simple sexual selection will eliminate such a mutation in short order.
And this is the point we’ve been hammering home. The mutation can give no benefit, it’s just parasitic. There must be an equal number of non-carriers for it to “infect” just to survive. There can never be more of these mutant males in the breeding pool than there are naturally resistant males that they can cuckold. As such it can never confer any more survival benefit than the mutant males already have. Even if it confers a thousand fold increase in survivability then it will simply have a thousand fold fewer hosts and will have to reduce its own population down to that of the host.
In short the parasite can’t produce actually more offspring than the number of non-infected males. All it can do is reduce the number of offspring the non-infected males have. In very short order sexual selection and plain old fashioned natural selection will eliminate it completely.
I can trace this out for you generation for generation if you wish.
Yes, but only to a degree that is compensated by increased diversity. OTOH a third sex of the type you propose a never compensate for the decreased diversity because it is entirely dependent on the existence of equal number of non-carriers.
Look, all sexually produced offspring has double the survival odds of an asexual one because they have double the gene pool and thus can very rapidly dominate the population. In short order the entire species can become sexual reproductives carrying sexual reproduction genes.
You tri-sex hypothetical requires that it always be in a minority. As soon as there are fewer standard males than the mutants the mutant population must decline to that level. It is a predator-prey or host-parasite relationship. When the host population crashes so must the parasite’s. It can never be more successful than its host because there is a direct 1:1 relationship to the host built in. The population can never, ever consist of more than ~30% of individuals.
In contrast any individuals carrying resistance genes without the parasite is 50% or 30% more succesful reproductively, and they can make up the entire population. IOW your third sex is at the mercy of the host population, but the host can and will try to eliminate the parasite.
Exacatly, and the entire honeybee population is now made up of such individuals.
But the entire population can never be made up of your third sex individuals or carriers thereofo, because by definition they need equal numbers of non-carriers to parasitise. In contrast no honeybee needs any honeybees that don’t cary the colony genes. they are entirely self sufficient and able to outcompete such individuals.
Simple questions:
What is the maximum percentage of the bee population that could be made up of colonial bees? Is it possible to have a landamass where 100% of bees carry the colonial genes?
What is the maximum percentage of the population that could be made up of your third sex? Is it possible to have a landmass where 100% of individuals carry the tri-sex genes? If so then what exactly are they mating with, since the males and female are mutually sterile?
If you can answer those questions you should see why such a trait, even if it could evolve, will be eliminated in short order and offers no reproductive advantage.
Nope, makes no sense. If a diploid male mates with a haploid larva the result will be triploid, which you have neglected to include. I assume when you say “mate” you mean use haploid sperm to fertilise. If that is so then all you’ve done is describe the normal human lifecycle using non-standard terms.
It’s not three sexes.There’s nothing creepy about this, it’s where we all come from.
The haploid larva (read polar body) is female and carries an egg (remember the ovum doesn’t complete mieosis until after fertilisation, so the unicellular polar body carries the ovum as a separate cell). When the egg is fertilised the individual enters the pupal stage, the haploid parts (polar bodies) are destroyed and the egg develops into a diploid adult. Diploid females always lay unfertilized eggs that develop into haploid female larva, and the diploid males only mate with the larva.
And there you go. This is not three sexes. It is the standard human life cycle with the standard alternation of generations common to almost all multicellular organisms.
The haploid larva (gametophyte) is female and carries an egg, when the egg is fertilized, the individual enters the pupal stage (sporophyte), the haploid parts are destroyed and the egg develops into a diploid adult. Diploid females always lay unfertilized eggs (spores) that develop into haploid female larva (gametophytes), and the diploid males only mate with the larva.
There is nothing in any way unusual about the situation you describe. I can’t think of any muticellular organism that doesn’t exhibit this lifecycle. But nobody considers that we have more than two sexes.
And once again, where this differs from your hypothetical above is that there is no competition between the “sexes”. Producing a polar body or gametophyte doesn’t reduce the fitness of the female or the male, and the entire population can be (and is) made up of individuals with genes coding for this lifecycle. Nobody depends on individuals with a third distinct genetic makeup.
I think the trouble is that you are thinking that alternation of generations somehow equates to a different sex. It doesn’t. We alternate between three. Some organism alternate between six different generational forms. Nobody describes the generations as sexes.
That’s the tricky part, I’ll have to agree. I’ll concede that it’s extremely unlikely that anything resembling terrestrial animals will ever evolve three sexes from two.
However, that doesn’t preclude something like it evolving from the same conditions that sexual reproduction evolved on Earth in the first place. Instead of 2 cells fusing to form a diploid organism, you could just have 3 instead. Over time these 3 cells might have differentiated into different roles, just as our sex cells have. Would you accept this setup as three genders?
Sure, but we have to remember that three gametes doesn’t mean three phenotypes. You could have an organism capable of producing all three types of gametes.
But the fatal flaw for any three gamete systems is that the optimum number of genders is probably lower than two. There are obvious advantages to asexual reproduction…all your offspring are 100% genetically identical. And with two genders, your offspring are only 50% genetically identical. And we see over and over again in the natural world that organisms that have sexual reproduction secondarily evolve asexuality, or preserve asexuality.
Sex is advantageous for several reasons. But you don’t need very much sexual reproduction to reap the benefits of sexuality. The only reason organisms frequently have two genders is that two genders is the minimum number of genders you can have and still have sexual reproduction. Adding more complexity than that is counterproductive. If there was a physically possible way to have sexual reproduction with fewer than two mating types I’m certain that we’d have fewer than two. So a three gendered species is logically possible, but any mutation that eliminated one of the genders would be so evolutionarily successful that it would rapidly spread through the species until only two mating types were left.
Thats why we don’t see three genders. It has nothing to do with physical impossibility, or logical impossibility. It is because mating systems with only two types will rapidly outcompete systems with three types, since mutants that support two types are 17% more successful (50% vs 33%).
About a system requiring a third gender acting as a host.
Just because a % of the offspring don’t reproduct, doesn’t make it an evolutionary disadvantage. What % of bees or ants ever reproduct? How about the wolves, where even though all animals CAN reproduct, mostly the alpha pair do? If that % of non-reproducting offspring increase the survival rate of the rest of the offspring, where is the disadvantage?
Let’s look at a male/female/host set up. What if the host was able to carry the fertilied eggs of more then one female. Let’s randomly pick 3. Offspring ratio - 3 male, 3 female, 1 host. You could claim a 14% loss. Or you could claim about a 1/3 gain, if it freed up the females to reproduct more frequently. On top of that, the females now no longer have to have the equipment to incubate the egg. So more energy and resources are available to her to product the next egg.
Or how about a set up where the host never leaves the hive, where it is well protected, thus reducing the loss of host and young. Kind of like a queen bee.
Maybe a host who is able to eat a different set of food. You could have light weight flying males and females, and a non-flying host. Put them on different foods, and you’ve expanded the resources available to the species.
Heck, how about different intelligence levels. Male and female gender are ephemeral, seasonal creatures. Little intelligence. They are gonads with legs. Maybe like a mayfly. The hosts are sturdier. Larger bodies, bigger brains, longer lives. They could carry the fertilied eggs through the non-optimal season, making longer gestation possible, as well as increased protection.
That’s quite a compelling argument. I don’t dispute that the advantages of 2 genders over 3, from both a genetic and a logistical standpoint. That being said, I think there are a few advantages in having 3 types of gametes in a triploid organism. The main advantage would be more possible combinations to increase variation. There may also be more redundancy against harmful mutations, but I suppose you can have that by the polypoidy alone. There might also be something in the fact that an organism can contribute more than 1 gamete to a given mating. With 3 gametes, one organism can theoretically provide 66% of the genetic materials. There might be some competitive advantages in that, though I’m not quite certain if my logic works. All in all. while I doubt that these are enough to overcome the disadvantages, it’s there.
Any existence of tri-gendered organisms is sure to be the exception rather than the rule. Nevertheless, Evolution is a chance driven process, and however unlikely a feature is on paper, given the wide varieties of biochemistries possible, and given some combination of environmental factors, I think it’s still possible for tri-gendered species to exist.
Whether it would be advantageous or not, the snakes in Vonda N. McIntyre’s novel, Dreamsnake, had three sexes, one of which contributed extra-neuclear genetic material.
Sure, there’s a big advantage in being a double parent in a tri-gamete system. Which is why just about every organism in a tri-gamete system is going to try to be a double parent and prevent everyone else from being a double parent. The evolutionarily stable outcome for this battle is a two gamete system.
Where am I “now referring to a situation where all three individuals make a genetic contribution”? Three types of individuals that are serving distinct and required reproductive functions does not mean they all pass on their genes. The ability to give birth to three sexes does not require the third sex to pass it on. The ability can be encoded in all three sexes, and as long as it gives no disadvantage.
I think I’m a fairly good reference for statements I believe are true, but I will bite:
Since we are discussing the concept of a ‘third sex’, we are left with a definition of ‘sex’ that does not rely on defining it as the differentiation between males and females. As such, we are left with something like:
from quick definition at OneLook.
The reproductive roles of worker bees, queens and drones are distinctly different. They are also organisms. What do you find difficult about such a belief?
As I have implied, sexual reproduction using two sexes results in 50% loss of genetic input, so why don’t you go scrap THAT idea instead?
I’m going to ask for a cite. That is a ridiculous statement.
Yes, but only to a degree that is compensated by increased diversity. OTOH a third sex of the type you propose a never compensate for the decreased diversity because it is entirely dependent on the existence of equal number of non-carriers.
No, they do not. On average over time the genetic contribution % of asexual reproduction are slightly lower. That tells you nothing about the survival odds of all sexually produced offspring, much less any sexually produced offspring. Any parent that outlives their child due to the child developing some genetic disorder for which the parent was only a carrier disproves that the offspring has double the survival odds. If the offspring was a clone of the parent they would be a lot less likely to die before the parent, no?
So? That’s a red herring. There’s a lot less queens than any other type of bees and they are a distinct minority. So what? Are they not a sex now?
Just like the entire population cannot be made up of males or carriers of the Y chromosome. Doesn’t stop us from having two sexes.
Good questions. Have nothing to do with feasibility.
No, it wouldn’t. Using your logic widespread genetic diseases would not exist, and every single feature we have offers a reproductive advantage.
That’s what one usually means when they say “mate”, yes. But no, I did not describe normal human life-cycle using non-standard terms. Neither sperm nor eggs are ever external individual organisms for humans. A haploid larva that eats, moves around and is clearly a distinct organism is a sex. If it pupates and turns into a diploid it is different genetically, a completely different individual, you cannot consider that just a ‘life stage’ like in case of normal metamorphosis.
I think the trouble is that you are begging the question. You want to define sex using male and female, and then reject any third sex because it obviously does not fit the definition. Physiological differentiation between different organisms of the same species that results in a distinct and different reproductive role is a sex.
You also keep implying that evolution will always find some better solution. Evolution has no goals and no reasoning. Everything that is not a sufficient detriment to be eliminated will stay. Nothing that doesn’t mutate by chance will evolve. Evolution is not a process that yields ‘most efficient’ or ‘most fit’. It always yields ‘most sufficiently fit’ from the set of ‘randomly mutated’ – which is very unlikely to be any sort of efficient.
