Speculate on why more animals haven't evolved the hermaphrodite model

I was looking at futanari porn and thinking about evolution the other day.

With the way evolution works, beneficial characteristics usually end up surviving in a species because it helps them adapt to their environment. If that is so, wouldn’t efficiency dictate that more animals evolve into hermaphrodites? Instead of having to find a mate, you’d reproduce with at a near 100% rate.

But then evidence shows this not to be so, and that got me wondering why that is.

My WAGs:

  1. While more efficient, its more complicated internally. Having to have 2 sets of sexual organs means twice as many things can go wrong.

  2. It’s more beneficial to the offspring with 2 parents. Even though not all animals stay together once the babies are born, or even conceived, in the cases that they do, it helps survival rates.

  3. Birth is dangerous for the mother. In a vulnerable time such as childbirth, it helps to have someone else guard the cave and hunt for food.

So is there some kind of scientific consensus that says why more animals aren’t hermaphrodites?

You’re missing the biggest point, which is the lack of genetic diversity. Asexual reproduction, which is essentially what you’re talking about, doesn’t allow for the mixing of genetic material from different lineages to create new combinations of gene alleles. This is a huge benefit, evolutionarily.

I should point out, too, that there are many many species out there across all of life that do reproduce asexually - think of plants, for instance, many of which will have both male and female reproductive organs and can self-fertilize. So it can work under the right conditions.

Hermaphrodism isn’t asexual reproduction; it’s having bot sexes in one individual.

I’m not a biologist; but an explanation I’ve read is that pure males and females outcompete hermaphrodites due to the advantages of being specialists. Males and females have different Darwinian strategies, which leads them to need different adaptations physical and behavioral. A single gendered creature can focus on one set of adaptations and take them farther than an individual built for both.

To use humans as an example; a man can be bigger, stronger, and more built for physical performance ( narrow hips for example ) than a woman because he’s not lugging around biologically expensive extra reproductive organs or specialized bone structures like wide hips. A woman can invest as much biological resources as she does in producing children - much more than a man does - without, say, dropping dead right after birth because she’s not lugging around a lot of excess muscle and bone and such.

I halted at the word usually in " … With the way evolution works, beneficial characteristics usually end up surviving …’ usually?

I am not to keen on these things but I rather suspect, guessing actually, that one could forth some argument against “usually”.

Lots and lots of animals, and most plants are hermaphrodites. Note that hermaphoditism is not the same as asexual reproduction, even if the hermaphrodite is capable of self-fertilization.

It’s true that among vertebrates hermaphroditism is very rare, but it isn’t as rare as you might think.

Clarify for me. Is your objection that I said “usually” instead of “always” or “usually” instead of “rarely”?

Granted asexual reproduction isn’t technically the same thing as self-fertilization, but the main point still stands – genetic diversity that comes from having two parents and thus more options for recombining DNA into new individuals is a huge advantage. A lot of genetic diseases are recessive, so that all you need is one parent without the damaged gene to not pass on the symptoms. On the other hand, if a carrier of a genetic disease self-fertilizes, proportionally more offspring will end up with two copies of the bad recessive gene.

That’s just one example. There’s also the fact that having a variety of genetic / physical characteristics in the population allows that population more flexibility in adapting to changing environmental conditions. If only a few individuals in the population have poor tolerance for cold, only a few will die off during the next ice age, as opposed to the entire species going extinct.

Again; none of that necessarily applies to hermaphrodites. A hermaphrodite is just as capable as mixing genes as any single sexed critter.

I simplified it to asexual reproduction because they’re basically the same thing from an evolutionary perspective. They both reduce your potential gene pool to whatever is contained in that one organism. Hermaphrodites can rearrange what they have via recombination, but they cannot gain any additional genetic material faster than through mutation. In a very real sense, each individual organism is its own evolutionary lineage, separate from all other individuals, assuming that this method of reproduction is the only one ever used. This is in contrast to true sexual reproduction, where you have a whole population’s worth of genetic information available to the next generation. Both systems can shuffle their cards, but in a true sexual species - ie with two parents - the deck of cards is exponentially larger. I should have spoken more precisely.

Ah, I just figured out what you are saying - hemaphroditic species could still interbreed between individuals, like slugs do, for instance. Yes, of course that’s true. Sorry. I had a bit of a blind spot there for some reason. My bad.

I think my thinking locked in because of this:

Which said to me that we were talking exclusively about self-fertilizing hemaphrodites.

So, why don’t we see more interbreeding hemaphrodites? Let me ponder on that one for a bit. I’ve got some ideas, I just need to think them through fully this time to make sure I make sense.

Incidentally, I’m a grad student in a lab that does a fair bit of evolutionary stuff, just for the record.

Sure, but the OP specifically said “Instead of trying to find a mate…” which is self-fertilization. So I responded as to why doing without finding a mate wasn’t better than needing to have one to reproduce, from an evolutionary perspective.

Since the OP wasn’t addressing fertilization occurring between two different individuals, neither was I.

Human sexual dimorphism is an extreme example, though. In many (most?) vertebrate species, males and females are much more similar than they are in our species.

Aside from the hermaphrodite issue:

That’s not how evolution works. You seem to be assuming that if a characteristic is “efficient,” it will appear, and be passed on to offspring. But why would it appear in the first place? Humans would be much more “efficient” if we had an extra hand, an eye on the back of our heads, and wings. But someone has to actually be born with these characteristics, and the characteristics have to exist for genetic reasons (as opposed to in the womb), and they have to be advantageous enough to survive. A whole lot of “ifs.”

The vast majority of animals are hermaphrodites. At a guess I would say that 80% of all animal species are hermaphrodites.

The real question is why a minority have abandoned the system in favour of single sexes. And the answer seems to be down to simple cheating. Normally a male or female gamete has the same approcimate chance of reaching reproductive maturity. However if an animals devotes itself exclusively to producing a single type of gamete it can become much more efficient and gain success rates above 50%. One that happens that gamete type starts to predominate, a which stage it becomes advantageous to cheat the other way, since there are more partners available.

Now I remember that theory.

You have a population of hermaphrodites. One hermaphrodite stops producing eggs, and starts producing only sperm. The sperm-only hermaphrodite can now produce more offspring because eggs take more energy than sperm. As males start to predominate, the value to the remaining hermaphrodites of producing sperm themselves becomes less and less, eventually they stop bothering because there are so many males.

The males and females stay in rough balance because if there are more individuals of a particular gender, members of the other gender have greater reproductive success. Too many females and each male fertilizes many females, so any mutation that increases the likelihood of being male is selected for. Too many males and the excess males can’t find females to mate with, so females are selected for.

And as for why there are only two genders instead of three or four or five, well, in some fungi there are such things as “mating types” which could be considered a type of gender. But there are the obvious disadvantages of sexual reproduction–your offspring are only 50% related to you, you need to find a mate, and so on. Sexual reproduction also has it’s advantages, but it is very common for species to switch off between asexual and sexual methods. Three offers no more benefits than two would. So even two genders seems about half a gender too many.

I probably worded my OP awkwardly but I didn’t want to come off as an arrogant know-it-all :smiley: I should have said that it seems billions of years is sufficient time for hermaphroditism to develope at least a couple of times, so why wasn’t it retained in more species?