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Imagine life evolving on a planet. First creatures crawl out of the ocean and start colonizing the land. Then one of the species gets into such conditions where it needs to specialize in two different tasks simultaneously and that specialization cannot be covered with sexual dimorphism alone.

In other situations this species would've become a "jack of two trades", find a symbiote or just go extinct, but a lucky mutation happens and this species gains an additional dimorphism capability based on a different pair of chromosomes. So now in addition to XX/XY variety they have, say, AA/AB variety while remaining a single species.

Problem solved. AA-type evolves into one task, AB-type evolves into another and the diverse, yet single, species gains an advantage over their rivals. So in a few million years all tetrapods on the planet have this non-sexual dimorphism feature.

So the questions are:

  1. What realistic circumstances could cause this dimorphism to appear?
  2. To stabilize and consolidate this feature the evolution has made it so that only cross-type couples can breed successfully, i.e. only XXAA/XYAB or XXAB/XYAA couples can have offspring. How to make this more plausible and what are possible evolutionary dangers here?
  3. What else should I take into consideration to make this scenario more scientifically plausible?
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    $\begingroup$ Occam's razor: what you describe can be achieved with current dimorphism, so there is no need to have a more complicated one. $\endgroup$ – L.Dutch Apr 15 '18 at 17:13
  • $\begingroup$ @L.Dutch Well, that's what I'm asking about: how to make a plausible situation that will be covered by this kind of dimorphism. I'm not saying that other solutions should be impossible, I'm trying to make this particular outcome look as scientifically solid as possible. $\endgroup$ – Verence Apr 15 '18 at 17:24
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    $\begingroup$ Do eusocual insects count? They’ve kinda cracked this already (ok, their males are mostly monomorphic, but hey) $\endgroup$ – Joe Bloggs Apr 15 '18 at 18:05
  • $\begingroup$ You mean exactly like bees and ants have on this Earth? $\endgroup$ – AlexP Apr 15 '18 at 18:40
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    $\begingroup$ You've stated you want this to be about genetic XX/XY AA/AB dimorphism but I just want to put this idea out there. There could be situations in which young of the species mature into different adults of the species. If I'm not mistaken, sea turtle eggs of specific temperatures become males or females. Maybe eggs laid in or mothers exposed to certain conditions could produce young that are radically different from one another. Like a cold weather and warm weather variety of a species that could still breed if they were to meet. $\endgroup$ – Dispenser Apr 16 '18 at 18:14
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The side-blotched lizard (Uta stansburiana) more or less does what you want, apart from the restricted pairings in your point 2. Wikipedia summary of the lizards

Males come in 3 types with 3 different behaviours:

  1. Orange throated. Aggressive. Pick fights a lot. Each male defends a territory and harem of females and really doesn't get along with his neighbouring males. But they are a bit dim and don't notice that yellow-throated males are male - they think they are female.
  2. Yellow throated. Don't hold territories. Tiptoe about sneakily mating with females when the orange throats and blue throats aren't looking. (Their sperm also survives inside the female for ages, so a female can mate with a yellow male once, and lay 3 different clutches of eggs fathered by him). However, yellows can't fool blue males, and will be chased away by them.
  3. Blue throated. They have a small territory with one female (monogamy). Get along quite well with their blue neighbours, as they are not very aggressive. They are smart enough to spot sneaky yellow males, but are beaten in fights by orange males.

It is called a rock-paper-scissors strategy.

Meanwhile the females also come in yellow and orange throated, with physiological/ecological differences:

  1. Orange females have large clutches of small eggs. If you need a population explosion, orange females are your go-to-girls.
  2. Yellow females have small clutches of large eggs. They can become 'egg bound' by the large eggs and die giving birth to them. If conditions favour a stable population, the yellows are the best choice.

EDIT: Oooh, just thought of a way to have your mating restrictions - make them marsupials (like kangaroos or seahorses). Both sexes of your creatures have pouches and can nourish the young in them, but AA have small pouches and AB have big pouches. You start the baby (or clutch of eggs) off in the small pouched parent and transfer them to the big-pouched parent later.

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  • $\begingroup$ There's a little fish in the African rift valley lakes which has a two-way split, with smaller "effeminate looking" males darting in and inseminating the eggs of females that belong to an alpha male's harem. $\endgroup$ – RonJohn Apr 17 '18 at 10:13
  • $\begingroup$ @RonJohn Thanks. Giant Australian cuttlefish 'female impersonators' do the same trick. $\endgroup$ – DrBob Apr 17 '18 at 15:57
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One could make a case that many tetrapods have exactly the dimorphic feature you describe. The morphs are juveniles and adults.

juvenile croc with mom

It is so familiar to us that we forget it is not necessarily so. But other animals have young born as essentially miniature adults: worms, echinoderms, mollusks.

A juvenile crocodile does not compete with adults for resources. The adults would not bother with the bugs and worms that nourish the juveniles. There are examples from the insect world too: I once raised a praying mantis that lived on fruit flies as a juvenile and ignored them completely as an adult, preferring crickets.

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    $\begingroup$ I'd say an even better example is insects that undergo metamorphosis. If you combined the concept of metamorphosis with mammalian puberty then you could have a species which changes radically at a certain point in it's life cycle, with both forms having different roles. You could probably even explain the inter breeding between the two forms $\endgroup$ – adaliabooks Apr 15 '18 at 23:09
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    $\begingroup$ @adaliabooks: you are so right about the insects! the metaphorphosis thing is extra cool too and that would make a fine SF concept. Post that as an an answer and I will take this one down when I see it. $\endgroup$ – Willk Apr 16 '18 at 1:28
  • $\begingroup$ No need, feel free to add it to your answer (that's why I commented rather than making a separate answer anyway). $\endgroup$ – adaliabooks Apr 16 '18 at 6:10
  • $\begingroup$ The three groups of animals you name all have significant different larval and adult forms: worms start out as a pilidium/trochophore, echinoderms have an echinopluteus or ophiopluteus, and mollusks have veligers. Perhaps you’re looking for something like the silverfish (Zygentoma) which has an ametabolous life cycle? $\endgroup$ – Dubukay Apr 17 '18 at 15:24
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several species have three forms with two different male forms, one built around combat and large with other built around sneaking and is small. This is sometimes referred to as trimorphism. Often in these cases the individuals carry all the relevant genes for both forms and conditions in development or childhood trigger one or the other suite of genes.

Then you have caste system in insects which can have several forms performing specific roles.

Multiple morphologies in biology is not that uncommon, Galapagos finch beaks are good example, it is just that if they are not attached to sex somehow they tend to form distinct species over time.

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The primrose is an example of non-sexual dimorphism (called heterostyly in this specific case): Both morphs have both male and female organs, but they differ in the positioning of the organs: One morph has a long stylus and low lying stamina, the other morph has a short stylus and high stamina. This guarantees cross-polination.

Upper left and right: Showing the two morphs of Primula veris

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I am not a geneticist, but let's take a crack at it.

This kind of non-sexual dimorphism is roughly (very roughly) akin to blood types. Ignoring for the sake of my argument the universal donor type (O-) and the universal recipient type (AB-), you can't transfuse different blood types without, well, basically killing the recipient.

Therefore, let's use the idea of blood types to forward the idea you're presenting. Let's suggest that your world has only two blood types, and that for successful reproduction it is required that both blood types be present. To avoid confusion, let's call them K+ and K-. Therefore, reproduction requires

  • A male of type K- and a female of type K+ or,
  • A male of type K+ and a female of type K-

No other combination of people will conceive children. This basically addresses your second bullet point.

Now let's try for bullet point #1.

Let's assume your planet has a bacteria or parasite that can only be rendered inert by the presence of both blood types.

(Remember, I'm not a geneticist, or affiliated with the medical world in any plausible way, so someone practiced in the medical arts might find this reasoning flawed.)

Let's run with a bacteria that is capable of feeding off of either blood type. However, once it starts feeding on one, it changes such that that one type makes it intolerant of the second blood type. This would protect the embryo until a viable immuno system develops that can naturally handle the bacteria (which is why no one is affected by the pestiferous bacteria after birth).

The bacteria survives because there are plenty of male.K-/female.K- and male.K+/female.K+ couplings to allow for its survival. The couplings may result in an infertile embryo, but an embryo is created nonetheless, one that the bacteria can feed on.

I can't address bullet point #3 because I have nowhere near enough insight into physiology and evolutionary genetics to even think about what you might be missing.

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