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I know that Tau society forbids *ahem* intimate relationships between members of different castes, but do we know if different castes are capable of producing offspring together? And if so, what the resulting children look like? We know that earth caste are basically always stocky, fire caste are tall and muscular, air caste are tall and willowy, and water caste are... none of the above. But what would the offpsring of a fire and air caste look like? Or earth and ethereal?

Given that tau are apparently capable of absurdly rapid evolution if not some sort of more overt genetic adaptation (see: air caste formerly having wings), I sort of wonder if combining the genetics of multiple castes might have a dramatic impact on the offspring's anatomy.

I doubt they've genetically diverged enough to have speciated and become reproductively incompatible.

But they have definitely diverged phenotypically, although so have humans if you look at geographical morphological groups on earth.

I would say they are probably similar to the neanderthal/denisovian/human speciations.

Remembering that horses and donkeys / lions and tigers are capable of breeding to produce infertile offspring and they are species separated by millions of years of divergence.

If tam psychology is similar to human, then it's a brain plasticity rather than genetic thing. which means upbringing is more important.

So tau behaviour would not be innate.

Physiology though would definitely be different.

The Air Caste has changed radically (see below). However, like Hellebore, I doubt they've speciated. Humans have Voidborn and it is implied they are still human, and so able to breed with non-Voidborn.
Air Caste

The Air Caste lives almost entirely off-world, and due to their constant exposure to zero and low-gravity environments, they have developed long, slender limbs and hollow bones.

This results in their possessing a skeletal and muscular structure of reduced strength compared to the T'au genetic baseline.

Prior to the unification of the T'au castes, during the time of the Mont'au, the Air Caste were said to possess membranous wings, allowing them to glide on warm air currents. They still possess vestigial remains of these membranes.

I think it's a stretch to think that Tau have diverged to such a degree that they aren't capable of procreating between castes anymore, considering we were capable of procreating with Neanderthals once.

Inter-caste Tau offspring would probably have similar results to interracial offspring among humans, like how the child of, say, a tall, pale Dutchman and a short, dark-haired Asian will have characteristics of both parents to varying degrees. So I imagine that a mixed Earth/Air caste Tau would be taller and slimmer than a pure-bred Earth caste but stockier than a pure-bred Air caste.

I think we could assume that among the various T'au's castes, is happened something similar to what happened among the Pink Iguana and the Yellow Iguana, which live on the Isabela Island in the Galapagos: they could interbreed without problems, but they don't because their chemical signature is so different that prevent this event.

Chemical signatures of femoral pore secretions in two syntopic but reproductively isolated species of Galápagos land iguanas

Like Hellebore says, it would probably be a zonky/liger situation, where they could breed but might result in genetic issues.

The_Grim_Angel wrote:
I think we could assume that among the various T'au's castes, is happened something similar to what happened among the Pink Iguana and the Yellow Iguana, which live on the Isabela Island in the Galapagos: they could interbreed without problems, but they don't because their chemical signature is so different that prevent this event.

Chemical signatures of femoral pore secretions in two syntopic but reproductively isolated species of Galápagos land iguanas

This is an excellent take! I'll buy into it. With ancient and deeply entrenched taboos against intermixing as cultural gravy on top.

Pheromones playing a role also rhymes well with certain Imperial theories as to Ethereal Caste control.

Also, I am certain that Tau prevents rare accidental mixed Caste members from passing their genes on in order to preserve the four distinct Castes, both for reasons of tradition, ideology* and pragmatic maintenance of Caste specialization. Whether it be by a ban on breeding, sterilization or discreet euthanasia. The latter seem very likely, since it is exactly the kind of secret shady dealings we should expect from the Greater Good, and would result in no one ever seeing inter-Caste freaks of nature.

___________
*

I could easily see Tau having no particular objection to mixed-caste couples, but having a complete lockdown on breeding, possibly to the point of regulating children by artificial insemination that completely ignores bonded pairs/groups.

And ethereal-run 'education centers' where children are educated and reared in batches... for the greater good.

I guess I'll be the only one.

Speciation can be complicated to define and use. And it isn't really binary.

5000+ years is a lot of time for sufficient genetic drift to cause issues with reproduction (hormonal/pheromonal incompatibility, physiological incompatibility, genetic incompatibility, social incompatibility, etc.), IF your Empire explicitly practices society- and species-wide eugenics with an EXTREMELY hands-on approach, with very distinct roles envisioned for each separate caste.

Some of those roles may well have been close enough that the diversion isn't as pronounced, while other diversions in role resulted in some pretty substantial extremes in phenotypical (and potentially genetic) structures.

It's entirely possible that some castes can interbreed while others can't, or that some castes can mate with at least some castes, which can then mate with the other castes that the original caste couldn't mate with.

The scale is absurd for natural evolution, don't get me wrong, but there's nothing natural about the control over the species exercised by the Ethereal caste.

As there's no evidence of the empire controlling internal caste reproduction, the only thing that will change caste genetics is genetic drift.

They don't seem to prevent reproduction by members of a caste for not being caste-y enough.

So the best you have is the genetic diversity found in the castes at the point they were culturally segregated, and then a few thousand years of genetic drift within the castes.

 Unusual Suspect wrote:
5000+ years is a lot of time for sufficient genetic drift to cause issues with reproduction (hormonal/pheromonal incompatibility, physiological incompatibility, genetic incompatibility, social incompatibility, etc.), IF your Empire explicitly practices society- and species-wide eugenics with an EXTREMELY hands-on approach, with very distinct roles envisioned for each separate caste.

Uh, no. 5000 years is barely 1/5 of the time Chinese, Europeans, and Americans were separated on their home continents. Last time I checked, we have no problems reproducing. Hell, Homo Sapiens could reproduce with other Homo species despite hundreds of thousands of years of separation (and likely owes our success outside of Africa to genes we got from them).

 BaconCatBug wrote:
Like Hellebore says, it would probably be a zonky/liger situation, where they could breed but might result in genetic issues.

No. These have millions of years of separation, not to mention being separated by whole continents. If even that vast gulf doesn't stop reproduction, nothing Tau can do naturally will.

And there are issues, yes, but modern medicine could easily solve it. There are fertile mules and ligers, even modern methods of genetic screening could ensure these would be the only embryos selected if we wanted to. Tau wouldn't even need to rely on screening, they could easily fix the problems themselves...

Based on the hard facts we know about the tau, one of two things is true: they operate under completely different rules of evolution, sociology, etc....or GW wrote some deeply implausible stuff. Given that one of the major races in 40k are sentient humanoid fungi, I'm gonna go the latter.