I'm retouching the idea discussed in detail here Evolution of species with separate sapient and non-sapient forms?. I like the discussion had there, but want to focus in on one specific question, the biggest difficulty of the question. Context and some ideas already come up with can be found there.
To give the short version, I had the idea of a species that had a separate sapient and non-sapient phase/lifecycle to combine R and K reproductive strategies. When the species mates, it produces lots of non-sapient young which are initially left to fend for themselves to live or die; I use the analogy of tadpoles produced by frogs, and in fact refer to these individual as T phase for tadpole. The species will eventually come back to pick a small number (1-4) of the 'strongest' of the remaining tadpoles and raise them as sapient adults. Those chosen to be raised by their parents will trigger a metamorphosis which leads to developing an increased brain and sapience, but requires years of rearing from the parents as any human child would. I refer to these as S-phase, for sapient.
However, those T phase that survived and were not chosen by their parents to be raised will continue to grow & develop on their own. A small number will grow into adults, while still being T-phase, meaning they are not sapient, being more animal the 'human', think the difference between semi-complex heard species like wolves and something between human and bonobo for difference in intellect between T and S phase, very roughly speaking.
The idea is that a batch of children can lead to both T and S phase young surviving and potentially later reproducing, thus allowing the parent a choice between S and T reproductive strategies. They can choose to produce lots of T phase young, or focus on S phase with fewer matings producing T phase as needed.
I'm specifically looking at the transition period where Sapience is evolving, so the S phase is at or beyond bonobo/chimp's level of intellect, However, they have not developed intellectually/culturally further then say caveman & stone users, they don't yet have sufficient technology to be modifying their mating strategies.
For this to work, T phase must be able to grow up and mate. Either phase is capable of mating with the other phase, and two T phase can mate to produce T phase children, but only S phase parents will be able to trigger the mutation that leads T phase young to grow into S phase.
My original question was very open-ended. I want to focus this question specifically on gene-flow; as such I'm less worried about the other (non-trivial) questions of how they evolved to reach this point, or the viability of the species (I acknowledge justifying a species like this is hard evolutionarily, though I don't think impossible with work). I want this species to have an evolutionary stable mating strategy, meaning it does not lead to either only S or T phase being produced and it does not lead to T and S phase becoming separate species. The biggest problem with this is having T phase adults able to produce S phase young. While S phase adults are capable of producing many T phase young if those young do not eventually produce now S phase then evolution will not 'reward' S phase adults for producing the T phase young and thus they would evolve to focusing on producing more S phase children and less T phase ones.
Thus I'm looking for good explanations for how to ensure gene flow between the two phases, specifically with T phase adults able to produce S phase young. A few presumptions:
- S phase have a much higher reproductive success, to justify the energy expenditure.
- An S adult would presumably prefer to have S children with an S mate, since it's easier to judge the fitness of s-relevant traits (like intelligence and social interactions) in a mate who is also S phase and thus demonstrating the traits. I'm more than open to S choosing to raise S young from a T mate, but some justification for why this offers genetic fitness over focusing on S children with S mates despite the lack of ability to judge relevant traits as easily must be provided.
- The vast majority of the time an S phase adult will be required to provide for a young in order for it to develop into S phase adults, K species simply are not self sufficient as young without parental investment.
- At least some T phase will mate with other T phase as adults to produce 2 gen T young.
- T phase are not dependent on S phase to care for them, unless part of an instinctual symbiotic relationship (ie, not something S phase engineered through technology & training or intentional domestication of T phase)
I'm looking for all possible methods to encourage gene flow, specifically from T adult to producing S young. I'll start with some key points which I likely will be using to get out of the way, though I don't consider them enough.
- Cuckoldry from T phase. T phase will attempt to fertilize the eggs of, or later add their own eggs into, the 'clutch' of S phase parents, in hopes that one of the S phase parents will raise the T children's child parent by mistake. This could be with support of the female ('cheating' on the male, possible to get a fit T phase child with help of an unfit S phase male in raising it) or without. I think this actually is a great potential starter towards justifying gene flow actually! But S parents would likely have evolved pretty effective methods of avoiding cuckoldry quickly, and cuckoldry will have to be limited in occurrence or the S phase would eventually disappear entirely (why invest energy in producing S phase young over lots of T if there is a non-trivial chance that energy is expended on a child that isn't yours?). So while I'll likely use this as at least a partial explanation I don't think it can be a full one.
- Adults, particularly males, who can not find or do not want a mate may mate with T phase and raise a smaller number as S children as a 'single parent' so to speak, most common with young males who can not yet convince females to mate with them or with males/females who have something 'wrong' with them to make it hard to convince an S adult to mate with them. However, it seems like it would be rare that committing the time investment of raising a S phase child alone is better than waiting a year or two to try to get an S phase mate.
- Occasional rare occurrence of S phase adult simply choosing to raise an unrelated T young (ie adoption); I don't see this adding up to a significant number to add much gene flow.
- However, a system when S adults raise T young that is closely related to them (grandchild, nephew, half-sibling) could potentially result in 'adoptions' being frequent enough to have an effect. Though I'm not certain how to justify this happening frequently, raising a half-sibling is only 50% as effective as raising your own, why would this become evolutionarily common? Only excuse I can think is in situations where the parent cannot produce children themselves. The only example f that to occur frequently enough to matter in less culturally advanced sapient would seem to be infertility due to old age; but wouldn't evolving to stay fertile longer, or assist your S phase children in producing children, make more sense then evolving an extensive system for tracking and choosing the young of T phase to raise?
- Some sort of quid-pro-quo where parents raise, or take a chance of raising, the child of a T phase in exchange for some resource the T offers. For instance a T male will guard the 'clutch' of S parents if it is allowed to fertilize a small number of the eggs in it, with the female then piking children to raise knowing some of them will be from the preferred S male and some from the possible less preferred T male, but the advantage of a 'clutch gaurd' for her clutch is worth the risk of picking the less desirable males child? There are likely a few variants of this idea, but I've yet to get one that felt viable and evolutionarily stable, particularly if one assumes males help in raising of S phase children since they would evolve to avoid agreeing to raise a child that isn't genetically theirs. Possibly combine this with cuckolding idea could work though..
Some more 'extreme' options I've debated but am not certain are ideal which someone may be able to expand on.
- Sequential hermaphrodites, with T phase all being female and S phase all male. Geneflow is guaranteed, but this would almost certainly result in 'wife husbandry' sort of harems being genetic/instinctual in males that I'm not certain rather or not I want in regards of making an interesting species I'd enjoy writing. I want the sapient species interacting socially as humans due, not in constant competition over winning the other male's females into his harem.
- Many other variants of unequal distribution of sexes I've considered. If I can get a system with s females, but more S males then females, it can work well, but fisher principle makes that difficult.
- Some natural symbiotic relationship between T and S such that the two are not mostly separate outside of mating season, though doing it as naturally symbiotic and not 'domesticated' T's being husbanded by technologically developed S is harder, and it doesn't solve gene flow as simply allow some interesting variants for how mating can occur due to the inherent closeness.