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.

  1. 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.
  2. 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.
  3. 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?
  1. 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.

  1. 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.
  1. 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.

2 Answers 2


I feel like this type of breeding behavior would make the most sense if there were specific points in time when T-phase offspring were optimal to produce and other points in time when S-phase offspring were optimal. If these two reproductive strategies were both critical to the success of the species, both would be preserved.

One way to obtain such a pattern would be with a cyclical climate, which altered between an incredibly lush climate, where plentiful resources made survival and reproduction easy and an incredibly harsh climate which favored highly intelligent individuals. In the plentiful season, rapid reproduction would give individuals a huge advantage in numbers, quickly outnumbering individuals who chose a slow reproductive strategy. During the 'fall' season, individuals would switch to a slow reproductive strategy to produce intelligent, competent offspring who could adapt to the challenges of winter. There would be some tradeoff in when individuals would switch strategy, so the two strategies would overlap to some extent in the spring and fall, while summer would be almost exclusively a time of fast reproduction and winter a time of slow reproduction.

If only S-phase individuals are capable of having S-phase children, the optimum behavior strategy would be to produce lots of T-phase individuals under good conditions, and then for S-phase males to produce lots of S-phase children with T-phase females before winter arrived.

While Earth-like seasons are too quick to lead to this sort of behavior, it could easily evolve on a planet with much longer years, where it would make sense for reproduction to continue year round, rather than being seasonally driven.

  • $\begingroup$ at first I liked this idea, but on second thought I'm not certain if it works. When 'winter' comes around and S parents decide to have S children why would they mate with T parents instead of S parents to have them? There is a limit to the rate one can raise children so mass producing S children wouldn't help (they wouldn't be K strategy at that point), and if there are as many S female as male why bother with T mates? $\endgroup$
    – dsollen
    Apr 21, 2016 at 20:28
  • $\begingroup$ At the beginning of fall, children would still be easy to produce, but only S type children would be valuable, so the optimal mating strategy would be to produce as many children as possible using the plentiful T mates, since a single T parent would be sufficient to raise a child. During the winter, only S mating would occur, and most of the Ts would probably die off. $\endgroup$
    – ckersch
    Apr 22, 2016 at 2:52
  • 1
    $\begingroup$ How about changing the reproductive strategies of the females of the two phases to match their overall reproductive strategy. That is, S females give livebirth to single offspring, T females still lay clutches of ~30 eggs. Now an S-phase family/clan group that is in a favorable resource situation could take advantage by bringing in a few T females to crank out a large number of offspring in short order. $\endgroup$
    – kingledion
    Jul 27, 2016 at 18:55

I was thinking about the mention of an S-phase adult choosing to adopt a T-born offspring, as a strategy to ensure the gene flow, and how to make that more likely. Adoption may be much more likely if there are advantages to having S-phase offspring that are more immediate than competitive genetic legacy. If your S-adults are slightly more social, with a social structure that's based around a slightly larger pack or family instead of individuals, then adoption becomes more likely to maintain a larger group.

One idea that struck me was the possibility of S children helping parents or family when ill, injured, or old. This behavior might include families helping each other forage for food, or banding together to ward off predators. This gives an advantage to raising a child even if unrelated, since it improves the adopting parent's chances of survival and later reproduction, or (depending on social structure) improves the chances of survival for a whole family.

Other factors that might play into adopting rather than depending on their own offspring - if there is a gender divide, perhaps those of one gender are 'adopted' into the family of their mates... so a family who tends to produce that gender may need to adopt to keep their own numbers up as their offspring mature and leave. Or those whose family becomes dangerously small, from predators or illnesses, trading the effort of raising unrelated offspring for numbers to survive long enough to reproduce later.

Another option may be that perhaps adults leave their parents once they are mature, and so only a still-dependent child will assist their parents... and so adoption will happen when an S-adult is already injured or ill, so it is not dependent on having had young of just the right age... and if the parent dies before the child is independent, the loss of the effort involved in raising an unrelated child is less expensive than all the effort lost if their own S-offspring were to be orphaned (including extra time maturing to adoptable age and the effort spent reproducing).

Or, to play off of ckersch's idea of cyclical climate - it might be that at certain times of the year, more pack-mates (or family members) are helpful to forage for food or fight predators, and at other times the fewer the better to conserve resources... adopting would therefore be equivalent to seasonal labor, unrelated t-born being adopted and raised when resources are plentiful and higher numbers are safer (perhaps more frequently than actual reproductive cycles allow?), and abandoned or driven away when resources become scarce.

This "less expensive loss" idea may also be a reason for a young adult to adopt T-born before attempting to reproduce and raise their own offspring, if a larger family increases the odds of the children's survival a great deal. The extra effort of raising the t-born (and making newbie mistakes) is balanced by a lesser cost if they are lost, and the extra protection when the young adult is ready to raise their own offspring in the larger family.

Alternatively, once you have advantages to raising a child regardless of actual parentage... maybe you can emphasize the differences in your R and K strategies. Tadpoles are, after all, rather difficult to trace the lineage of - they all exist in the same ponds, and don't look much like their parents. R strategy species don't tend to track their own lineage, its about reproducing many offspring and letting the fittest survive... so maybe the R phase of your species likewise can't tell whose tadpoles are whose.

Rather than having your curiously mixed stage, of producing lots of offspring but also tracking the lineages (and/or caring for them) until adoptable age, have your tadpoles simply be indistinguishable from each other, and the T-born or S-born lineages unclear at this stage. S-adults will then choose from all the tadpoles of adoptable age, looking for those most likely to have successful S-traits, and choosing successful offspring is then as much a survival strategy as reproducing lots of offspring. The families, or clans, are then solely based on adoption and mutual support - and the better survival of the clan means more chances for each individual to breed, and therefore more chances for their offspring to be adopted by any clan or survive as T adults. There might be some benefit to identifying relatives (by scent? or markings?) and adopting those preferentially, but it would not be required. Credit where it's due, I think I might have read a similar scenario in a book, maybe in the "Reteif!" or "Interstellar Patrol" series.

This scenario would probably lead to fairly quick evolutionary process - the tadpoles are selected for survival fitness, the survivors evaluated for S-traits, the adults (both S and T) are selected for survival fitness again. The free intermingling of genes during tadpole phase would keep the populations intertwined, and the S-adults selecting among all the offspring for S-traits would increase the selection pressure for those traits (selecting for individuals, not just families). Also, I would expect a heavy cultural emphasis (once they get that far) on being strong or useful, competing heavily, and surviving at all costs - since cooperation is not rewarded through a major portion of their lifestyle.

Side note - whatever mechanism you choose, you might want to relax your requirement that only S-adults can trigger S-offspring, otherwise the question of where the first S metamorphoses came from is quite puzzling. Having the possibility of T-born occasionally managing to hit the right cues for the metamorphosis would answer that question, while sneakily helping to keep the genes intermingling. That these occasional converts have a much lower chance of survival due to lack of training and care while young, keep the possibility from disadvantaging your S-families too much, while giving a slight advantage to T-adults having S-trait offspring and answering how the whole setup came to be.


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