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I am specifically interested in a form of reproductive cycle something like that of aphids on earth, where several generations of female-only parthenogenesis go by, followed by a generation being born consisting of both males and females capable of sexual reproduction. That is, the type of complex reproductive cycle known as heterogamy, although confusingly this term is also used for other things in biology.

On earth so far as I know this particular type of cycle only takes place among insects, although I am aware that you can get occasional "virgin births" among some vertebrates e.g. turkeys and komodo dragons.

We are talking about an alien species on an alien world, so you wouldn't expect to see things like DNA, chromosomes, etc. However I am assuming that they have evolved mechanisms to do broadly the same jobs, and have something like male and female.

The question is, what would be different about that world compared to Earth, or the evolutionary history of that species compared to humans, in order for a parthenogenetically reproducing species to become dominant sentients like homo sapiens did on Earth? I suppose a nearly-equivalent question is why parthenogenesis is so rare among larger and more intelligent animals on Earth.

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    $\begingroup$ Actually, alien life will use DNA and chromosomes. It is one of (if not the) most efficient systems that can naturally form. $\endgroup$
    – Jax
    May 1, 2015 at 17:08
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    $\begingroup$ Or at least RNA, which has the potential to shift to DNA over many generations. $\endgroup$
    – Jax
    May 1, 2015 at 17:09
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    $\begingroup$ @DJMethaneMan citation not in link given (if anything it hints towards the opposite with "the truly captivating idea, the one that makes my head spin, is that we might find something based on a completely different molecule than our much-beloved DNA"). $\endgroup$
    – Alex Celeste
    May 1, 2015 at 19:01
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    $\begingroup$ Sorry to bump an old thread, but you also might find a wealth of information in response to my WorldBuilding question Why would a parthenogenetic species invent males? and the Discussion that it spawned. $\endgroup$
    – Ayelis
    Apr 25, 2016 at 18:24
  • $\begingroup$ No need to apologise, Ayelis. On the contrary I am very grateful to you for pointing out such an interesting and relevant discussion. $\endgroup$
    – Lostinfrance
    Apr 25, 2016 at 18:38

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It's rare to see a reproductive system which does not provide crucial benefits to the society, so I would expect to see a substantial reflection of this deep rooted reproductive strategy in their society.

If asexual reproduction was enough, complicated techniques like sexual reproduction would almost certainly not occur. Given that the system you want is even more complicated than sexual reproduction, it's not going to occur in a world where asexual reproduction is good enough.

Sexual reproduction has a lot of advantages. It's why, generally speaking, higher life forms all use it. Accordingly, we're going to need to find a reason in this alien environment why sexual reproduction has too many problems to be the one-size-fits-all solution.

You mentioned aphids, which unfortunately I know very little about. Fortunately, aphids are alone. Enter yeast, my brewer's little buddy. Like your aphid buds, yeast reproduces asexually quite often. Give it a nice big vat of soon-to-be-beer wort, and it'll replicate like mad. However, in high stress situations, yeast shifts reproductive approaches towards sexual reproduction and the creation of spores. (I'm handwaving a bit. Check out the real lifespan of yeast, its awesome!)

So while I don't think your creatures are going to be hyper-intelligent yeast cultures, I do think we can take something from this story. The yeast had to deal with a tremendous dynamic range of environments, from "doin' the backstroke in the wort" to swimming in an ethanol-derived antiseptic bath of their own creation. They vary the reproductive approach based on the strain of the environment.

So let's consider a few possible worlds where the stress could be so high as to warrant such a complicated solution:

  • The society is usually female, except in times of famine. In a scenario like this, it makes sense to have the males be small lightweight drones. Normally, it's an all female culture, but when times get tough reproduction shifts to making large numbers of resilient but otherwise useless males which can go into the world and keep their genetics going.
  • The society is usually female, except in times of great violence. At this point, they begin to birth males, which are genetically structured to be vicious warriors. This would piggyback on any interesting reproductive strategy, but the cultural ability to create tools of war in a few short years might actually dwarf the reproductive benefits.
  • There is no society, most of the time. The world is too inhospitable to allow it. Everyone lives in isolation as hermits. However, every once in a while a society survives long enough to warrant switching over to to sexual reproduction, while the opportunity lasts.

In all cases, expect to see a dramatic environment which inspires dramatic approaches to survival.

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    $\begingroup$ "The society is usually female, except in times of famine." I'd flip that about. In times of excess, you have spare resources to experiment (evolutionarily speaking) on risky adaptations. In times of famine, you want to be making surer bets. Females represent the surer bet because, hey, they've survived this long! $\endgroup$
    – user6511
    May 5, 2015 at 4:29
  • $\begingroup$ @user6511 I think both approaches can make sense. The version I have, being based on how yeast does it, is more of a "if things are working, why muck with it by doing anything but business as usual." Yeast, being single celled, does play different games than we do with larger creatures. $\endgroup$
    – Cort Ammon
    May 5, 2015 at 4:58
  • $\begingroup$ "Given that the system you want is even more complicated than sexual reproduction, it's not going to occur in a world where asexual reproduction is good enough." I liked all the answers, but found this thought particularly useful. $\endgroup$
    – Lostinfrance
    May 21, 2015 at 20:52
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    $\begingroup$ @Ayelis You would be amazed at how many ways mother nature has evolved to do sex determination on earth. We usually think of XY, because that's what humans use, but a wikipedia trip will dredge up others such as ZW X0, UV, and a funny hodgepodge of Haplodiploidy. And that doesn't even begin to cover some strange systems, like the yeast particles I mentioned. $\endgroup$
    – Cort Ammon
    Jul 17, 2015 at 21:10
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    $\begingroup$ @Ayelis Asexual reproduction is considered, by every biology opinion I know of, to be vastly simpler than sexual reproduction. It takes a lot of evolutionary strain to reach the point where sexual reproduction is advantageous enough for evolution to have a high likelyhood of achieving it. Sure, its possible sexual reproduction could occur first, but its unlikely. I would draw an analogy to trying to invent a i7 intel processor without first inventing something similar to a 368 along the way. It can be done, but its easier to go through the 386 first. $\endgroup$
    – Cort Ammon
    Jul 17, 2015 at 21:25
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Parthenogenesis tends to be a "last resort" reproductive mechanism. When you have plenty of males around, there's no need to asexually reproduce since they're all vying for your attention/favor. It's more plausible that your aliens are relatively solitary creatures who aren't DOMINANT per se, but rather intellegent creatures in a hostile environment.

Think cave men in the times of the dinosaurs, and yes, I know that's an anachronism.

The combination of a relatively solitary nature and small prey status keeps them from forming large communities (which makes them targets for predators) for the purposes of mating. This pushes parthenogenesis (and the resulting Amazonian matriarchal structure) to the fore.

In these circumstances, expect radical changes once they master weapon tech that allows them to dominate their environments. That's why it's so rare among large animals on Earth - it isn't a necessity once you're big and/or intelligent enough to stop worrying about predators all the time.

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    $\begingroup$ To put it another way, it's a delaying mechanism that extends "one generation" to the lifespan of more individuals. Under the simplistic layperson's version of genetics most of us think in terms of, it contributes nothing either way to the evolutionary process. $\endgroup$
    – Alex Celeste
    May 1, 2015 at 19:03
  • $\begingroup$ I agree with all but the last paragraph. Pathnogenisis would not be required once you are smart enough to best your enviroment, but it would still exist, evolution won't do away with it for thousands of years. Thus it will have a cultural value in this community. Look at all the single mother's by choice now, using anonymous sperm or other methods, imagine them all reproducing via pathogenesis. The choice to be a single mother will be far more common, if still not the primary state, in this culture. $\endgroup$
    – dsollen
    May 6, 2015 at 14:43
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We are intelligent because in order to reproduce this way we needed to find and convince our pairs that we are the right ones.

And because we got more intelligent, we were able to choose more ideal mates.

Also heterogamy results in a much much wider biodiversity(Without this you usually* don't get any biodiversity at all.)

The way you could achieve your goals is through controlled mutations/biological engineering, which could produce sufficient biodiversity for survival and perhaps even more.

If you choose biological engineering it would be very hard to get to that point without mutations or heterogamy.

Controlled mutations could be on the other hand an alternative evolutionary way, tough its hard to reason how and why would such a thing win against heterogamy.

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