Imagine we have a multi-cellular 'mammal-like' species which is K-select and usually has only one young at a time. Now imagine that this single young regularly has multiple fathers. Specifically the mother may mate with multiple males and the resulting embryo having half of the mother's DNA but the other half of the child's DNA may come from multiple males the mother mated with. This species would likely have to have slightly different genetic encoding then our DNA, but you could imagine something along the lines of certain chromosomes came from male X, others from Y and others from Z, all combining to make up one half of the child's DNA.

I'm wondering how beneficial such a mechanism is, and how hard it would be to evolve in the first place. The species does not need to evolve on earth or be tied specifically to earth's method of encoding DNA, or even using DNA, but should evolve under similar pressures to earth and result in something similar to a mammal in basic functionality.

I'm mostly wondering if this species could exist, or if there is any evolutionary hurdle that makes it impractical I'm unaware of, though any thought into how a species like it could work would be welcomed. For example the main questions that come to mind are:

  1. Would such a species be able to evolve and gain evolutionary advantage from having more diverse genetics?
  2. Is there a limit to how many fathers could provide DNA or how intertwined different parental DNA strands can be?
  3. Could such a species exist with either conscious or subconscious mate selection, ability to prefer use of DNA of a preferred male?

3 Answers 3


Two-parent sexual reproduction gives a maximum of a roughly 50% increase in the progeny's genetic variability over binary fission. To add a parent and have three parents, you get at most a 66.6% increase in variability, with four parents, you get a 75% increase in variability, with five you get an 80% increase in variability, and so on. This approaches a limit of 100%, but never gets there.

The actual increase in variability depends on the differences between the parents.

However, having all these additional parents has a cost in that they must be mustered to provide their genetic material at the appropriate time, and the more parents, the harder to arrange.

So, from an evolutionary point of view, the greatest advantage comes from 2-parent reproduction, and with the diminishing returns and additional complexity of higher numbers of parents, having higher numbers of parents is of a lower degree of fitness than just two.

However, evolution is 'survival of the adequate', so if a three-sexed reproductive system occurred in the absence of a two-sexed system, it would likely provide sufficient advantage to be retained in future generations. However the likelihood of this is quite low, but not beyond the bounds of all probability.

In addition, such a three-sexed reproductive system would be vulnerable to competition by newly occurring two-sexed species which can reproduce faster and more easily.


It is generally not possible for a female to provide 50% of DNA and for an arbitrary number of males to provide parts of the other 50%.

You may have a situation with a genome having a ploidy of 2N, where male and female contribute equal amounts of DNA (N each).

With a ploidy of 3N, the female may contribute 2N and one male contributes N, or the female and each of two males contributes N, or the male 2N and the female N.

With a ploidy of xN, (where x > 3), there are many combinations of what multiple (m where sum(all m) = x) of N the female and each male involved (up to x-1 males) contributes. You cannot have an arbitrary number of parents; the system will dictate that there are specific numbers of each sex involved.

To violate ploidy typically results in either death of the offspring or a greatly reduced fitness.

Consider bananas: the ones we eat have a ploidy of 3N, resulting in the seeds being very small and infertile. Otherwise, if we were eating 2N ploidy bananas, we'd be spitting out the pips. This means that cultivated bananas cannot generally reproduce without human assistance. This is a typical minimal consequence of violated ploidy.

Another example is chickens: They are typically 2N, but they can reproduce parthenogenetically, resulting in N offspring, which are - on the rare occasion when they hatch at all - considerably less healthy than their parent.

Since evolution will typically configure an organism to have a certain amount of genetic material, having 50% from a female and 50% from an arbitrary number of males would require some sort of mechanism to select which parts of the male 50% to take from each of the arbitrary number of males. This is highly unlikely to evolve naturally.

If such a reproductive system was to evolve, having 2 parents would provide a 50% advantage in variability as normal, 3 would provide at most 62.5% advantage, 4 would provide a 61.11% advantage, 5 would provide a 51.56% advantage, 6 would provide a 50.16% advantage, and so on, approaching 50%.

I had to run these numbers several times to verify that they were correct; it seems that the advantage is greatest with two fathers, and actually decreases thereafter. It seems counter-intuitive, but that's how the math works out.

It is more likely that if an arbitrary number of males contribute to the genetic makeup of a female's offspring, having 1 male parent would result in 50% of the offspring genome originating with the female, but having 2 males would mean that the female and each of the males contributes 33.3%, or 3 males means that each parent contributes 25%, and so on.

In the latter situation, the more males fertilise the female, the less the offspring would be related to the female, and hence it would not make evolutionary sense for the female to allow more than one male to mate with her.

  • $\begingroup$ I'm not certain, but your answer seems to be implying something different then my stated intent. The mother would always provide 50% of her DNA, so the genetic variability provided by each male she mates with is less, since 50% of the DNA is always hers. However, that also means that the only sacrifice she makes to increase genetic variability is the low cost of allowing another male to mate her. I'm not sure I see how that cost is significantly high to counteract the benefit of increased genetic variability? $\endgroup$
    – dsollen
    Commented Jan 20, 2016 at 14:58
  • 1
    $\begingroup$ Have a look at my edits. $\endgroup$
    – Monty Wild
    Commented Jan 21, 2016 at 0:33

Taking the questions one at a time:

  1. Would the species be able to evolve and gain evolutionary advantages?

Yes, but evolution would likely be even slower than it is on Earth. Any time that you're passing on genetic information, evolution can happen, but the problem here is that we don't know whether information IS being passed on. Consider the case where there are three fathers, just as an example. Each father gives about a sixth of the offspring's genetic code (a third of the male half). Now, suppose that one of the fathers had some evolutionary advantage from sheer random happenstance. Even supposing it's a dominant trait, there's only a 1/6 chance that it gets passed on - if the code for it was in an sequence that a different father provided, it's not helpful for the kid. An interesting corollary here is that evolutionary changes go faster when the mutations occur in the females, because then they're more likely to be passed on.

  1. Is there a limit to how many fathers there could be?

Theoretically, we'd have to go into deeper detail on how exactly the reproductive process works -- does the paternal mixing bowl take place on a chromosomal level? A sequence level? A base pair level? Whatever the minimum level is, the upper bound on the number of fathers is however many "blocks" of that minimum level there are. If it's on a chromosomal level, then (assuming similar genetics to humans) there could be 23 fathers (one for each chromosome). If it's on a base pair level, there could be a hell of a lot more. Practically it comes down to how much sex the mother can have -- how many sperm can she get to her egg.

  1. Could such a species exist with conscious or subconscious mate selection, the ability to prefer use of the DNA of a specific male?

Realistically, doubtful, if only because that (as far as we know) isn't how genetics works -- no amount of wishing is going to manipulate the genetic code of the fetus growing inside of you. However, if you're making this world, you make the rules, so if you wanted that to happen then go for it. I would say that subconscious would probably work better than conscious choice, if only because that way you don't end up with people crafting humans for specific purposes, but it's your choice.

  • $\begingroup$ insects show mate choice when it comes to how many of their young carry a male genetics quite often. The trick is usually about deciding what sperm gets to reach eggs, not trying to change the genetics after sperm and egg meet. $\endgroup$
    – dsollen
    Commented Jan 19, 2016 at 22:08

I think it would make more sense if you had one "female" gender and a previously selected number of "male" genders. These males would each provide some parts of the child's DNA, while the mother gives the child the rest of the DNA and births it. However, you would need to have some way to differentiate between these "male" genders.

Each different gender could have something it's better at. Now, while this isn't totally true, bear with me. Men are generally more muscular and stronger than women. Women are more articulate and detail oriented. For your species, one male gender could be physically strong, another extremely intelligent, and a third would have heightened senses. Then you'd have the mother, who would be more like a regular human woman.

  • $\begingroup$ This runs into the arrangement issues pointed out in Monty Wild's answer. $\endgroup$
    – zeta
    Commented Jan 20, 2016 at 17:08
  • $\begingroup$ @sumelic Well, considering the different genders here have different purposes, kind of. I would say that there is an advantage to this system. But he is right that there would be more possible issues with children. $\endgroup$ Commented Jan 20, 2016 at 17:36

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