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A hypothetical organism is tetraploid and isogametous. To reproduce, four haploid gametes must meet and fuse into a tetraploid zygote. Any four individuals are sexually compatible, barring self-incompatibility or mating types.

What selection pressures would favor this method of reproduction over the fusion of a pair of ovum and sperm?

EDIT: near as I can tell, this would be a defense against recessive traits, frequent population bottlenecks and frequent mutations. The downside would be a slower overall rate of evolution, I think.

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    $\begingroup$ How are young raised? Is it egg laying followed by abandonment or do the four parents stay together to raise live birth young? How many young are usual? A few more details would be cool. $\endgroup$
    – Matthew
    Commented Feb 22, 2017 at 18:49
  • $\begingroup$ @Matthew: At this point I haven't thought past unicellular organisms similar to yeast or diatoms (the simplest organisms I am aware of which reproduce sexually). $\endgroup$
    – Anonymous
    Commented Feb 22, 2017 at 19:06

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Not as likely as Will's answer but:

  1. If conditions are such that it takes more than two to guard or feed the young, having more than two "parents" with an actual stake in the outcome (by sharing DNA) could do it.
  2. In a very fluid environment, having multiple choices of DNA (from multiple parents) could increase the randomization of the offspring. If the young are produced in litters, it would make it more likely that some would have what it takes to survive the conditions they find themselves in.
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Imagine a scenario where there is rampant DNA parasitism by transposons https://en.wikipedia.org/wiki/Transposable_element or similar genetic parasites. If there was a good chance that any given parental gene donated to the zygote might be inactivated or damaged by a DNA parasite, then getting more copies would increase the chance that at least one of them would work the way it is supposed to and make a functional gene product.

This is not super far fetched. Transposons are a (longstanding!) problem for DNA based life. Anything that uses DNA to reproduce has mechanisms to limit damage from transposons. This is a benefit of meiosis based reproduction generally over mitosis-type self cloning or budding based reproduction.

Superficially it seems to me that the same might be true in a situation where there is a lot of mutagenesis for other reasons - maybe from radiation?

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    $\begingroup$ While this idea makes sense in the short term it would almost inevitably lead to the extinction of the species. By going to 4 gene copies you aren’t actually dealing with the issue of increased mutations. You are merely temporarily masking them. The deleterious mutations will build up in the gene pool, hidden by the good copies until the majority of copies in the genome are non-functional. The rate of viable births will decrease dramatically and the species will go extinct. In the face of active transposable elements or increased radiation a species would be better served becoming monoploid. $\endgroup$
    – Mike Nichols
    Commented Feb 22, 2017 at 20:07
  • $\begingroup$ You may be right about radiation mutations but maybe not about transposons. If there were a process like lyonization en.wikipedia.org/wiki/X-inactivation to inactivate genes which did not make a product that would weed out these damaged genes from the organism. The actual method by which animals defend against transposons has to so with small noncoding rna whatisepigenetics.com/non-coding-rna which as I understand them, cut the transposon and with it the gene it is in. $\endgroup$
    – Willk
    Commented Feb 22, 2017 at 21:19
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    $\begingroup$ Epigenetic silencing of transposable elements is intended to prevent them from spreading further, but if a transposon has directly disrupted a gene by inserting into one of its exons there is no way to recover the function of that gene through epigenetic means. Inactivating a transposon or a non-functioning gene won't remove it, nor will it create a functioning replacement. The only way to remove the deleterious allele will be through natural selection, which increased ploidy will work against. $\endgroup$
    – Mike Nichols
    Commented Feb 22, 2017 at 21:43

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