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This question already has an answer here:

Do bisexuals have any evolutionary advantage over straights and gays? Bonobos exhibit bisexuality extensively. Why so?

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marked as duplicate by Remi.b, kmm, another 'Homo sapien', Bryan Krause, David Mar 5 '17 at 11:26

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    $\begingroup$ Welcome to Biology.SE. You should have a look at Is homosexuality an adaptation in humans? and How can homosexuality evolve despite natural selection?. If insights from these posts don't answer your question, can you please elaborate on why they don't. $\endgroup$ – Remi.b Mar 2 '17 at 22:29
  • $\begingroup$ those questions are about homosexuality vs heterosexuality, mine is bisexuality vs homosexuality/heterosexuality. $\endgroup$ – akm Mar 2 '17 at 22:44
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    $\begingroup$ @AmitMaurya Can you point out what elements of the answers of those questions do not apply to your question? $\endgroup$ – Bryan Krause Mar 2 '17 at 22:53
  • $\begingroup$ @BryanKrause you tell me which element is applicable? is it established that bisexuality and homosexuality caused by same gene(s)? $\endgroup$ – akm Mar 2 '17 at 23:10
  • $\begingroup$ The genetic basis of either is not well established. The research cited also doesn't look at all at particular genes, it looks at reproductive rates among relatives. It also doesn't look at purely binary genders or sexual preferences and actually groups people into "heterosexuals" and "non-heterosexuals", the latter clearly including bisexuality. Please at least make an effort to read the answers and citations, like this one. $\endgroup$ – Bryan Krause Mar 2 '17 at 23:34
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Like many things, whether bisexuality is advantageous for an individual's reproductive success or not depends on the environment. The fact that a wide variety of sexual systems and behavior exists in the animal kingdom suggests that. Also, "advantageous for an individual's reproductive success" doesn't necessarily imply "will spread via the processes of evolution"; for a trait to spread via natural selection+random mutation you need the trait to promote the individual's reproductive success, and for it to be reliably passed down to its offspring. And as the causes of sexual orientation in humans aren't well understood there is always the possibility it isn't strongly or genetically inherited.

As far as bonobos go, I am given to understand they use sex as a social tool, much more so than we do. In that context it does seem like bisexuality would be advantageous; it's silly to only be able to use your most powerful tool for conflict-resolution and social bonding on half of your community. This particular advantage won't be true, or will be much reduced, in species that don't use sex in the same way, i.e. all or most other species that we know of.

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  • $\begingroup$ are you saying bonobos' bisexuality is because of social learning rather than being genetically determined which is naturally selected? $\endgroup$ – akm Mar 3 '17 at 10:25
  • $\begingroup$ "Like many things, whether bisexuality is advantageous for an individual's reproductive success or not depends on the environment" so what environmental characteristics make bisexuality advantageous in bonobos but not other animals? $\endgroup$ – akm Mar 3 '17 at 10:38
  • $\begingroup$ @AmitMaurya I'm not saying that no; I don't know why Bonobos are bisexual. I certainly think it's plausible that bisexuality is subject to natural selection, and that it was naturally selected in Bonobos because of their social use of sex, but I don't know whether those things are true or not and I don't think research has demonstrated anything on the subject. $\endgroup$ – Oosaka Mar 3 '17 at 10:41
  • $\begingroup$ @AmitMaurya "so what environmental characteristics make bisexuality advantageous in bonobos but not other animals?" I described them in my answer; things like "other organisms of the species", "the social structure of a social species", etc are part of the environment. $\endgroup$ – Oosaka Mar 3 '17 at 10:42

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