12.8 Ultimate causes: Why is homosexuality an evolutionary “puzzle?”

What do biologists mean by an “evolutionary puzzle?”

Some biologists have referred to homosexuality as presenting a puzzle within evolutionary biology. This is because same-sex sexual behavior is non-reproductive, yet homosexuality occurs in relatively high numbers—enough to support an adaptive function for homosexuality. So the big question is: How can natural selection promote on a trait that seems unable to increase an individual’s fitness?

What do you think about this question? As mentioned in Section 12.2, referring to homosexuality as a “puzzle” can be harmful because of historical and, sadly, modern-day human rights abuses towards homosexual people. Biologists don’t use the term “puzzle” to mean that homosexuality is something negative, but that it is an interesting scientific puzzle to study.

Key assumptions for the puzzle to exist in the first place

Think back the requirements of natural selection that we introduced in Chapter 2. In order for a trait to evolve via natural selection, there must be variation, that variation must be heritable, and different traits must correspond to differences in reproductive success. We can use these three requirements to assess whether homosexuality really presents a puzzle. So, here’s what we need to know.

1) Do people vary in their sexual orientation? We can safely answer yes, people do have different sexual orientations.

2) Is sexual orientation heritable? What do you think? Look back to what we’ve learned about possible genetic proximate causes of homosexuality.

3) Does reproductive success vary depending on sexual orientation? Biologists make this assumption when framing homosexuality as a puzzle. What do you think? Do you think that homosexual people have fewer biological offspring than heterosexual people?