Insights into the evolution of homosexuality

How has homosexuality survived evolution? Genes associated with same-sex relationships may potentially have multiple functions that benefit heterosexuals, according to research from the University of Queensland.

“A long-standing mystery is why same-sex sexual behaviour, which is known to be influenced by genes, has survived evolution,” says Brendan Zietsch, who led the study. “This is mysterious because same-sex sex doesn’t lead to reproduction, which is the currency of natural selection.”

In the study, published in Nature Human Behaviour, researchers conducted a genome-wide association study – a technique that scans for genes that are common in people who exhibit a specific trait – to establish whether a set of genes was prominent in people who had same-sex partners. Interestingly, once the common genes were found, the team also discovered that they may have been advantageous to heterosexuals by helping them have more offspring.

“We found that the genetic variants associated with having had same-sex partners are also associated with having more opposite-sex partners in people who’ve never had a same-sex partner,” says Zietsch.

“Our findings provide a possible solution to this mystery – the genes for same-sex sexual behaviour may have survived because they provide a mating advantage in heterosexuals, which during evolution would have increased reproduction.”

This means that some genes may have had more than one function – a genetic phenomenon called antagonistic pleiotropy.

“Antagonistic pleiotropy is when the same genes affect different traits,” explains Zietsch.

“In this case, the same genes that increase the likelihood of having same-sex partners also increase the likelihood of having numerous opposite-sex partners in heterosexual carriers of those genes.”

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The authors note that there were some limitations to the study, because the cohort studied were from the UK and US and of European descent.

“The most important (caveat) is that our sample is limited in time and location, so we can’t be sure the genetic associations we found would be the same in other places, including different ethnic groups, and at other times, for example in the distant past,” explains Zietsch.

“Attitudes towards homosexuality differ greatly across time and place, so this could affect the genetic associations.”

In addition, genome-wide association studies present a correlation, and not necessarily causation. Behaviours are often influenced by both genetics and society, especially as this study estimated sexual behaviours, not sexual orientation, which are highly societally regulated.

However, further studies could reveal the prevalence of these genes in other cultures and countries.

The paper was released with an accompanying editorial that highlighted how LGBTQ+ advocacy groups were consulted during the editing process, to ensure it was ethically sound and sensitive.

“This is not a trivial research topic,” the editors wrote. “Same-sex behaviour is a human behaviour that huge swathes of the world (71 jurisdictions) consider to be a crime, warranting prosecution and imprisonment.

“These consultations (with advocacy groups) caused us to pause and reflect, and – we hope – to learn.

“They shaped the way in which we edited the article, and they helped us to identify the key issues that we feel are most crucial for our readers to understand the societal context of this work.”