Male Homosexuality a ‘Darwinian paradox’

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Male Homosexuality a `Darwinian paradox'

Sexually Antagonistic Selection in Human Male Homosexuality

Camperio Ciani1*, Paolo Cermelli2, Giovanni Zanzotto3

1 Dipartimento di Psicologia Generale, Università di Padova, Padova,

Italy2 Dipartimento di Matematica, Università di Torino, Torino,

Italy3 Dipartimento di Metodi e Modelli Matematici per le Scienze

Applicate, Università di Padova, Padova, Italy

Abstract

Several lines of evidence indicate the existence of genetic factors

influencing male homosexuality and bisexuality. In spite of its

relatively low frequency, the stable permanence in all human

populations of this apparently detrimental trait constitutes a

puzzling `Darwinian paradox'. Furthermore, several studies have

pointed out relevant asymmetries in the distribution of both male

homosexuality and of female fecundity in the parental lines of

homosexual vs. heterosexual males.

A number of hypotheses have attempted to give an evolutionary

explanation for the long-standing persistence of this trait, and for

its asymmetric distribution in family lines; however a satisfactory

understanding of the population genetics of male homosexuality is

lacking at present.

We perform a systematic mathematical analysis of the propagation and

equilibrium of the putative genetic factors for male homosexuality in

the population, based on the selection equation for one or two

diallelic loci and Bayesian statistics for pedigree investigation.

We show that only the two-locus genetic model with at least one locus

on the X chromosome, and in which gene expression is sexually

antagonistic (increasing female fitness but decreasing male fitness),

accounts for all known empirical data. Our results help clarify the

basic evolutionary dynamics of male homosexuality, establishing this

as a clearly ascertained sexually antagonistic human trait.

Citation: Camperio Ciani A, Cermelli P, Zanzotto G (2008) Sexually

Antagonistic Selection in Human Male Homosexuality. PLoS ONE 3(6):

e2282. doi:10.1371/journal.pone.0002282

Editor: , The University of New South Wales, Australia

Received: December 11, 2007; Accepted: April 6, 2008; Published: June

18, 2008

Copyright: © 2008 Camperio Ciani et al. This is an open-access

article distributed under the terms of the Creative Commons

Attribution License, which permits unrestricted use, distribution,

and reproduction in any medium, provided the original author and

source are credited.

Funding: The authors have no support or funding to report.

Competing interests: The authors have declared that no competing

interests exist.

* E-mail: andrea.camperio@...

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0002282