Homosexuality – Models of evolution
Posted by Agnivo Niyogi
Despite its widespread occurrence in the animal kingdom, homosexuality is still perceived unnatural. Evolutionary scientists have long been confused why an apparently non-contributing trait like homosexuality has been preserved throughout the evolutionary tree. Homosexuals cannot mate and are thus reproductively useless. Hence, for long geneticists have trashed homosexuality as an anti-thesis to Darwin’s theory of natural selection. Had homosexuality been a lethal trait it would have been erased out in course of selection. But not only has it been preserved but over time it has shown no signs of weakening. In this post I discuss a few models which try to throw some light on how homosexuality evolved.
Here it would be wise to mention a few terms in genetics and reiterate a few concepts so that people can identify with the following models. The genetic makeup of an individual is called genotype and the physical expression of it is the phenotype. The genes have many loci which govern different traits. Every locus has two alleles which give rise to two different forms of the phenotype. According to Mendel’s law of dominance, one of the alleles (dominant) masks the effect of the other (recessive) allele. There are exceptions to this rule. Mendel also said that during formation of the gametes, the alleles segregate and come together again in the offspring. This is the law of segregation. A major exception to this rule is linkage. In linkage many loci together determine the phenotypic expression of a particular gene. Then there is also the phenomenon of multiple allelism (or two or more alleles at the same locus determining a single trait).
Scientists developed theory to make contrasting predictions about the genetic characteristics of genes influencing homosexuality including: (i) chromosomal location, (ii) dominance among segregating alleles and (iii) effect sizes that distinguish between the two major models for their polymorphism: the overdominance and sexual antagonism models.
There is evidence that homosexual males and females have lower lifetime offspring production in some modern Western societies (up to 80% lower; Bell et al. 1981), and that this may also have been true in human ancestors (reviewed in Pillard & Bailey 1998). Second, there are two lines of evidence that homosexuality is influenced by polymorphic genes: (i) twin studies indicate that there are both genetic and environmental factors that contribute to the expression of the homosexual phenotype (Pillard & Bailey 1998; Bailey et al. 1999; Dawood et al. 2000), and (ii) male homosexuality appears to be inherited more frequently from the matriline (Pillard et al. 1981, 1982; Pattatucci 1998; Camperio-Ciani et al. 2004), suggesting the existence of polymorphic, heritable maternal effects and/ or polymorphic X-linked genes influencing male homosexuality. Third, even if one assumes only a small fitness cost to the expression of homosexuality, it appears to be more common in both males and females than can be plausibly explained by mutation–selection balance (Kinsey et al. 1948, 1953; Gebhard 1972; Diamond 1993; Sell et al. 1995).
Scientists have speculated a relationship between birth order and homosexuality. Among sibs, the occurrence of homosexuality depends on the number of older brothers but not on older sisters. It reflects the progressive immunization of some mothers to unspecified male-specific antigens with each
successive male foetus and the increasing effects of such immunization on sexual differentiation of the brain with each successive male foetus (Blanchard & Klassen 1997; Blanchard 2004, but see Bearman 2005).
The inheritance pattern of homosexuality in pedigrees is complex. This is indicative that homosexual trait is not a simple Mendellian character, thus making the mapping of homosexual gene very difficult. A quantitative trait locus mapped the homosexual trait to the X chromosome (Xq28 or the 28th position of the longer arm of the X chromosome) (Hamer et al. 1993; Hu et al. 1995), but the methodology used in these studies was questioned later (McKnight 1997) and the findings have been difficult to replicate (Bailey et al. 1999; Rice et al. 1999). Recently a genome wide QTL study has shown three “nominally significant linkage peaks”, indicating three autosomal genes that may influence male sexual orientation, as well as limited support for the previously reported X-linked QTL (Xq28). These initial results are only preliminary and require confirmation from additional genetic studies (Mustanski et al 2005).
Following these studies two models were proposed to account for the polymorphism in the genes that govern homosexuality. These are over dominance and frequency dependant selection via kinship. Over dominance theory assumes the over expression of certain genes in the heterozygous condition (having two different alleles rather than a double copy of the same allele) confer greater fitness to that individual. For example, the sperms of heterozygous homosexuals may have competitive advantage over other males. The Kinship theory holds that during the course of human evolution homosexual individuals may have helped family members, through direct or indirect provision of resources, to reproduce more successfully than they would have otherwise. Thus, genes for homosexual behaviour would have been propagated indirectly through relatives. The theory has been criticized for a variety of reasons including reliance on a number of false assumptions and a lack of supporting evidence. Consequently, it has been rejected as an explanatory model. Another theory that has been doing the rounds is the sexually antagonistic selection theory which states that traits which decrease the relative fitness of one sex are maintained in the population because they are useful to the other sex.
I hope what i discussed was not Greek to you all. For those who are interested to know more can refer to these papers (available for free download):
- Genetic models of homosexuality: generating testable predictions (Sergey Gavrilets and William R Rice) published in the Journal of Proceedings of the Royal Society of Biological Sciences (2006)
- Homosexual Orientation in Males: Evolutionary and Ethological Aspects(Frank Muscarella, Bernhard Fink, Karl Grammer & Michael Kirk-Smith) in the journal of Neuroendocrinology (2001)
[P.S. This article was first published in the June issue of Gaylaxy]