Natural selection is the process by which favorable heritable traits become more common in successive generations of a population of reproducing organisms, and unfavorable heritable traits become less common, due to differential reproduction of genotypes. Natural selection acts on the phenotype, or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and reproduce than those with less favorable phenotypes.

Genes handed down by one's mother and having a large number of older brothers may determine whether someone is a homosexual, according to a study published.
Put together, these two factors may account for perhaps 20 percent of the prevalence of homosexuality, although social and cultural influences probably make up most of the rest, it suggests.
Psychologists at the University of Padova asked 98 homosexual men and 100 heterosexual men in northern Italy to fill out a confidential questionnaire detailing their sexual orientation and that of their siblings, first cousins, parents, aunts, uncles and grandparents.
Among the homosexuals, 22 out of the 396 male relatives on the maternal line of their family were gay, as were 12 out of 593 on the paternal side.
Among the heterosexuals, none of the 370 male relatives on the maternal line was gay; on the paternal line, the number was eight out of 604.
Just as striking was the relationship between birth order and sexual orientation.
In large families, homosexuals were likelier to have been born second, third, fourth or later, and were far likelier to have older male siblings rather than older sisters.
The study, led by Francesca Corna, says this adds statistical support to hypotheses about possible genetic causes for homosexuality, although it did not investigate homosexuality in women.
Previous research, carried out among gay brothers, suggests a link between homosexuality and a genetic sequence called Xq28 on one of the arms of the X chromosome, one of the chromosomes that determines sex.
Men have an X chromosome, which comes from their mother, and a Y chromosome, from their father. Women have two X chromosomes, one from each parent.
Research published in the mid-1990s bred the theory, strongly contested by some, that the male foetus presents an antigen, a molecule that triggers a response from the woman's immune system.
With each successive male birth, the mother is successively immunized against this antigen and the subsequence chemical change in the uterus has an effect on the sexual differentiation of the fetus, according to this idea.
An architect of this hypothesis, Canadian scientist Ray Blanchard, has calculated that each additional older brother increases the odds of homosexuality in the next male by some 33 percent...
While acknowledging that the Nature versus Nurture debate about homosexuality will continue to rage, the authors believe they may have resolved one of the enigmas about homosexuality.

This is the so-called Darwinian paradox: if homosexuality is conferred in part by genes, why haven't these genes been progressively eliminated over the millennia by natural selection -- the process that prefers genes which are useful for reproduction and survival?
The answer could lie in Xq28, for the mothers of homosexuals could be exceptionally fertile.
In other words, this particular genetic variation is a Darwinian tradeoff -- there is low or zero fecundity among men because they are homosexuals, but high fecundity among women.
The study appears in Proceedings of The Royal Society B, a journal published by the Royal Society, Britain's leading scientific association.