Courses/CS 461/Winter 2006/Rick Strom/Week Four
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[edit] Week Four
[edit] Mating Advantage in Homosexuals
This week, I was reading a book called Sperm Wars: The Science of Sex, in which the author, Robin Baker, attempts to counter the argument that certain forms of sexual behavior (homosexuality, rape, group sex, wife swapping, etc) are abberant because they do not contribute to the primary purpose of living beings -- reproduction. Baker argues that these forms of sexual expression actually do offer reproductive advantages for their participants, and case by case he analyzes the behavior and attempts to explain the reproductive advantage gained by the behavior. Overall, a very interesting book.
In the chapter on homosexuality, he makes a few claims that seemed to make a lot of sense. First, he offers the suggestion that homosexuality may be genetic, passed on the mother's side. He then offers that when the gene is passed in such a way that the offspring are actually bisexual (receiving the gene for homosexuality along with the gene for heterosexuality), the bisexual offspring have a reproductive advantage during their fertile years. He offers three possible explanations for this advantage:
- Early learning with other males provide homosexual males with experience at the mating game, before females of their age can catch up. Consequently, when their peer females do reach sexual age, the homosexual males are much more competant and confident, and thus more likely to successfully mate with them.
- As in (1), homosexual males have more experience with a wider variety of personalities than their heterosexual counterparts, giving them a greater ability at verbal interaction during courtship.
- Bisexual males are much more able to successfully cheat on their female partners with other males without detection (presumably because their partners are looking for evidence of another woman, not another man), leading to more skillful infidelity with females when they choose to do so.
Explanation (1) seemed to me to be the most plausable, but in any case, the argument is the same: that males with a tendency towards homosexuality will have an advantage when it comes to mating with others. Baker concludes that this reproductive advantage keeps the homosexuality gene in the pool, and the level at which homosexuality is present in a population should be exactly the level at which it is most advantageous to the population. A number I remember hearing was around 10%, although baker puts it at around 6% of the population.
Baker's analysis made a lot of sense, so I decided to model this and watch it in action.
[edit] The Model
On setup, the world is populated with a number of agents, each of which is randomly assigned a sexual orientation (heterosexual, homosexual, bisexual or asexual). Heterosexual males are blue, heterosexual females are red, all homosexual or bisexual agents, regardless of sex, are green. All asexual agents are yellow.
On go, each agent moves randomly around the world. If an agent finds a compatible mate on his patch (according to his preference), he attempts to mate with him or her. His chances of success are given by the scoring-odds slider. Additionally, agents with the is-homosexual? property are given an additional percentage advantage, given by homosexual-advantage. If an agent successfully mates, both partners enter a refractory period, given by refractory-period, during which they are not interested in mating and will refuse all offers. This refractory period is entered whether the sex is heterosexual or homosexual.
In the event that the mating is between a man and a woman, the woman will additionally enter a pregnancy period, given by gestation-period, during which she will refuse all offers. When this pregnancy is over, she will spawn one new agent, and if she carries the homosexuality gene, she will pass it to her offspring with a probability given by homosexuality-propogation-likelihood. Similarly, the mother also passes the heterosexuality gene according to heterosexuality-propogation-likelihood. As on setup, there is a chance of asexual offspring.
All offspring will die when they have reached life-span days.
[edit] .nlogo file
[edit] Applet
Homosexuality Applet (at stromcode.com)
[edit] Source
[edit] Screenshot
http://img2a.glowfoto.com/images/2006/02/03-2207458880T.jpg
[edit] Conclusion
No matter how many times I run this simulation, or what I set the sliders to, the homosexual population dies out completely pretty rapidly, leaving only a majority of heterosexuals and a small minority of asexuals. Although Baker's arguments made sense to me when I read them, it is really difficult to simulate them. According to the rules I programmed into the model, it makes perfect sense that the homosexual gene would disappear rather quickly, and it did. I don't know if I would conclude that he is incorrect based on this model, but I would say it is likely that there is more at work here than a simple genetic transfer.
[edit] Additional Notes
The netlogo code makes pretty extensive use of self and myself, and also forced me to understand the different ways of creating new turtles (in this case, using hatch-<breed>.
