Originally Posted by Lusankya
Hmm, I'm not so sure. If humans are the first species to encounter such environments, then there wouldn't have been any opportunity for such a trait to evolve. It's possible rats are just less particular about gender and, while a male rat would prefer a female rat, if they get sexually starved enough they'll go after anything that's alive. Thus, the behavior wouldn't necessarily be intended to reduce population density. Maybe something in the study makes this theory obsolete though.
Yeah, the paper mentioned that a portion of the mice were actually just not interested at all. It's actually not that difficult to think that the genes themselves could be neutral until a certain amount of crowding happens. That is, you don't commonly (obviously this still happens, just at a lower rate) see mice exhibiting different behaviour under "normal" circumstances. There needs not be an opportunity for a trait to evolve so much as there only needs opportunity for the trait to express itself.
Think of this as antibiotic resistance. All
known antibiotic resistances are currently not new, but are locked away in large plasmid/chromosomic chains of genes (i.e. operon) in different bacteria. It is simply that due to the evolutionary pressures involved that some aspects of that operon is in motion, but it also means that there are hundreds of thousands of other genes in that operon that hasn't had any chance to do anything yet. Perhaps in humans, epigenetic changes (i.e. methylation, acetylation, etc.) control our homosexual genes in a similar fashion in utero
as a response to crowding? Of course, I must admit that it is conjecture, but it's not that far of a stretch, especially since we know
humans use epigenetic controls in utero
in response to the environment.
There is a documentary from BBC called "Ghost in your genes" that documents some of the current (well a couple years ago) understanding of the still new field of epigenetics. It's a recommended viewing for a broad overview, especially with the interesting cases of Angelman/Prader-Willi (sp?) syndromes.