Sure, it might be optimal for the species; however, individuals are not going to adapt "for the good of the species" [1]. If your gas smelling genes have no disadvantages and they help you survive in situations where you might not otherwise, they might as well just spread to everyone and the "slightly different behavior" gets exhibited everywhere. In other words, it doesn't really matter if something is optimal for the survival of the species since no one is optimizing at that level.
As an aside, our capability for rationality means we have the ability to optimize for the survival of our species, but it doesn't seem like we do so.
Individuals need not voluntarily adapt for the good of the species, and that's not what I am suggesting. I am suggesting an emergent phenomenon that leans towards behavioral diversity within a species.
An example: Ant colonies exhibit wide diversity in behavior between workers, keepers and queens. Something similar but subtler might be at work in all species.
As an aside, our capability for rationality means we have the ability to optimize for the survival of our species, but it doesn't seem like we do so.
[1] http://evolution.berkeley.edu/evolibrary/misconceptions_faq....
Edit: I don't know how to quote so I removed it.