I reason about it this way, I observe that natural systems seek the lowest possible energy state. Selection forces on systems like these select for the most gain with the smallest investment in energy, which is an analog for lowest possible energy state. The rationale for this force on selection is that an organism is more successful the more food it gets vs the energy it expends to get the food. So any small change that increases net food energy returned contributes to the success of the organism, and an organism that is fed, reproduces to the next generation.
Is it optimal for all possible cases? We can't say, just like a rock rolling into a gully has achieved the lowest potential energy it can when considered as a section of terrain it sits in, may not have the lowest energy possible when you consider the whole planet. So for the carpenter ants, which have a worst case generation length of roughly 15 years [1] over a million years you have at least 67 thousand generations in which to find the lowest energy solution. So for the food gathering problem I would expect its pretty close to optimal.
Is it optimal for all possible cases? We can't say, just like a rock rolling into a gully has achieved the lowest potential energy it can when considered as a section of terrain it sits in, may not have the lowest energy possible when you consider the whole planet. So for the carpenter ants, which have a worst case generation length of roughly 15 years [1] over a million years you have at least 67 thousand generations in which to find the lowest energy solution. So for the food gathering problem I would expect its pretty close to optimal.
[1] http://www.orkin.com/ants/carpenter-ant/carpenter-ant-queens...