Two stories from the Research Highlights section of a recent Nature podcast got me thinking about the wide range in human behaviors, e.g. extraversion and introversion, risk appetite and aversion, optimism and pessimism.
It seems that selective breeding has shaped dog behavior (duh). For example, brain regions involved in movement and navigation were bigger in dogs bred for coursing, such as Greyhounds, than in dogs bred for companionship, such as the Maltese. Presumably Homo Sapiens has shaped its own behavior (and thus brains) in the same way.
There are no optimal traits though, so variability will persist. Researchers found that white barn owls have more hunting success during the full moon -- but brown ones did better the rest of the month, explaining the large variability in owl color.