I was thinking about the now supported imprinted brain theory, which shows that autistic and psychotic conditions are at least somewhat opposed, with autism spectrum conditions resulting from a paternal bias in gene expression, and psychotic or mood disorders from a maternal bias. I wondered whether most variability can be explained by graded characteristics with an optimal inbetween, an optimal point which may covary with the individual's other traits or his environment. For instance, purely anti-autistic selection might have eliminated autism. Instead, perhaps nature wants the right mix of autism and psychosis, and our variability results from nature's inability to get this balancing act exactly right every time. This could also apply to other graded characters such as size, strength, or skin tone. In summary, if a deleterious trait persists, there is an equally deleterious opposite, a situation which maintains genetic variability. What do you think?

Edited June 20, 201510 yr by MonDie

More Examples

 

Without seeing the research, I would guess that depression is deleterious. This case might involve a U-shaped curve rather than a depression-opposite, but it's the same situation if you think in cytokine levels.
http://discovermagazine.com/2014/julyaug/9-depressions-dance-with-inflammation

Among the participants with high inflammation, those who received Infliximab experienced a reduction in their symptoms. Good news so far. But when Miller’s team analyzed the group with normal inflammation, they were taken aback.
[...]
Yirmiya and his colleagues have done studies in mice for the past 20 years suggesting that a specific level of cytokines is needed to ward off symptoms of depression.

Skin tone presents another balancing act where it's either vitamin D absorption or UV resistance.

Edited June 21, 201510 yr by MonDie