That conversion of fructose to energy in the liver appears to create metabolic byproducts that (at sufficient quantities) lead to fat accumulation in the liver,

That conversion of fructose to energy in the liver appears to create metabolic byproducts that (at sufficient quantities) lead to fat accumulation in the liver, and consequently to insulin resistance. The “just-so” evolutionary story behind this goes: when fructose calories are available in excess, you want to make sure you’re storing all you can, because a fruit bonanza is often associated with an upcoming lean period. So, creatures whose bodies treated large amounts of fructose as a metabolic trigger to fatten up and reduce energy expenditure were more likely to make it through the winter.