It is theorized that certain genetic disorders are caused by selection pressure for intelligence and overdominance, e.g. a recessive mutation may boost IQ in heterozygotes despite being deleterious in homozygotes. Examples include Tay-Sachs or Gaucher's disease. See Ectopic dendrite initiation: CNS pathogenesis as a model of CNS development (Siegel et al., 2002).
The putative mechanism for elevated intelligence is reinitiation of primary dendrites on mature cortical neurons. My question is, if dentritogenesis on mature cortical neurons promotes learning and boosts IQ why hasn't the human organism evolved to have common non-pathological alleles that do this already without the need for deleterious mutations?