The modern evolutionary synthesis assumes that traits are transmitted via genetic inheritance, an inherently vertical transmission mechanism. But non-genetic and non-vertical transmission mechanisms are common, including social learning, microbiome transmission, and horizontal gene transfer.
I describe results from our PNAS paper "Evolution of vertical and oblique transmission under fluctuating selection". We found that oblique transmission (i.e. from non-parental adults) increases phenotypic diversity by protecting maladapted phenotypes from selection. We further found that the population optimal strategy is for most individuals to use oblique transmission, but only low rates of oblique transmission can evolve, unless environmental changes are very rapid.