CreamCloud0
Hero
i was only referring to the half elf as an example in that they have notable features derived from both speciesI dont consider a "Half Elf" to be a "species". That said. I am fine with the Eberron setting having the Khorvaire as a separate species originate from a sotospeak magical gene splicing between Elf and Human species.
Normally a Human and an Elf cannot reproduce offspring. In most stories where such offspring did happen, it was the Elf parent who magically shapechanged into a Human. The offspring are Human but there is evidence of the elven magical inheritance. An origin feat accommodates this narrative trope, even taking levels in a caster class can.