Bearded Dragon

Why did beards evolve (in us and these lizards)? Were the lizards in Holes real? Will bearded dragons changing sex decimate the species?

Bibliography: https://docs.google.com/document/d/1Gi3tAGnuhqMVB4w9i-bjB804WPaluKvb3TdrD7bpVDE/edit?usp=sharing