Alnus seem to have originated as a monophyletic genus, and a sister genus to Betula, within the family Betulaceae. It may have originated at or before the Late Cretaceous and developed further in the Tertiary Paleocene and Eocene. The major biodiversity focus is (now at least) in Eastern Asia. The American species, including those in Central and South America, appear to have crossed from Asia mainly via the Beringian Land Bridge. Currently there are 18 - 23 species in Asia, 4 - 5 in Europe and 9 in America. The only one that strays into Africa is the Holearctic Alnus glutinosa.
As well as the Frankia actinomycete nodules, seedlings may have AM mycorrhizae, and ECM mycorrhizae are often found throughout the life cycle.
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