Extracts the stem age from the phylogeny when the a species is known to belong to a genus but is not itself in the phylogeny and there are members of the same genus are in the phylogeny. The stem age can either be for the genus (or several genera) in the tree (stem = "genus") or use an extraction algorithm to find the stem of when the species colonised the island (stem = "island_presence), either 'min' or 'asr' as in extract_island_species(). When stem = "island_presence"the reconstructed node states are used to determine the stem age.
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