Nice paper about willow trees
Regional patterns of genetic structure and environmental differentiation in willow populations (Salix humboldtiana Willd.) from Central Mexico https://onlinelibrary.wiley.com/doi/pdf/10.1002/ece3.5475
The willows grow along rivers in Mexico. The study asked how much the genetic structure of the populations matched the rivers the trees grew along - and boy howdy does it! Trees growing along the same river are more related than trees growing along different rivers.
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