Helsen P., Browne R. A., Anderson D. J., Verdyck P., Van Dongen S., Galápagos' Opuntia (prickly pear) cacti: Extensive morphological diversity, low genetic variability, Biological Journal of the Linnean Society, 2009, 96(2):451-461, doi: 10.1111/j.1095-8312.2008.01141.x
A comparison of morphological diversity in Galápagos prickly pears with sequences from four DNA regions. The paper shows low variation in some of the markers studied, discusses local gene flow and identifies the value of considering plant traits and genetic data together.