Parasitic plants have aroused curiosity among scientists for centuries, yet they remain one of the most poorly understood groups of flowering plants (angiosperms), and much of their evolutionary biology remains a mystery. There are approximately 4,000 species of parasitic plants (in 19 families) which occur in all major biomes, from arctic islands to tropical forests. The evolutionary shift to parasitism has been associated with the degeneration of morphological features traditionally used in plant classification to infer evolutionary relationships, making systematic studies of the relationships between parasitic plants and their photosynthetic ancestors extremely difficult. Therefore until very recently, our understanding of the evolutionary origins of parasitic angiosperms has lagged behind that of other major groups of angiosperms. Parasitic plants show considerable variation in their host specificity. For example, some species infect hundreds of species from taxonomically diverse families, while others are restricted to a single host species. Host ecology may isolate parasite populations and facilitate genetic divergence. For example in leaf-eating (phytophagus) insects which, like parasitic plants, are discriminate users with respect to the host plants on which they feed, host specificity has been demonstrated to drive genetic divergence, and ultimately speciation. However speciation in parasitic plants has, until recently, remained relatively unexplored by evolutionary biologists, and our understanding of host specificity as a potential catalyst for speciation in these plants has lagged behind that of phytophagus insects. Now, in light of recent research, a similar pattern of host-driven speciation in parasitic plants is emerging. This chapter reviews recent research into the host specificity of parasitic plants, which appears to be an important and underestimated promoter of genetic divergence and speciation. © 2013 by Nova Science Publishers, Inc. All rights reserved.
