That perception is wrong, say Indiana University Bloomington scientists in this week's Nature. Their analysis of 882 plant and animal species and 1,347 inter-species crossings -- the first large-scale comparison of species barriers in plants and animals -- showed that plant species are just as easily categorized as animal species.
The study also yielded a surprise. The hybrid offspring of different animal species are more likely to be fertile than the hybrid offspring of plant species.
"Not only are plants just as easily subdivided into species as animals when analyzed statistically, plants are more likely to be reproductively isolated due to hybrid sterility," said evolutionary biologist Loren Rieseberg, who led the study. "Most plant species are indeed 'real.' The problem has been that botanists have been way over-attracted to the plant species that readily hybridize and where the hybrids perpetuate themselves asexually. While it's true that dandelions and blackberries pose problems, these horror stories only make up 1 percent of the whole."
The scientists did find categorization problems with nearly half of the plant and animal species they surveyed.
Rieseberg and his co-authors, doctoral student Troy Wood and postdoctoral research associate Eric Baack, examined hundreds of peer-reviewed papers reporting the measurement of various plant and animal characteristics, or reporting on the success or failure of hybridization of plant and animal species with similar species. The scientists culled the papers for information, grouped and combined data for each given species, and then looked at how often characteristics clustered in accordance with named species.