On April Fool's Day it seems appropriate that I chase a fool's errand: defining a plant species. While Ernst Mayr's Biological Species Concept is still widely taught, its many weaknesses in non-avian organisms have been apparent for decades. This week I read in the Grey Lady that African elephants are now considered to be comprised of two species with the smaller population of forest elephants considered critically endangered. Clearly conservation efforts like IUCN red listing can be undercut by taxonomic shenanigans.
Current phylogenetic work based heavily on DNA studies is finding interesting phenomena, but there is likely more going on. We often don't know the functional meaning of DNA differences. How much divergence in DNA sequences makes a species?
Clearly, if a single change can instantly make a plant reproductively isolated from others (I'm lookin' at you, polyploidy), then you've got new species by one way of thinking. Larrea forms a polyploid complex across the southwest. Generally speaking, L. tridentata in the Chihuahuan Desert is diploid, while in the Sonoran creosote bush is tetraploid and Mohave Larrea is hexaploid.
Recently (O'Connor, Laport, Whiteman, 2019) showed some interesting distribution patterns with gall midges on creosote. "Contact zones between plant cytotypes are dispersal barriers for many Asphondylia species due to plant-insect interactions. The distribution of L. tridentata cytotypes therefore shapes herbivore species ranges and herbivore community structure across North American deserts. Our results demonstrate that polyploidy in plants can affect the biogeography of ecological communities."
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