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The foundation of a taxonomy of glomalean fungi is stable and hierarchical patterns of morphological divergence, and for that reason alone a detailed and thorough analysis is warranted. Because of the simplicity of fungal design and organization, most morphological change has occurred in asexual spores, both in mode of formation and differentiation of subcellular structure. Even though arbuscular fungi are asexual organisms, immediate fixation and strong heritability of whatever neutral or positive genotypic and phenotypic traits evolved over time assures that all progeny generations can be grouped diagnostically as a genealogically conserved morphospecies. Asexual species of fungi in Glomales are cohesive taxonomic entities (whether monophyletic or polyphyletic) because of strong constraints on the direction and magnitude of morphological variation and these are independent of other processes, ecological or otherwise. The study of morphology provides insights into initial hypotheses of speciation in arbuscular fungi. The number of species in Glomales has long been considered to be low (154), especially relative to that observed in other symbiotic fungal groups such as ectomycorrhizal fungi (20,000) as well as plant groups (250,000). This difference is attributed to asexuality, a rather homogeneous environment within plant roots for much of the fungal life cycle, and possibly other factors about which we know little. However, ecology really has little to do with the origin of morphological variation; rather it acts on that variation so that only the beneficial attributes persist from one generation to the next. It is the design and complexity of an organism that determines the extent to which new variants can be integrated into the developmental program without causing harm. Since speciation in arbuscular fungi involves divergence only in component parts of single somatic cells that become spores, the number of species known today actually seems miraculously large. We adopt the view that the ancient origin of the arbuscular mycorrhizal symbiosis and cospeciation between glomalean fungi and their plant hosts has afforded both partners expanded evolutionary potential that would not have been realized if each had evolved separately.
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