Organismal rules with no evidence (Rensch’s rule, Foster’s rule)
Rensch’s rule describes a pattern of increasing sexual size dimorphism with increasing body size in species where males are larger than females and decreasing sexual size dimorphism where females are larger than males (Rensch 1950). The rule appears widespread in animals and particularly birds, where strength of sexual selection is the strongest predictor of Rensch’s rule (Dale et al. 2007). This rule has not been tested in microorganisms, to our knowledge, because most microorganisms lack distinct sexes. Prokaryotic microbes such as bacteria are predominantly asexual, while eukaryotic microbes such as fungi and protists may reproduce clonally or through diverse recombination strategies (Narra & Ochman 2006; Ni et al. 2011; Weedall & Hall 2015). Furthermore, body size can be difficult to assess among microorganims due to their small size and body plans that may change across the lifecycle (i.e. filamentous fungi, which possess differing vegetative and reproductive forms).
Another macroecological rule that examines body size patterns is Foster’s rule, where taxa show increased or decreased body size on islands compared to mainland populations (Foster 1964; Van Valen 1973). Foster’s rule has been studied extensively among mammals and a variety of other animals, but support for the rule is mixed even for mammals and particularly low for non-mammalian species (Lokatis & Jeschke 2018). No study, to our knowledge, has examined Foster’s rule for microorganisms.