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.