Cope's Rule in a modular organism: Directional evolution without an overarching macroevolutionary trend

Abstract

Cope's Rule describes increasing body size in evolutionary lineages through geological time. This pattern has been documented in unitary organisms but does it also apply to module size in colonial organisms? We address this question using 1169 cheilostome bryozoans ranging through the entire 150 million years of their evolutionary history. The temporal pattern evident in cheilostomes as a whole shows no overall change in zooid (module) size. However, individual subclades show size increases: within a genus, younger species often have larger zooids than older species. Analyses of (paleo)latitudinal shifts show that this pattern cannot be explained by latitudinal effects (Bergmann's Rule) coupled with younger species occupying higher latitudes than older species (an “out of the tropics” hypothesis). While it is plausible that size increase was linked to the advantages of large zooids in feeding, competition for trophic resources and living space, other proposed mechanisms for Cope's Rule in unitary organisms are either inapplicable to cheilostome zooid size or cannot be evaluated. Patterns and mechanisms in colonial organisms cannot and should not be extrapolated from the better‐studied unitary organisms. And even if macroevolution simply comprises repeated rounds of microevolution, evolutionary processes occurring within lineages are not always detectable from macroevolutionary patterns.

From zygote to a multicellular soma: body size affects optimal growth strategies under cancer risk

“…This adds ontogenetic challenges to the reasons why Cope's rule, the increase of body sizes in macroevolutionary lineages, is expected to be a rather stochastic 'rule' (see e.g. Liow and Taylor, 2019).…”

Trait–fitness associations do not predict within-species phenotypic evolution over 2 million years

“…However, it is known from microsatellite studies of the closely related C. hyalina that larvae settle randomly with respect to kin [ 54 ]. We used one to several small random ‘spots’ of preserved colonies to estimate morphological traits and fecundity (electronic supplementary material, tables S1 and S2), but note that much of the within-species variation is captured by only measuring few zooids within few colonies [ 24 ]. We also assumed (see electronic supplementary material, figures S7–S9) that we have captured much of the 30–50% of trait variation that is external to within-colony variation [ 55 ].…”

“…We were able to make measurements for 311 distinct colonies (electronic supplementary material, table S2), recognized by the geometry and direction of growth. The median number of zooids measured per distinct colony is 13, but we note that within-colony morphometric variation can be captured by measurement of as few as three zooids [ 24 ]. Figure 1 shows how these traits were measured.…”

“…Overgrowth has the effect of covering exit points (orifices) of feeding lophophores and hence cause partial to complete death of the overgrown colony. Whereas there is no large-scale trend in increasing autozooid size over the evolutionary history of cheilostome bryozoans despite apparent advantages, there is a tendency for ancestral species to give rise to descendent species that have larger autozooid sizes in the same lineage [ 24 ]. Hence, our naive prediction for A. tongima is that there should be a positive trait–fitness association for autozooid size.…”

Skeletal resorption in bryozoans: occurrence, function and recognition

“…Approximately 6600 living species have been described, as well as over 15000 extinct species, dating back as far as the early Paleozoic (McKinney & Jackson, 1989; Bock & Gordon, 2013; Orr et al ., 2018). The extensive, diverse and character‐rich fossil record of the Bryozoa has made this phylum an important group for stratigraphic studies, palaeoenvironmental analyses and for testing ecological hypotheses and evolutionary theories, such as punctuated equilibrium and clade replacement (Cuffey, 1967; Nelson et al ., 1988; Jackson & Cheetham, 1994; McKinney et al ., 1998; Liow & Taylor, 2019). All of these applications focus on bryozoan skeletal morphology.…”