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.
Highlights
Body size variations closely reflect important individual differences in life history traits
Many of our insights on life history traits in general, and especially macroevolutionary patterns of changes in size, have come almost entirely from unitary metazoans rather than modular colonial metazoans (Alroy 1998; Hunt and Roy 2006; NovackGottshall and Lanier 2008; Heim et al 2015). This dearth of insights and theory from clonal or colonial perspectives (Hamilton et al 1987) is in part because body size, a trait relatively measured in unitary organisms, can be difficult to quantify in modular colonial organisms, such as cnidarians, ascidians, and bryozoans, as these often show indeterminate growth (Heino and Kaitala 1999)
To test the possibility that the AD patterns might be an artifact of older species from lower latitudes giving rise to younger species that moved into higher latitudes and, had larger zooids, we modeled changes in zooid size in AD pairs as a function of the change in latitude using all the AD pairs we could generate (“AD pair uncertainty treatment 1”)
Summary
Body size variations closely reflect important individual differences in life history traits. Many of our insights on life history traits in general, and especially macroevolutionary patterns of changes in size, have come almost entirely from unitary metazoans rather than modular colonial metazoans (Alroy 1998; Hunt and Roy 2006; NovackGottshall and Lanier 2008; Heim et al 2015) This dearth of insights and theory from clonal or colonial perspectives (Hamilton et al 1987) is in part because body size, a trait relatively measured in unitary organisms, can be difficult to quantify in modular colonial organisms, such as cnidarians, ascidians, and bryozoans, as these often show indeterminate growth (Heino and Kaitala 1999). TAY L O R size in corals scales in a predictable way, where species with larger polyps exhibit a higher reproductive output per polyp, albeit coupled with a lower reproductive output relative to their investment in somatic tissues (Leuzinger et al 2003)
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