Friday, April 10, 2015

Do Marine Animal Lineages Evolve Toward Larger Body Size Over Time

aka Cope's Rule-

I like these big questions about the history of life and I am fascinated and very impressed when palaeontologists take up such questions. It is incredibly laborious and time consuming work, to go through archival data on fossils and often generate new data from museum specimens and older compilations describing fossil taxa.

A recent study in Science Magazine:

Cope’s rule in the evolution of marine animals - Noel A. Heim, Matthew L. Knope1, Ellen K. Schaal, Steve C. Wang, Jonathan L. Payne

Cope’s rule proposes that animal lineages evolve toward larger body size over time. To test this hypothesis across all marine animals, we compiled a data set of body sizes for 17,208 genera of marine animals spanning the past 542 million years. Mean biovolume across genera has increased by a factor of 150 since the Cambrian, whereas minimum biovolume has decreased by less than a factor of 10, and maximum biovolume has increased by more than a factor of 100,000. Neutral drift from a small initial value cannot explain this pattern. Instead, most of the size increase reflects differential diversification across classes, indicating that the pattern does not reflect a simple scaling-up of widespread and persistent selection for larger size within populations.

What that means is that the size increase is not due to a uniform increase across all animal groups. Rather, groups that were larger very early in animal evolution have diversified disproportionally more than smaller sized groups. Why should that happen? The authors suggest that there may be advantages to being larger, such as, ability to move faster, to capture larger prey and to burrow deeper for protection and exploiting additional food resources.

That would seem to make larger animals more resilient to background extinction and make larger sized lineages longer lived. But why would that make larger animals more speciose? i.e. why would larger sized animals species split into more new species than smaller sized animal species? ..because that is what is the claim, that throughout the history of animal evolution larger sized species gave rise to more new species than smaller sized ones (differential diversification). In fact, one could make arguments favoring higher rates of speciation in smaller sized organisms, such as, their ability to disperse over greater geographic area resulting in greater chances of populations getting reproductively isolated resulting in new species, their ability to survive better during environmental crises (survivor fauna after mass extinctions tend to be smaller bodied, mass extinctions seems to kill of larger bodied species disproportionately). If mass extinctions differentially kill off larger bodied species, then is the observed trend really a series of trends, each reset at the aftermath of the crises, resulting in small pioneer /survivor fauna evolving towards larger size. There could be a physical limit to how small one could become and the only direction for size to vary (either through drift or natural selection) would be towards a larger size. I am just speculating without even reading the paper, the authors do mention that drift from a small initial size does not explain their findings, but it would interesting to know what role mass extinctions might be playing in disruption or amplifying trends.

So although a trend is apparent, the answers are not all clear cut. It would also be interesting to group the trends according to life habits, i.e. planktonic versus benthic, sessile versus mobile  and see if any of these life styles particularly favors evolution towards larger size.

Eurekalert has a summary of the study

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