A study reported in the journal Science provides new support for the so-called Cope’s rule – a theory in biology that states that animals tend to evolve toward larger sizes over time. It reveals that over the past 542 million years, the mean size of marine animals has increased 150-fold.
The blue whale (Balaenoptera musculus) is the largest extant animal and the heaviest that has ever existed. Image credit: NOAA / CC BY 2.0.
Named after American paleontologist Edward Drinker Cope, Cope’s rule was formulated in the 19th century after scientists noticed that the body sizes of terrestrial mammals such as horses generally increased over time.
Researchers have attempted to test the rule in other animal groups, but the conclusions have been mixed.
Corals and dinosaurs seem to follow the rule, for example, but birds and insects do not.
As a result, some scientists have wondered whether the pattern observed in land mammals is a real evolutionary phenomenon or merely a statistical one resulting from random, non-selective evolution, also known as neutral drift.
To test Cope’s rule across all marine animals, the authors of the study – scientists from Stanford University and Swarthmore College – compiled a data set of body sizes for 17,208 genera spanning the past 542 million years and five major phyla (arthropods, brachiopods, chordates, echinoderms, and mollusks).
To compile such a vast dataset, the scientists relied heavily on the Treatise on Invertebrate Paleontology, a 50-volume book set that includes detailed information about every invertebrate animal genus with a fossil record known to science.
Using photographs and detailed illustrations of fossils in the Treatise, they were able to calculate and analyze body size and volume for the marine animals.
A pattern soon became apparent – not all classes of marine animals trended toward larger size, but those that were bigger tended to become more diverse over time.
The researchers suspect this is due to advantages associated with a larger size, such as the ability to move faster, burrow more deeply and efficiently in sediment, or capture larger prey.
To investigate what might drive these trends toward larger body sizes, the scientists entered their measurement data into a computer model designed to simulate body size evolution.
Beginning with the smaller species from each phylum, the model simulated how their body sizes might change as they evolved into new species.
“As time marches forward, each species is assigned some probability of producing a new species, of remaining the same, or of going extinct, at which point it drops out of the race,” said Dr Noel Heim from the Stanford University’s Department of Geological and Environmental Sciences, the first author on the study.
When a new virtual species was created, the model assigned the new creature a body size that could be bigger or smaller than its ancestor.
Dr Heim and his colleagues ran multiple simulations, each with different assumptions.
One scenario assumed a neutral drift model of evolution, in which body size fluctuates randomly without affecting the survival of the species.
Another scenario assumed natural selection of body size, in which having a larger body size confers certain survival advantages and is thus more likely to propagate through the generations.
The team found that the neutral drift simulation could not explain the body size trends observed in the fossil record.
“The degree of increase in both mean and maximum body size just aren’t well explained by neutral drift. It appears that you actually need some active evolutionary process that promotes larger sizes,” Dr Heim said.
The scientists believe that the database they compiled will be useful for studying other questions related to body size, such as whether or not organisms near the equator are, on average, bigger or smaller than those living at higher latitudes.
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Noel A. Heim et al. 2015. Cope’s rule in the evolution of marine animals. Science, vol. 347, no. 6224, pp. 867-870; doi: 10.1126/science.1260065
