Bigger is better for successful evolution in the ocean

Photo credit: The Biologos Forum  The reconstructed skeletons (black) from modern whales (top) and various ancestral skeletal forms (series below) are in chronological order (from Pakicetus up). Relative body size, to scale, is indicated by the gray shapes at the right of each animal.

Photo credit: The Biologos Forum
The reconstructed skeletons (black) from modern whales (top) and various ancestral skeletal forms (series below) are in chronological order (from Pakicetus up). Relative body size, to scale, is indicated by the grey shapes at the right of each animal.

A recent study published on February 20th 2015, in the Journal Science, reveals that evolution appears to follow certain rules for at least one trait, body size. Research shows that evolution of marine organisms has been favouring growth for more than 500 million years.

This realisation provides support for Cope’s rule (the idea that animal lineages evolve towards larger body sizes over time). Previous research on terrestrial animals has shown the rule holds true, but it was unknown whether or not this was the case in the ocean.

A data set of body sizes was compiled, containing 17,208 genera of marine animals spanning the past 542 million years. Five major phyla were represented – arthropods, brachiopods, echinoderms, molluscs and chordates, making up over 74% of all the animal genera ever to have lived.

The minimum body size for marine animals has decreased by only 10 percent over time whereas the maximum body size (the blue whale) has increased by a factor of 100,000 since the Cambrian period. The huge data set revealed that while some groups like crustaceans have gotten a little smaller, marine organisms as a whole are getting bigger. These results mean that not every single genus evolved to grow larger. Alternatively, larger animals diversified and expanded, producing a greater variety of bigger organisms.

The findings show that the mean size of marine animals has increased 150-fold over the last 542 million years. There clearly has been an evolutionary preference for larger body size which cannot be described by neutral drift.

It has been suggested that size may offer marine animals advantages such as being able to capture larger prey and being easier to avoid becoming prey by being faster and better at burrowing in sediment.

Air breathing could explain the rapid and consistent large size in marine reptiles and mammals. Larger air breathing animals can have faster metabolic rates, permitting more active lifestyles by breathing air rather than water. Air has a greater concentration of oxygen than water and the diffusion rates of oxygen through membranes are 300,000 times faster so they would be delivered extra oxygen. While these findings are important, they are not driving the trend as there are low numbers of these individuals. There is a huge diversity of other phyla and these together sustained a trajectory of growing size.

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Lucy Grable

Lucy Grable

MSc Species Identification and Survey Skills student at Reading Uni | BSc Marine Zoology | Website Editor MARINElife | Zanzibar humpback whale researcher|Marine wildlife enthusiast

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