When it Comes to the Evolutionary Progress of Snails, Size Does Matter...
Were Niles Eldredge and Stephen Jay Gould wrong about their evolutionary theory of Punctuated Equilibrium (PE)? When it comes to the endemic land snail subgenus Poecilozonites of Bermuda, a pillar of their 1972 PE theory, new research by Paul J. Hearty and Storrs L. Olson suggests that this example may be flawed. Using the rock record and novel dating methods to study the same snail fossils examined by Gould, Hearty and Olson have postulated an alternative process to explain striking size changes observed in the Bermuda snails at key intervals over the last half million years.
To understand and interpret the geology of Bermuda, Hearty, an adjunct research associate professor in the Department of Environmental Studies at the University of North Carolina Wilmington, and Olson, paleobiologist and senior scientist emeritus at the Division of Birds, Smithsonian Institution, collected land snails and other fossils for dating, and used amino acid racemization (AAR) to reconstruct the morphological changes in fossil forms over time. AAR measures the decomposition of proteins and amino acids in the carbonate shells of fossils, which allowed Hearty and Olson to rank the age of every snail form in order to evaluate Gould’s work.
Poecilozonites is of great importance to PE, considered by many to be the prevalent modern theory explaining the nature and tempo of evolution. In his earlier work, Gould compared the evolution of these snails to the adaptive radiation of the Galapagos finches, which evolved different bill types perfectly adapted to their particular habitat and food type. Gould defined three major lineages that co-existed on Bermuda over several hundred thousand years, postulating that larger shell species evolved when carbonate was minimal in the landscape (such as during low sea levels)
Hearty and Olson’s research on Poecilozonites revealed a different evolutionary story. When more than 400 shells representing all known forms from Bermuda were dated and sequenced using AAR, the data did not confirm Gould’s hypothesis of tree-like adaptive radiation(like Darwin’s finches). Instead, the data showed the evolution of a single lineage, changing in size and shape in response to colonization or extinction of predators. Those changes coincided with dramatic, order of magnitude changes in island size driven by interglacial-glacial sea-level changes.
With the ups and downs of sea level, Bermuda shrank in size to less than 50 km2 during highstands and expanded to more than 650 km2 during glacial lowstands. Although no ice ever came near Bermuda, lower sea levels exposed an oceanic island plateau with lush habitats and a greater land biodiversity.
“The labile Poecilozonites quickly grew large during the lower sea level periods in response to an increase in predators,” said Hearty (left). “So it was predation, not the availability of carbonate or other factors, which drove increases in shell size. Greater shell size reduced predation by cranes, rails, ducks and tortoises, allowing the species to survive.”
As the lowstand plateau was inundated by rising seas, according to Olson and Hearty, the biodiversity and land area available to terrestrial organisms was vastly reduced. Large predators died out due to loss of their luxuriant habitat, reducing predation of the snails, which then became several times smaller.
These findings attribute the entire succession of Poecilozonites to a single anagenetic lineage and not a genetically-driven adaptive radiation as promulgated by Gould. Thus, Hearty and Olson’s research largely invalidates Gould’s best example of PE. Although the theory may be still alive and well, new examples are needed to demonstrate this evolutionary process.
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