Seeley, Robin Hadlock. \"Intense Natural

Seeley, Robin Hadlock. "Intense Natural Selection Caused a Rapid Morphological Transition in a Living Marine Snail." Proceedings of the National Academy of Sciences of the United States of America, 83.8 (Sep. 15, 1986): 6897-6901.

One of the most controversial issues within the scientific community surrounding evolution is the extent to which rapid evolution is possible. While there is no doubt that there are seismic 'gaps' in the existing fossil record that is available for study, the causation of these apparent 'jumps' in the evolutionary process is not self-evident. To fill in some of these gaps, evolutionary biologists study apparent evidence of evolution within the contemporary animal kingdom. This article is an examination of such a review.

Specifically, the article deals with a snail native to Northern New England known as Littorina obtusata. Researchers observed that the snail's shell shape and shell thickness had apparently altered in a noticeable fashion between the years 1871-1984. Because of the information available from previous research, scientists were able to discern that the shells of the snails that were harvested before 1900 were almost exclusively characterized by high-spierd, thick walls, versus the shells collected of a far more recent duration from 1982-84. These more recently gathered shells were largely characterized by low-spierd, thick walls. In one instance, the snails shells collected in Nahant, Appledore Island, or Isle au Haut prior to 1900 were higher- spierd and thinner than those collected in 1982-1984, showing a deviation of .9 within at most 86 generations, which is highly unusual in terms of the rapidity of morphological change.

The specific controversy the study of the snail was attempting to address was the contention by some evolutionary biologists that the gaps in fossil records are not due to natural selection within a species but are instead due to the development of entirely new species. Other evolutionary biologists suggest that environmental pressures can create a phenomenon whereby natural selection creates a rapid transformation within a species, not simply gradual change over time. It was believed that the rapid internal natural selection during 1982-1984 was due to the fact that the animal that fed upon the snail, the crab Carcinus maenas (L.) had entered into the area. Snails with high- spierd shells were found to be far more vulnerable to this predator, as demonstrated in both field and research studies, which observed the crab's behavior with the two types of snails. Snails with low-spierd shells, because of the increased overlap in their shells, are better defended from crab attacks. The crab had no presence within the area under study before 1900 but its numbers began to steadily increase after that date. The fact that the snail populations of high and low-spierd shells are not genetically isolated further confirmed the notion that rapid evolution through natural selection had taken place within the snail population.