The Shape And Thickness Of Intertidal Snail Littorina...

Charles Pham Evolution Dr. Hellack Feb. 4th, 2016 Between 1871 and 1984, the shape and thickness of intertidal snail Littorina obtusata has changed dramatically in northern parts of New England. Shells that were collected in the 19th century were observed to possess characteristics of being high-spired with thin walls, while those collected between 1982 and 1984 were low-spired with thick walls. It was noted that an intertidal crab Carcinus maenas which preys on L. obtusata had ironically expanded its range into the northern parts of New England around 1900. (Seeley, 1986). Considering the significance of these two various characteristics, one may hypothesize that the various shell morphology is due to speciation (Ha) or, with careful experimentations, another proposed hypothesis (Ho) can be that the change in L. obtusata shell morphology was due to natural selection in response to the predation of C. maenas. The shape and thickness of the shell (dependent variable) is a function to the presence of C. maenas (independent variab le). Rapid morphological transitions in organisms have been difficult to witness, especially for ecologist and paleontologist. This has raised concerns for some evolutionary biologist whom suggest that transitional forms are not visible in the fossil record and represent speciation events. Seeley (1986) reported that a rapid morphological transition did indeed occur in the intertidal snail Littorina obtusata between 1871 and 1984 in response