Darwin's Theory of Evolution by Natural Selection

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Roxann Dorodian
1215612
BIOE 109E
June 23, 2012

L. obtusata a Model Organism for Understanding Darwin’s Theory of Evolution by Natural Selection ABSTRACT:
The mechanisms for individuals in a population to survive heterogeneous habitats have been a strong debate within the scientific community. The native European Littorina obtusata are a great model organism to study Darwin’s Theory of Evolution by Natural Selection. This is because of their vast phenotypic characteristics, particularly their evolving shell thickness throughout history due to predatory pressures of the new arrival of Carcinus maenas. Here we test whether L obtusata three most critical tenants of Darwin’s theory of evolution by natural selection; variation within a population, heritability, and selection. By observing the shell thickness of East Coast of N. America L. obtusata where predatory C. maenas are common versus the West Coast of N. America where C. maenas are not found, it will further provide evidence whether these shell thickness differences are due to natural selection predatory or phenotypic plasticity occurring. This was tested by placing banded crabs in both East and West populations of L. obtusata and observing how their shell thickness decreased, further proving that L. obtusata cannot sense or smell the presence of C. maenas. These findings provide further evidence for Seeley’s research that the accelerating morphological changes of L. obtusata shell thickness was in response to strong selection by C. maenas and not because speciation occurring. INTRODUCTION

Scientists have long been fascinated by the variation in nature but there has been controversial debate over the mechanisms that produce the pattern in life’s history. Some believe that Darwin’s theory of evolution by natural selection is operating the morphological breaks found in fossil records within a population. Darwin’s book On the Origin of Species listed four basic postulates to natural selection; there must be variations among individuals in a population, variations must be heritable, survival and reproduction must be variable and non-random (Darwin, 1859). As a result, every generation there will be slight changes in the population. One scientist in particular, Robin Seeley, tried understanding Darwin’s theory of Natural Selection by studying the thickness of shells of flat periwinkles, Littorina obtusata, of Appledore Island and the new arrival of predatory green crabs, Carcinus maenas. Seeley noted acceleration in natural selection that normally is not observed within the L. obtusata population in response to strong selection by C. maenas. This is because shell thickness affects the survivability and reproduction of L. obtusata when C. maenas claws break them to meal (Trussel, 1996). The observation confirmed that when C. maenas were introduced in the East Coast of N. America, L. obtusata range of shell thickness increased compared to the West Coast of N. America L. obtusata populations where predatory C. maenas were not found (Seeley, 1986). These vast morphological differences in their shell thicknesses was thought to occur because of the C. maenas selection for the thinner-shelled L. obtusata. Seeley’s research further showed that speciation was not occurring within the intertidal snails as many previous scientists claimed (Seeley, 1986). Some scientists prefer to restrict the concept of phenotypic plasticity for producing the geographic variation within the intertidal snails (Trussell, 2001). Phenotypic plasticity is the ability for an individual to change its phenotype at a single genotype in response to changes in the environment (Whitman, 2009). Although Seeley’s research is a great model to follow it lacked evidence in one of the three important conditions of Darwin’s theory of evolution by natural selection to occur, heritability. Seeley’s data demonstrated that when C. maenas were present, shell thickness increased but it did not provide evidence that the...
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