In a species of plant-eating land snail, the shell color is variable and ranges from very light to intermediate to very dark in color. The plants are dark in color. The snails are preyed upon by birds that use sight to find their prey. A small population of these snails is moved to an island where the food plants in their preferred habitat are either very light in color or very dark. The predator is the same. Required:
a. What type of selection do you predict for the snails initially?
b. How about when they are moved to the island?
c. Provide support for your answers.
d. Graph what is the population of the snail species looks like over time, before they are moved to the island.
e. Graph what they will look like over time when they aremoved to the island. Make sure to label your axes

Answer and Explanation:

Natural selection is the result of the phenotype-environment interaction which determines gene destiny in space and time, selecting beneficial alleles and increasing their frequency in the population. There are different types of natural selection: sexual selection, stabilizing selection, directional selection, frequency-dependent selection, and disruptive selection.  

a. What type of selection do you predict for the snails initially?

    Initially, Directional Selection was acting.

Directional selection increases in the proportion of individuals with an extreme phenotypic trait, in the exposed example this trait is the dark shell. This selection occurs more frequently in those cases in which interactions between living organisms and the environment modify in the same direction. Initially, snails were living in an area where plants were all dark-colored, and in order not to be seen by the birds that preyed on them, directional selection acted on the snails to turn them dark-shelled, so they camouflaged better with vegetation and avoided being eaten by birds.    

b. How about when they moved to the island?

  When they moved to the island, Disruptive Selection was acting.

The disruptive selection causes an increase in the two types of extreme phenotypes over the intermediate forms. In the exposed example the extreme phenotypes were light-shelled and dark-shelled. Limits between one extreme and the other are frequently very sharped. Individuals belonging to one phenotype can not live in the same area as individuals belonging to the other phenotype, due to the traits differences between them, competition, predation, etcetera.  In the exposed example, when snails went to the island, where there was light and dark vegetation, and there were also birds as predators. As snails ate both types of plants, they had to camouflage by turning either light-shelled or dark-shelled. So those dark individuals could only live on dark plants, and light individuals could only live od light plants. In this way, they could avoid predation by birds, because they could not be seen.

d. Graph what is the population of the snail species looks like overtime before they are moved to the island.  In the attached file you will see that the curve tends to be in one of the extremes, the one that corresponds to the dark phenotype.  The red arrow is indicating the direction of the change. Intermediate individuals tend to be in the middle of the space, while light individuals are located in the other extreme. The Y-axe represents the percentage of individuals of each phenotype that got to survive over time. The chart shows that only dark individuals survived, while light and intermediate decreased to near cero probably due to predation.

e. Graph what they will look like over time when they moved to the island. In the attached file. You will see that the curve tends to be divided, showing maximums in the extremes of the X-axe, which corresponds to light individuals in one of the extremes and dark individuals in the other extreme. The red arrows are indicating the direction of the change. Intermediate individuals tend to be in the middle of the space, where you can see that the percentage of surviving individuals is almost cero. The intermediate phenotype did not get to survive to predation. The chart shows that only dark and white individuals survived.

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