Part 4 - Natural Selection and Adaptations 3 of 113 of 11 Items

Item 3
A scientist studied a snail population that had a variation of shell colors. The number of each differently colored snail shell was recorded. The graph shows the data the scientist collected.

Over time, the rocks on which the snails lived became covered with dark-colored seaweed.

Which graph best represents the range of colors in the snail population that resulted from natural selection after the dark-colored seaweed covered the rocks?

There are different types of natural selection. When one of the extremes phenotypes is being favored, directional selection is going on. Meanwhile, balancing selection increases the proportion of intermediate phenotype.

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Since I do not have the graphs, I will set different potential scenarios

Scenario 1: More light brown individuals than brown and dark brown

Probably after the dark-colored seaweed covered the rocks, snails needed to camouflage to avoid predation. So they must have suffered a change in color from light brown to dark brown.

Since this new trait is beneficial, natural selection favored it.

So animals change from being light brown to dark brown.

The type of natural selection acting here is directional selection.

A population that is under the effects of directional selection, experiences an increase in the proportion of individuals expressing an extreme phenotype, different from the original one.

In these cases, interactions between living organisms and the environment modify in the same direction.

So, the most abundant individuals turned from being light brown to being dark brown.

After dark-colored seaweed covered the rocks, the number of dark brown individuals increased, and the light brown individuals decreased.

                                            **********************

Scenario 2: More brown individuals than light brown and dark brown

This is the same situation as before. In this case, there were more brown individuals than snails expressing extreme phenotypes.

                                            **********************

Scenario 3:

Let us suppose that the original number of individuals was higher for one of the extreme traits, either yellow or grey.

After the seaweed-covered rocks, snails needed to camouflage to avoid predation, so they changed their color from yellow or grey to brown.

In this scenario, the intermediate phenotype is being favored by natural selection over the extreme phenotypes.

Balancing selection -or Stabilizing selection-, decreases the number of individuals expressing the extreme phenotype and favors individuals that exhibit medium-range characteristics. These are the ones that get to survive. Selective pressure is against h0m0zyg0tes.

In this case, the number of individuals expressing yellow or grey shells decreases, while individuals with brown color increase.

In the attached files you will find graphs for the three scenarios.

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because photosynthesis could not occur (apex)