Problem 15 - Campbell Biology Concepts & Connections

Question

A population of snails is preyed on by birds that break the snails open on rocks, eat the soft bodies, and leave the shells. The snails occur in both striped and unstriped forms. In one area, researchers counted both live snails and broken shells. Their data are summarized below: $$ \begin{array}{|lcccc|} \hline & & & \underline{\phantom{xxx}} \\ & \text { Striped } & \text { Unstriped } & \text { Total } & \text { Percent Striped } \\ \text { Living } & 264 & 296 & 560 & 47.1 \\ \text { Broken } & 486 & 377 & 863 & 56.3 \\ \hline \end{array} $$ Which snail form seems better adapted to this environment? Why? Predict how the frequencies of striped and unstriped snails might change in the future.

Step-by-Step Solution

Answer

Unstriped snails are better adapted due to lower predation rates. Their frequency will likely increase, while striped snails' frequency will decrease.

Step 1 - Understand the data

The given table presents data on two forms of snails – striped and unstriped. It shows counts for living snails and broken shells for both forms.

Step 2 - Calculate the predation rate

Calculate the rate at which birds prey on each form of snails:\(\text{Predation Rate}_{\text{Striped}} = \frac{486}{264 + 486} = \frac{486}{750} \approx 0.648 \ (or \ 64.8\% )\)\(\text{Predation Rate}_{\text{Unstriped}} = \frac{377}{296 + 377} = \frac{377}{673} \approx 0.560 \ (or \ 56.0\% )\)

Step 3 - Compare the predation rates

Compare the predation rates calculated in step 2:The predation rate is higher for striped snails (64.8%) compared to unstriped snails (56.0%).

Step 4 - Reason the adaptation

Since unstriped snails have a lower predation rate, they are better adapted to this environment than striped snails.

Step 5 - Predict future snail frequencies

In the future, because unstriped snails are less preyed upon, their frequency in the population is likely to increase, while the frequency of striped snails is likely to decrease.

Key Concepts

Predation Rate

In the given exercise, predation rate means the percentage of snails that were preyed upon by birds. It tells us how often a certain form of snail, either striped or unstriped, is eaten.

To find the predation rate for striped snails, we use the formula: \[\text{Predation Rate}_{\text{Striped}} = \frac{486}{264 + 486} = \frac{486}{750} \approx 0.648 \ (or \ 64.8\% )\] \ and for unstriped snails: \[\text{Predation Rate}_{\text{Unstriped}} = \frac{377}{296 + 377} = \frac{377}{673} \approx 0.560 \ (or \ 56.0\% )\]

Using these calculations, we see that striped snails have a predation rate of 64.8%, while unstriped snails have a rate of 56.0%. Therefore, striped snails are preyed on more often than unstriped ones.

A higher predation rate indicates that birds find striped snails easier to catch or more attractive to eat compared to unstriped snails.

Adaptation

Adaptation refers to the traits that help an organism survive better in its environment. In our exercise, we analyze which form of snail—striped or unstriped—is better adapted to avoid becoming prey.

Because unstriped snails have a lower predation rate (56.0%) compared to striped snails (64.8%), we can infer that unstriped snails are better adapted to their environment. Their trait, being unstriped, likely helps these snails to blend in better with their surroundings or makes them less visible to predators.

Adaptation is crucial because it directly influences an organism's chances of survival and reproduction. In our case, the unstriped snails, with their adaptation of being less spotted by birds, are more likely to survive and reproduce. This will, over time, strengthen this trait in the population.

Frequency Change

Frequency change describes how common a particular trait—in this context, being striped or unstriped—becomes within the population over time. It is directly impacted by natural selection, as seen through predation rates.

Since birds prey more on striped snails, we can predict that the frequency of striped snails in the population will decrease. Conversely, because unstriped snails are less preyed upon, their frequency is likely to increase.

As generations pass, the beneficial trait—in this case, being unstriped—becomes more common. Meanwhile, the less advantageous trait, being striped, will become less common. This change in frequencies shows how populations evolve over time due to differential survival and reproduction, driven by natural selection.

So, to sum it up: unstriped snails, having a survival advantage, will likely become more frequent in the population, showcasing how natural selection operates to shape the characteristics of organisms in a population.