BIOL 101 Homework Critical Thinking Case Types of Natural Selection solutions complete answers

BIOL 101 Homework Critical Thinking Case Types of Natural Selection solutions complete answers

 

Biological Concepts Overview

In this case, you will explore the fundamental concepts underlying natural selection and will apply those concepts to the problem of antibiotic resistance in a clinical setting by examining a case involving methicillin-resistant Staphylococcus aureus (MRSA). The case has four separate activities that may be assigned individually or together:

·         Evolution via Natural Selection

·         Types of Natural Selection

·         Methicillin Resistance in a Bacterial Population

·         How Gene Transfer Contributes to Evolution

The problem of antibiotic-resistant infections has gained much attention in recent years. While it may seem like these infections have come out of nowhere, they are, in fact, the result of evolution via natural selection. The misuse of antibiotics clinically and the overuse of antibiotics in applications such as livestock feed have created environments rich in the selective pressure required for antibiotic-resistant populations to expand.

In this case, you will explore MRSA is one of the most common healthcare-associated infections. Ranging from wound infections to pneumonia, approximately 80,000 people acquire a MRSA infection each year, and about 12,000 people die as a result. MRSA is recognized by the Centers for Disease Control (CDC) as a serious public health threat.

 

© Kateryna Kon/ShutterStock.com

Biological illustration of methicillin-resistant Staphylococcus aureus (purple) on the skin surface (red)

Learning Objectives for “Types of Natural Selection”

After completing this activity, you will be able to:

·         Distinguish directional, stabilizing, and disruptive selection

·         Analyze graphical representations of selection and extrapolate to antibiotic resistance in bacteria

 

·         A. Population before selection occurs.

·         B. With directional selection, forms of a trait at one end of a range of variation are adaptive.

·         C. With stabilizing selection, extreme forms of a trait are eliminated, and an intermediate form is maintained.

·         D. With disruptive selection, a midrange form of a trait is eliminated, and extreme forms are maintained.

Evolution results in a change in the frequency of a particular trait within a population. Because traits are encoded for by alleles, the frequency of the alleles that encode that trait change within the population as well. There are three modes explaining the shifts in allele frequency that can occur as a result of natural selection. Panel A depicts the distribution of alleles prior to selection.

In directional selection (panel B), traits at one end of the spectrum are selected (green arrows) while the other traits are selected against (red arrows). The evolution of a predominantly black-colored peppered moth during the English Industrial Revolution is an example of directional selection. Light or peppery-colored moths were selected against in the dark environment that was created by soot pollution.

Disruptive selection (panel D) actually selects for traits at both ends of the spectrum. Imagine an environment such as a rocky beach that features light-colored sand and dark-colored rocks. Within this environment, light-colored mice are selected for (i.e., protected from predators) by the light sand, and dark-colored mice will be selected for by the dark rocks. Medium-colored mice, however will be visible to predators in both the light- and dark-colored environments and will be eliminated by predators.

In contrast to directional and disruptive selection, stabilizing selection (panel C) favors intermediate traits. A good example would be height in plants in a particular environment. Plants that are very tall are susceptible to damage by wind, while plants that are too short may not be able to gain enough access to sunlight for photosynthesis. The intermediate type, plants of medium height, is favored, as it allows plants to resist wind damage while still having access to sunlight.

Now check your understanding by answering the related assessment questions.

 

The following graphs depict three representations of modes of natural selection.

 

 

In the figure, choose the correct label for the graph

 

Which of the graphs best depicts the evolution of antibiotic resistance?