BIO 182 Exam 2 Questions and Correct
Answers, 179 Q&A. 100% Correct.
2024/5.
How many modes of selection are there?
3
What are the modes of selection?
Stabilizing selection, directional selection, and disruptive selection
Stabilizing Selection
The mean of the trait remains similar over time but the standard deviation decreases
Directional Selection
The mean phenotype shifts in a particular direction
Disruptive Selection
The standard deviation of the trait increases as individuals with the mean value disappear.
Stabilizing Selection Explained
,The stronger the selective pressure against extreme phenotypes, the narrower the distribution of
phenotypes will be in future generations. In the end, the size of the standard deviation will reflect both
the strength of selection against extremes and the frequency of mutations creating new variation. Thus,
when we observe a wide range of phenotypes in a population, we might infer weak selective pressure
against the extremes.
Directional Selection Explained
The population's environment changes. The mean phenotype in the population may no longer be the
phenotype that reproduces the most. Over time, we should expect the mean phenotype to shift higher
or lower, depending on the direction of selective pressure. Once the mean phenotype catches up to the
change in the environment, stabilizing selection will resume. The environment must change slowly
enough for the population to persist. If the environment changes too quickly, no members of the
population will be able to reproduce, leading to extinction.
Disruptive Selection Explained
A population can occupy an environment in which individuals with extreme phenotypes reproduce more
than individuals with the mean phenotype.
This mode of selection occurs when several distinct strategies confer greater reproductive success than
other strategies do. Can produce new species when individuals choose to mate only with similar-looking
individuals. This non-random mating prevents recombination from restoring intermediate phenotypes.
Evolution by Natural Selection
Heritable variation leads to differential reproductive success
Selection and Fitness
Directional Selection Equation
y=bx+c
y=relative fitness
x= trait value
b= strength of selection
c=parameter
True or False? If the slope of the relationship between the phenotype and fitness is positive, natural
selection would decrease the phenotype over generations.
False, it would increase the value of the trait over time
Stabilizing Selection Equation
, y=ax^2+bx+c
y=relative fitness
x= trait value
a/b= strength of selection
c= parameter
True or False? If the parameter for the squared term is negative, natural selection will likely stabilize the
phenotype at an intermediate value.
True, depends on how large the parameter is for the squared term
What is fitness?
The relative number of viable, fertile offspring
What does natural selection do?
Natural selection not only changes mean phenotypes but also increases or decreases deviations from
mean phenotypes.
What type of selection is most common?
All three can happen to some extent. Both the direction and the strength of natural selection varies
greatly among traits and species.
True or False? Directional selection is more likely to be weak than strong.
True
True or False? Stabilizing selection occurs more often than disruptive selection.
False
The next 5 questions are based on the following story:
A researcher studied the relationship between the phenotypes of snakes and their fitness. First, he
measured the color patterns and the escape behavior of hundreds of snakes. For color pattern, snakes
were scored as being either checkered or striped (see photos below). For escape behavior, snakes were
chased by simulating a predator attack; some snakes moved in a straight line while fleeing, but others
tended to turn frequently. After recording these phenotypes, the researcher released the snakes in their
natural environment. A year later, he returned to see which snakes had survived. The survival of snakes
was used as a proxy of fitness. The figure below shows the relationships between color pattern and
relative fitness for snakes that flee straight and snakes that flee by turning.
Answers, 179 Q&A. 100% Correct.
2024/5.
How many modes of selection are there?
3
What are the modes of selection?
Stabilizing selection, directional selection, and disruptive selection
Stabilizing Selection
The mean of the trait remains similar over time but the standard deviation decreases
Directional Selection
The mean phenotype shifts in a particular direction
Disruptive Selection
The standard deviation of the trait increases as individuals with the mean value disappear.
Stabilizing Selection Explained
,The stronger the selective pressure against extreme phenotypes, the narrower the distribution of
phenotypes will be in future generations. In the end, the size of the standard deviation will reflect both
the strength of selection against extremes and the frequency of mutations creating new variation. Thus,
when we observe a wide range of phenotypes in a population, we might infer weak selective pressure
against the extremes.
Directional Selection Explained
The population's environment changes. The mean phenotype in the population may no longer be the
phenotype that reproduces the most. Over time, we should expect the mean phenotype to shift higher
or lower, depending on the direction of selective pressure. Once the mean phenotype catches up to the
change in the environment, stabilizing selection will resume. The environment must change slowly
enough for the population to persist. If the environment changes too quickly, no members of the
population will be able to reproduce, leading to extinction.
Disruptive Selection Explained
A population can occupy an environment in which individuals with extreme phenotypes reproduce more
than individuals with the mean phenotype.
This mode of selection occurs when several distinct strategies confer greater reproductive success than
other strategies do. Can produce new species when individuals choose to mate only with similar-looking
individuals. This non-random mating prevents recombination from restoring intermediate phenotypes.
Evolution by Natural Selection
Heritable variation leads to differential reproductive success
Selection and Fitness
Directional Selection Equation
y=bx+c
y=relative fitness
x= trait value
b= strength of selection
c=parameter
True or False? If the slope of the relationship between the phenotype and fitness is positive, natural
selection would decrease the phenotype over generations.
False, it would increase the value of the trait over time
Stabilizing Selection Equation
, y=ax^2+bx+c
y=relative fitness
x= trait value
a/b= strength of selection
c= parameter
True or False? If the parameter for the squared term is negative, natural selection will likely stabilize the
phenotype at an intermediate value.
True, depends on how large the parameter is for the squared term
What is fitness?
The relative number of viable, fertile offspring
What does natural selection do?
Natural selection not only changes mean phenotypes but also increases or decreases deviations from
mean phenotypes.
What type of selection is most common?
All three can happen to some extent. Both the direction and the strength of natural selection varies
greatly among traits and species.
True or False? Directional selection is more likely to be weak than strong.
True
True or False? Stabilizing selection occurs more often than disruptive selection.
False
The next 5 questions are based on the following story:
A researcher studied the relationship between the phenotypes of snakes and their fitness. First, he
measured the color patterns and the escape behavior of hundreds of snakes. For color pattern, snakes
were scored as being either checkered or striped (see photos below). For escape behavior, snakes were
chased by simulating a predator attack; some snakes moved in a straight line while fleeing, but others
tended to turn frequently. After recording these phenotypes, the researcher released the snakes in their
natural environment. A year later, he returned to see which snakes had survived. The survival of snakes
was used as a proxy of fitness. The figure below shows the relationships between color pattern and
relative fitness for snakes that flee straight and snakes that flee by turning.
