Assignment Example: Natural Selection

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Middlebury College
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Describe what is happening in figures 1-3. Is the population of mice different in figure 3 than in figure 1? Explain why.

Yes, due to natural selection. The few mice in figure 3 are those, which have been able to adapt to cold temperatures.


Living things that are well adapted to their environment survive and reproduce. Those that are not well adapted dont survive and reproduce. An adaptation is any characteristic that increases fitness, which is defined as the ability to survive and reproduce. What characteristic of the mice is an adaptation that increased their fitness?

The color of their fur

The table below gives descriptions of four female mice that live in a beach area, which is mostly tan sand with scattered plants. According to the definition given for fitness, which mouse would biologists consider the fittest? Explain why this mouse would be the fittest.

The mouse with the tan colored fur is the fittest. This is because the color of the fur enables it to live longer and thus reproduce more pups.

Color of fur Black Tan Tan and Black Cream

Age at death 2 months 8 months 4 months 2 months

# pups produced by each female 0 11 3 0

Running speed 8 cm/sec. 6 cm/sec. 7 cm/sec. 5 cm/sec.

If a mouse's fur color is generally similar to its mothers color, what color fur would be most common among the pups?


A characteristic that is influenced by genes and passed from parents to offspring is called heritable. Over many generations, heritable adaptive characteristics become more common in a population. This process is called evolution by natural selection. Evolution by natural selection takes place over many, many generations.

Evolution by natural selection leads to adaptation within a population. The term evolution by natural selection does not refer to individuals changing, only to changes in the frequency of adaptive characteristics in the population as a whole. For example, for the mice that lived in the beach area with tan sand, none of the mice had a change in the color of their fur; however, due to natural selection, tan fur will become more common in subsequent generations than it is for the mother mice in the table above.

In summary, a heritable characteristic that helps an animal or plant to have more offspring that survive to reproduce will tend to become more common in a population as a result of evolution by natural selection.

Explain why a characteristic that helps an animal to live longer will generally tend to become more common in the population as a result of evolution by natural selection.

A characteristic that enables an animal to adapt increases the animal's overall fitness. In turn, the animal is able to reproduce pups that share the same characteristic. This characteristic is passed on to future generations of mice through evolution.

Not all characteristics that contribute to longer life become more common in the population. Some characteristics contribute to long life, but not more offspring. For example, a female cat that is sterile and cannot have any offspring may live longer because she will not experience the biological stresses of repeated pregnancies. Explain why a characteristic like this, which contributes to a long life, but with few or no offspring, would not become more common as a result of evolution by natural selection.

The characteristic will not become more common because it has not been passed on to any offspring.

The following example will illustrate that evolution by natural selection cannot occur if the variation in a characteristic is not heritable. This example also illustrates a more complete definition of fitness, which is the ability to survive and produce offspring who can survive and reproduce.

Name George Dwayne Spot Tyrone

Age at death 13 years 16 years 12 years 10 years

# cubs fathered 19 25 20 20

# cubs surviving to adulthood 15 14 14 19

Size 10 feet 8.5 feet 9 feet 9 feet

(Adapted from Michigan State University, Occasional Paper No. 91, Evolution by Natural Selection: A Teaching Module by Beth Bishop and Charles Anderson, 1986)

According to this definition of fitness, which of the four male lions described above would biologists consider the fittest?


Explain why Dwayne is not the fittest even though he lived the longest and fathered the most cubs.

Out of the 20 cubs he fathered, only 14 survived to adulthood.

Below is a series of pictures representing changes in a population of cacti. Pictures 1 and 2 show what happened when a deer came to eat, picture 3 shows the cacti a few weeks later (notice the flowers on the right-hand cactus), and picture 4 shows the situation a few months later.

Recall that the three conditions listed below are necessary for natural selection to take place.

Variation in characteristics within the population: In picture 1, what is the main difference between the cactus on the left and the cactus on the right?

The cactus on the Left is not thorny like the cactus on the right.

Differences in survival and reproduction, fitness: Why would a deer be more likely to eat the cactus on the left than the cactus on the right?

Because it does not have thorns which make cactus survive and reproduce.

What effect does the deer's behavior have on the survival and reproduction of these two types of cactus?

The deer's behavior will affect the survival and reproduction of the smooth cactus. However, it will not have any effect on the survival and reproduction of the thorny cactus.

Heritability of characteristics from parent to offspring: The difference between the cacti is a heritable characteristic (see picture 4). Do you think that evolution by natural selection is occurring in this cactus population? Explain why or why not.

Yes. The thorny cactus has sprouted flowers. Hence, the cactus can pass on its heritable characteristic.

Basic Requirements (the assignment will not be accepted or assessed unless the follow criteria have been met):

Assignment has been proofread and does not contain any major spelling or grammatical errors.


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