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C2.4 · Compare natural and artificial selection through a case study

Learn to compare natural and artificial selection through a case study through clear examples and targeted practice.

Ontario Grade 11 Biology

Evolution

A case study in how selection changes which inherited traits become common

In SNC2D, you studied living things as parts of systems and used evidence to investigate questions. Here, the system is a population: members of the same kind of organism living in an area. Individuals in a population can differ. Some differences can be inherited, meaning they can be passed from parents to offspring. Selection happens when some inherited differences affect which individuals leave more offspring. In natural selection, environmental conditions are linked to this difference. In artificial selection, people choose which organisms reproduce. A comparison of peppered moths and domestic dogs shows what the two processes share and how they differ.

What you will learn

Describe natural selection and artificial selection in clear terms.
Use evidence from examples to compare how each type of selection works.
Distinguish an observed pattern from an explanation for that pattern.
Recognize that selection affects populations over generations, not the certainty of an individual’s outcome.

From variation to a biological question

A useful starting point is to compare two dogs. One may have a short coat and another a long coat. A dog breeder may choose parent dogs because they have a desired coat or body size. Over generations, the traits breeders choose can become more common in that group.

Now imagine a population of moths with light and dark wing patterns. If the conditions where they live make one pattern harder for predators to notice, moths with that pattern may be more likely to survive and reproduce. No person needs to choose them.

These examples raise the same biological question: how can the traits seen in a population change over generations? A population is not a single organism. A change in the population means that a trait becomes more or less common among its members.

Variation means differences among individuals.
An inherited trait can pass from parents to offspring.
Selection is about differences in survival or reproduction associated with traits.

Two kinds of selection

Natural selection occurs when conditions in an environment are associated with different chances of survival and reproduction for individuals with different inherited traits. The environment does not make a plan or choose with intention. For example, predators, food availability, or weather can affect which individuals leave more offspring.

Artificial selection occurs when people choose which organisms reproduce because of traits people want. A breeder might choose dogs with a particular coat or body shape. Over many generations, the chosen traits may become more common in the bred population.

Both kinds involve inherited variation and differences in which individuals reproduce. The key difference is what is doing the selecting. In natural selection, environmental conditions are involved. In artificial selection, people make the choice. Selection does not mean that an individual changes its inherited traits because it needs to.

Natural selection: environmental conditions are linked to reproductive differences.
Artificial selection: people choose organisms to reproduce for desired traits.
In both cases, inherited differences can affect the traits common in later generations.

Case study: peppered moths and domestic dogs

Peppered moths provide an example of natural selection. The moths include light and dark forms. Historical observations in parts of Britain recorded an increase in dark moths during the period when industrial pollution darkened many tree surfaces. Later, as pollution decreased and surfaces became lighter, the dark form became less common in some studied areas.

The observed pattern is the change in the relative frequency of the two forms over time. A proposed explanation is that the background affected how noticeable moths were to predators. If one form was less noticeable in a particular setting, it could have had a better chance of surviving to reproduce. The observation alone does not directly show every step of that explanation, so it is important to distinguish the recorded pattern from the model used to explain it.

Domestic dogs provide an example of artificial selection. People have chosen dogs to reproduce for traits such as size, coat, or ability to perform particular tasks. When people repeatedly choose parents with a desired inherited trait, that trait can become more common among their descendants. Different breeding choices have produced many kinds of dogs.

The comparison is clear: both examples involve differences among organisms and changes in traits across generations. In the moth example, environmental conditions are linked to which forms may leave more offspring. In dog breeding, people deliberately choose parents. Neither example means that every individual survives or reproduces in the same way, and neither guarantees a particular trait in every offspring.

The moth pattern is an observed change in the relative frequency of light and dark forms.
Predator visibility is an explanation for the moth pattern, not the same thing as the observation.
Dog breeding is artificial selection because people choose which dogs reproduce.

Using the model carefully

A simple model for either case is: inherited variation exists; some individuals reproduce more than others; and the traits they pass on can become more common in later generations. This is a way to organize evidence, not a claim that every population changes in the same way.

When you evaluate a selection example, ask what trait varies, whether the trait is inherited, and what affects which individuals reproduce. Then identify whether people made the breeding choice. If the evidence only shows that a trait became more common, do not automatically claim that you know exactly why it happened.

Selection works across generations. It does not mean that an individual moth changes its colour to match a tree, or that a dog develops the trait its breeder wants simply because the breeder wants it. Also, selection does not make a particular outcome certain for each offspring.

Identify the trait, the source of selection, and the evidence for a change over generations.
Separate an observed pattern from an explanation of that pattern.
A population-level pattern does not guarantee an individual outcome.

Comparing the two types of selection

Feature	Natural selection	Artificial selection
What is associated with which individuals reproduce?	Environmental conditions	Human choices
Example in this lesson	Peppered moth forms	Domestic dog breeding
What can change over generations?	How common inherited traits are in the population	How common chosen inherited traits are in the bred group
Does each individual outcome have to be the same?	No	No

Worked example

Interpreting the moth case

A report describes an increase in dark peppered moths in an industrial area, followed later by a decrease as pollution fell. Classify the selection and separate the observation from a possible explanation.

Identify the observed pattern
The report records that the relative frequency of dark moths rose and later fell. That is the observation; it does not, by itself, establish the cause.
Identify the kind of selection
No person is described as choosing moths to breed. The case is therefore an example of natural selection, where environmental conditions may be associated with differences in survival and reproduction.
State a possible explanation
A possible explanation is that changing backgrounds affected how visible each form was to predators. If visibility affected survival and reproduction, the form with an advantage in that setting could become more common. This is an explanatory model, not a direct restatement of the observation.

Answer: This is natural selection. The observed pattern is the changing relative frequency of dark moths. A possible explanation is that changing backgrounds affected visibility to predators and, in turn, survival and reproduction.

Check: The answer keeps the observation separate from the proposed explanation.

Worked example

Classifying a dog-breeding plan

A breeder repeatedly chooses parent dogs with a particular coat trait, and that trait becomes more common in the bred group over generations. Explain why this is artificial selection and name the important condition for the trait to be passed on.

Find who makes the choice
The breeder chooses which dogs reproduce. Human choice makes this artificial selection.
Connect choice to later generations
The trait can become more common if it is inherited and the selected dogs pass it to some offspring. The description concerns a pattern across generations, not a guaranteed result for every puppy.

Answer: This is artificial selection because a person chooses the parents. The coat trait must be inherited for selection to make it more common across generations.

Check: A breeder’s preference alone cannot ensure that every offspring has the preferred trait.

Worked example

Comparing two explanations

In one population, a trait becomes more common because environmental conditions favour individuals that carry it. In another, people repeatedly choose parents with a desired trait. Compare the selection in the two populations without claiming that individuals change because they need to.

Name the shared features
Both populations contain variation, and the traits in question are inherited. In both cases, differences in reproduction across generations can affect which traits become common.
Name the difference
The first case is natural selection because environmental conditions are associated with reproductive differences. The second is artificial selection because people choose parents for desired traits.
Avoid an individual-level claim
Neither explanation says that an individual changes its inherited trait in response to need. The model describes a population pattern across generations.

Answer: Both types involve inherited variation and differences in reproduction. Natural selection involves environmental conditions; artificial selection involves human choice. Neither means that individual organisms change a trait because they need to.

Check: The comparison identifies both the shared process and the difference in what affects reproduction.

Common mistakes and how to avoid them

Saying that natural selection happens because organisms change when they need a trait.

Correction: Natural selection describes how inherited differences can be linked to different reproduction across generations. It does not mean that individuals change their inherited traits because of need.

Calling any change in a trait an example of artificial selection.

Correction: Artificial selection requires people to choose which organisms reproduce. If environmental conditions are associated with reproductive differences and no human choice is involved, the example is natural selection.

Treating a proposed explanation as if it were the observation itself.

Correction: State what was observed first. Then describe a possible explanation and make clear that it is a model for the pattern.

Assuming that every offspring will have the trait a breeder wants.

Correction: Selection can affect how common inherited traits become across generations. It does not guarantee a particular trait in every offspring.

Lesson summary

Natural selection occurs when environmental conditions are associated with differences in survival and reproduction among individuals with inherited variation.
Artificial selection occurs when people choose organisms to reproduce for desired traits.
Both forms can change how common inherited traits are in a population over generations.
Use evidence to distinguish an observed pattern from an explanation, and avoid treating population changes as certainty for individuals.

Check your understanding

Question 1

Which detail makes dog breeding an example of artificial selection?

People choose which dogs reproduce because of desired traits.
Dogs live in different environments.
Some dogs have traits that differ from other dogs.
A trait may be inherited.

Show answer and explanation

People choose which dogs reproduce because of desired traits.

Artificial selection involves human choice of which organisms reproduce. Variation and inheritance matter too, but they do not alone distinguish artificial selection.

Question 2

A study records that one moth form became more common. Which statement is the observation?

The form became more common in the population.
Predators caused the pattern because they could see one form more easily.
Each moth changed its form to match its surroundings.
Every offspring had the same form as its parent.

Show answer and explanation

The form became more common in the population.

The change in how common the form was is the recorded pattern. Predator visibility is a possible explanation, while the other statements are not supported by that observation.

Question 3

Which statement best compares natural and artificial selection?

Both can affect which inherited traits become common; environmental conditions are involved in natural selection, and people choose in artificial selection.
Natural selection is always planned, while artificial selection happens without human choice.
Both guarantee that every offspring has the trait that is becoming common.
Artificial selection changes individuals because they need a trait.

Show answer and explanation

Both can affect which inherited traits become common; environmental conditions are involved in natural selection, and people choose in artificial selection.

Both processes can shift trait frequencies across generations, but the source of selection differs. Neither guarantees a specific trait in every offspring.

Key terms

Population: Members of the same kind of organism living in an area.
Variation: Differences among individuals.
Inherited trait: A characteristic that can pass from parents to offspring.
Natural selection: A process in which environmental conditions are associated with differences in survival and reproduction among individuals with inherited traits.
Artificial selection: A process in which people choose organisms with desired traits to reproduce.
Relative frequency: How common one form or trait is compared with others in a population.

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