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C3.4 · Compare mechanisms including variation, selection, and genetic drift

Learn to compare mechanisms including variation, selection, and genetic drift through clear examples and targeted practice.

Ontario Grade 11 Biology

Evolution

How differences among organisms can shape a population over generations

In Grade 10 science, you learned to look for patterns in living systems and to separate observations from explanations. That skill is useful when studying changes in populations. Imagine a group of beetles with different colours. If the group’s colour pattern changes over generations, what caused the change? The answer may involve natural selection, genetic drift, or both. This lesson compares these ideas and shows how variation provides differences that can be passed on.

What you will learn

Explain what variation is and why inherited variation matters to changes in populations.
Compare natural selection and genetic drift as different ways that inherited traits can become more or less common.
Use evidence from a simple population model to distinguish a pattern from a possible explanation.

From differences in organisms to a population question

A population is a group of organisms of the same species living in the same area. A species is a group of organisms that can reproduce with one another. Individuals in a population are not always identical. They may differ in a trait, which is a feature such as colour or body size. These differences are called variation.

Some variation can be inherited. Inherited means passed from parents to offspring. Other differences may be caused by an organism’s surroundings and may not be passed on. For this lesson, focus on inherited differences because they can affect which traits become common over generations.

A generation is the group of individuals born around the same time. A population-level change is a change in how common an inherited trait is across generations. One individual does not evolve by changing its inherited traits during its lifetime. Instead, the mix of inherited traits in a population can change as generations pass.

Consider beetles that vary in colour. If dark beetles become more common, that is an observed pattern. The pattern alone does not prove why it happened. Scientists use evidence and models to compare possible explanations.

Variation means differences among individuals.
Inherited variation can be passed from parents to offspring.
Evolution in this lesson refers to a change in the frequency of inherited traits in a population over generations.

Selection: differences linked to reproductive success

Natural selection is a process in which individuals with certain inherited traits are more likely to survive and reproduce in a particular environment. Reproduce means to have offspring. If offspring inherit the trait, it may become more common over generations.

A useful model has three parts. First, individuals vary in a trait. Second, the environment is linked to differences in survival or reproduction. Third, some of the successful individuals pass the relevant inherited trait to offspring. When all three apply, the trait can increase in frequency.

Selection is not a choice made by an organism, and it does not happen because a population needs a trait. The environment does not create the useful inherited difference on demand. Instead, existing differences can lead to different outcomes in that environment.

For example, suppose some beetles are dark and some are light. On a dark surface, birds may more easily spot the light beetles. If dark beetles are more likely to reproduce and their colour is inherited, dark colour may become more common. The explanation depends on evidence for the differences in spotting, reproduction, and inheritance—not only on a change in colour counts.

Selection is linked to differences in survival or reproduction.
The trait involved must be inherited for it to become more common in later generations.
A trait can be helpful in one environment and not in another.

Genetic drift: chance changes in a population

Genetic drift is a change in how common inherited traits are because of chance. Chance events can affect which individuals survive or reproduce, even when their traits do not give them an advantage. As a result, a trait may become more common or less common by chance.

Drift can have a stronger effect in a small population. If only a few individuals contribute offspring, their inherited traits may make up a large share of the next generation simply because of who happened to reproduce. This does not mean that those traits were better suited to the environment.

A sudden event can leave only a small group from a larger population. The surviving group may not have the same mix of traits as the original group. This is one way chance can change the population’s inherited variation. The important point is that the change is not necessarily caused by a trait’s advantage.

Selection and drift can both change how common inherited traits are. To compare them, ask whether the evidence connects a trait to different reproductive outcomes in that environment. If so, selection may help explain the pattern. If the change is due to chance differences in which individuals leave offspring, drift may help explain it. Both processes can occur in the same population.

Genetic drift is change due to chance, not because a trait is necessarily advantageous.
Chance can shift trait frequencies, especially when relatively few individuals contribute offspring.
A change in trait frequency by itself does not identify the mechanism.

Comparing explanations and reading evidence

A population model can help organize an explanation. It is not a direct view of every event in nature. In the table, imagine a population with inherited dark and light colour differences. The rows show possible explanations for dark colour becoming more common.

For selection, look for a link between colour and survival or reproduction in a stated environment. For drift, look for a chance process that affects which individuals contribute to the next generation. If the available evidence only shows a change in colour counts, the cause remains uncertain.

The same observed pattern can fit more than one explanation. For example, dark beetles might become more common because they are less often spotted, or because a chance event leaves more dark beetles among the breeders. Further evidence about survival, reproduction, and chance events would help compare these models.

Variation matters to both explanations. Without inherited differences in the trait, selection cannot favour one version over another, and drift cannot change the relative representation of different inherited versions of that trait. Variation is therefore essential to these examples, while selection and drift describe different ways a population’s mix can change.

Selection: a trait is linked to different reproductive outcomes.
Drift: chance affects which inherited traits are represented in later generations.
Evidence about the process is needed; a pattern alone is not a complete explanation.

Three ideas used to explain population change

Idea	What it means	Clue to look for
Variation	Individuals differ; some differences can be inherited.	Different inherited forms of a trait are present.
Natural selection	An inherited trait is linked to different survival or reproduction.	Evidence connects the trait to reproductive success in that environment.
Genetic drift	Chance changes which inherited traits are represented.	A chance event affects who survives or leaves offspring, without a stated trait advantage.

Worked example

A trait linked to survival

In a hypothetical beetle population, light and dark colour are inherited variations. On dark ground, birds catch more light beetles. Dark beetles leave more offspring, and offspring can inherit colour. Which mechanism best explains a later increase in dark beetles?

Identify the variation
The beetles differ in colour, and the problem states that colour is inherited. This supplies a difference that can be passed to offspring.
Connect the trait to reproduction
Dark beetles are caught less often and leave more offspring in this environment. The difference in reproductive success is linked to colour.
Choose the explanation
This is natural selection because an inherited trait is associated with greater reproduction. The increase is not explained merely by chance in the information given.

Answer: Natural selection best explains the increase in dark beetles.

Check: The conclusion relies on inherited colour and a stated link between colour and reproductive success.

Worked example

A chance change in a small group

A small, isolated group begins with 5 dark and 5 light beetles. A storm happens to leave 4 dark and 1 light beetle alive. Assume their colours are inherited and there is no evidence that colour affected survival. What mechanism best explains the changed mix among survivors?

Compare the groups
At first, dark and light beetles are equally represented. After the storm, dark beetles are more numerous among the survivors.
Check for a trait advantage
The problem gives no evidence that colour affected survival. It describes the storm’s outcome as a chance event.
Name the mechanism
Genetic drift is the best explanation in this model. The survivors are a small group, and the chance event changed which inherited colours are represented. This does not show that dark colour was advantageous.

Answer: Genetic drift best explains the changed mix among the survivors.

Check: The counts change from 5 dark and 5 light to 4 dark and 1 light, but no colour-linked advantage is stated.

Worked example

When the pattern does not identify the cause

After several generations, dark beetles are more common than before. The only evidence is the change in colour counts. Can you conclude that selection, rather than drift, caused the change?

State what the evidence shows
The observation shows that dark beetles became more common. It does not show why their frequency changed.
Compare possible models
Selection could explain the pattern if dark colour were linked to greater reproduction in that environment. Drift could explain it if chance affected which beetles left offspring.
Set a limit on the conclusion
The evidence is not enough to choose between these explanations. More information about reproductive outcomes or chance events would be needed.

Answer: No. The observed pattern alone cannot distinguish selection from genetic drift.

Check: A sound explanation separates what was observed from the model proposed to explain it.

Common mistakes and how to avoid them

Saying that an individual evolves by changing its inherited traits because it needs to survive.

Correction: Individuals may survive or reproduce differently, but population-level change in inherited traits occurs across generations.

Calling every increase in a trait natural selection.

Correction: A frequency change alone does not reveal its cause. Look for evidence that the trait is linked to reproductive success.

Assuming that genetic drift means a trait is harmful or useful.

Correction: Drift is change due to chance. The trait can become more or less common without giving an advantage.

Treating variation, selection, and drift as the same process.

Correction: Variation describes differences. Selection and drift are different processes that can change how common inherited differences are.

Lesson summary

Inherited variation supplies differences among individuals that can be represented in later generations.
Natural selection occurs when an inherited trait is linked to differences in survival or reproduction in an environment.
Genetic drift changes the representation of inherited traits through chance.
Selection and drift can both change a population’s trait mix, and both may occur together.
A pattern in trait counts is an observation. Evidence about reproductive success or chance events is needed to compare explanations.

Check your understanding

Question 1

A group of inherited colour variations is present. Birds more often catch one colour, and the other colour becomes more common among offspring. Which mechanism is best supported?

Natural selection, because colour is linked to different reproductive success
Genetic drift, because every change in colour frequency is random
Variation, because the beetles have different colours
No population change, because individuals do not change colour

Show answer and explanation

Natural selection, because colour is linked to different reproductive success

The evidence links an inherited colour to different survival and reproduction, which supports natural selection.

Question 2

A chance event changes which beetles leave offspring. No evidence links survival to colour. Which mechanism does this describe?

Natural selection
Genetic drift
Variation alone
A change within one individual

Show answer and explanation

Genetic drift

A chance change in which inherited traits are represented in offspring is genetic drift.

Question 3

A study reports only that dark beetles became more common. What is the most careful conclusion?

Selection definitely caused the change.
Drift definitely caused the change.
The pattern is observed, but the cause is not identified by that information alone.
Dark colour must have appeared because the population needed it.

Show answer and explanation

The pattern is observed, but the cause is not identified by that information alone.

Trait counts show a pattern, but evidence about reproductive success or chance events is needed to compare selection and drift.

Key terms

Population: A group of organisms of the same species living in the same area.
Trait: A feature of an organism, such as colour.
Variation: Differences among individuals in a population.
Inherited: Passed from parents to offspring.
Natural selection: A process in which inherited traits linked to greater survival or reproduction can become more common over generations.
Genetic drift: A change in how common inherited traits are because of chance.
Generation: A group of individuals born around the same time.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation C3.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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