DoAssignment study guide

C1.2 · Evaluate environmental change, natural selection, and species vulnerability

Learn to evaluate environmental change, natural selection, and species vulnerability through clear examples and targeted practice.

Ontario Grade 11 Biology

Evolution

Ontario Grade 11 Biology — Study topic C1.2

In SNC2D, you learned that organisms are living systems and that changes in their surroundings can affect them. This lesson uses that foundation to ask a bigger question: when an environment changes, why might some organisms leave more offspring than others? A population is a group of the same species living in an area. Natural selection is a model that explains how inherited differences can become more or less common in a population over generations. To evaluate this model, separate what was observed from the explanation, and consider whether the evidence supports the explanation.

What you will learn

Describe how an environmental change can affect organisms and their chances of survival and reproduction.
Explain natural selection as a change in the traits common in a population over generations.
Evaluate how evidence supports an explanation of natural selection while recognizing its limits.
Assess factors that can make a species more vulnerable to environmental change.

From an environmental change to a biological question

An environmental change is a change in living or non-living conditions in an organism’s surroundings. Examples include a shift in temperature, a change in available food, or a new predator. The effect depends on the species and the conditions it faces. A change that creates difficulty for one species might have little effect on another.

A trait is a feature of an organism, such as body colour or the time of year it is active. Individuals in a population can differ in their traits. Some differences can be passed from parents to offspring; these are inherited traits. Other differences may be caused by an organism’s surroundings and may not be inherited. Natural selection concerns inherited differences that affect success in a particular environment.

For example, imagine that a forest becomes darker after a change in its plant cover. A population of insects has both light and dark individuals. If birds more often find the light insects against the darker background, dark insects may be more likely to survive and reproduce. This is a possible explanation, not a conclusion to assume before examining evidence.

The biological question is not whether an individual chooses to change. It is whether individuals with an inherited trait tend to leave more offspring under the changed conditions, and whether the trait becomes more common over generations.

A population contains individuals of one species in an area.
Environmental change can affect survival or reproduction, but effects depend on the species and the conditions.
Natural selection acts on differences among individuals; populations change over generations.

The natural-selection model

Use this simple sequence to organize an explanation. First, individuals in a population vary in a trait. Second, some of that variation is inherited. Third, an environmental condition affects how likely individuals with different traits are to survive and reproduce. Fourth, individuals that leave more offspring can pass the associated inherited trait to some of those offspring. Across generations, that trait may become more common.

Reproductive success means how much an organism contributes offspring to later generations. It does not mean that an individual is stronger, better, or more advanced in every way. A trait is advantageous only in relation to particular conditions. If conditions change again, a trait that was helpful may no longer be helpful.

The pattern in the population is the observed result. Natural selection is an explanatory model for how that pattern could arise. Evidence for the model is stronger when it supports the steps in the explanation—for example, when a trait is inherited and individuals with that trait have greater reproductive success in the conditions being studied.

Environmental change does not create a needed inherited trait because organisms need it. Instead, individuals already differ, and conditions affect which differences are linked to greater reproductive success. The model predicts a change in how common inherited traits are, not a planned change in each individual.

\text{inherited variation} + \text{different reproductive success} \rightarrow \text{trait frequencies may change}

Variation, inheritance, differences in reproductive success, and change across generations are central parts of the model.
A trait’s advantage depends on the environment.
A change in trait frequency is a population pattern; natural selection is a possible explanation that needs evidence.

Evaluating evidence and environmental change

To evaluate a claim, identify what was measured or observed, then ask what conclusion it supports. Suppose a survey finds that dark insects are more common after a forest becomes darker. That observation shows a pattern. By itself, it does not prove why the pattern occurred. The insects might differ in survival, or another factor might be involved.

Evidence becomes more useful when it links the proposed steps. Relevant questions include: Did the individuals differ in the trait? Is the difference inherited? Were individuals with one trait more likely to survive or reproduce under the changed conditions? Was the trait more common in later generations? The more of these links the evidence supports, the more convincing the selection explanation is.

Also consider other possible explanations and the limits of the evidence. A comparison across time may show that a trait changed in frequency, but it may not show what caused the change. A study in one place or over a short time may not represent other places or later conditions. A careful conclusion states what the evidence supports, rather than claiming more than it can show.

A species is vulnerable when environmental change puts it at a greater risk of harm or makes it harder for its populations to persist. Vulnerability depends on the species and on the type and scale of change. Consider whether individuals have inherited differences that could matter under the new conditions, whether the change is rapid, and whether the species can persist in the affected conditions. These considerations guide an evaluation; they do not guarantee a particular outcome.

A pattern is not automatically proof of its cause.
Evaluate how well evidence supports each part of the natural-selection explanation.
Vulnerability is relative to a particular environmental change; avoid treating every species as equally affected.

A careful conclusion

When writing an answer, name the environmental change, the trait that may matter, and the evidence for differences in survival or reproduction. Then explain whether the trait is inherited and whether its frequency changed over generations. If one link is missing, say so. This keeps the conclusion accurate.

Do not say that every individual changes or that every advantageous trait will spread. A trait may not be inherited, the environmental conditions may change again, or the evidence may be too limited to support a firm conclusion. Natural selection is a useful model, but an evaluation must include both what it explains and what remains uncertain.

Tie conclusions to specific evidence and conditions.
Use cautious wording when evidence supports a pattern but not its cause.
Explain vulnerability in relation to a stated environmental change.

A checklist for evaluating natural selection and vulnerability

Question	Why it matters
What changed in the environment?	The effect of a trait depends on the conditions.
What trait differences are present?	Selection requires differences among individuals.
Is the relevant difference inherited?	An inherited difference can be passed to offspring.
Is the trait linked to reproductive success in these conditions?	This supports a selection explanation.
What changed across generations, and what are the limits of the evidence?	A population pattern and its cause are not the same claim.

Worked example

Evaluating a change in insect colour

A forest becomes darker after a change in plant cover. In surveys taken before and several generations after the change, dark insects make up a larger share of the sampled population. No information is provided about inheritance or survival. What can you conclude, and what evidence would strengthen a natural-selection explanation?

State the observation
The surveys report that dark insects make up a larger share of the sampled population later. This is a change in the observed pattern. It does not by itself identify a cause.
Compare the observation with the model
The darker background could favour dark insects if colour is inherited and if dark individuals leave more offspring under those conditions. The information given does not establish either point, so selection is plausible but not demonstrated.
Name useful additional evidence
Evidence that colour is inherited, along with evidence that dark insects have greater survival or reproductive success in the changed conditions, would support the model. Repeated observations across generations would help show whether the trait’s share continues to change.

Answer: The sampled share of dark insects increased, but the observations alone do not show that natural selection caused the increase. Evidence about inheritance and differences in survival or reproduction would strengthen that explanation.

Check: A change in trait frequency is an observation. Evidence linking the trait to inheritance and reproductive success is needed to support natural selection.

Worked example

Assessing vulnerability after food conditions change

A species depends on one kind of food. The food becomes less available. Individuals vary in a feeding trait, but the available evidence does not show whether the variation is inherited. Explain why the species may be vulnerable and what can—and cannot—be concluded about natural selection.

Connect the change to the species
Reduced availability of a food source may make it harder for individuals to obtain what they need. Dependence on that food is a reason to consider the species potentially vulnerable to this change.
Check the variation and inheritance links
Individuals differ in a feeding trait, but the evidence does not establish that the difference is inherited. Without that link, we cannot conclude that the trait will become more common through natural selection.
Make a limited evaluation
The species may be at risk because an important food source is less available. More evidence would be needed to assess whether some individuals reproduce more successfully because of an inherited feeding difference, and whether that difference becomes more common over generations.

Answer: The species may be vulnerable because it depends on a food source that has become less available. Natural selection is possible only if relevant differences are inherited and linked to reproductive success; the stated evidence does not establish those conditions.

Check: Vulnerability can be identified as a concern without claiming that natural selection will solve it.

Worked example

Separating a pattern from its cause

After a local environmental change, a survey finds that a trait is more common in a population than it was earlier. The survey covers one location and does not measure survival or reproduction. Write a balanced evaluation.

Identify what the evidence shows
The survey supports the claim that the trait was more common in the sampled population at the later survey. It does not directly show how or why the difference arose.
Assess the selection explanation
Natural selection could explain the pattern if the trait is inherited and individuals with it have greater reproductive success under the changed conditions. Neither inheritance nor reproductive success was measured, so the explanation remains unconfirmed.
State a limitation and a next question
Evidence from one location may not apply elsewhere, and the survey did not measure the proposed link to reproductive success. Further evidence about inheritance and reproductive success would make the explanation easier to evaluate.

Answer: The survey documents a change in the trait’s frequency in one sampled location. It is consistent with natural selection but does not establish it, because inheritance and differences in reproductive success were not assessed.

Check: A balanced evaluation states both what the evidence supports and what it cannot establish.

Common mistakes and how to avoid them

Saying that organisms develop a useful inherited trait because they need it.

Correction: Individuals already vary. Environmental conditions can affect which inherited traits are linked to greater reproductive success.

Treating a trait that is helpful in one setting as helpful everywhere.

Correction: Explain the conditions in which the trait may be advantageous. A different environment may change its effect.

Claiming that a survey showing a trait became more common proves natural selection.

Correction: The survey shows a pattern. Evaluate whether evidence also supports inheritance and differences in reproductive success.

Assuming natural selection means every individual changes.

Correction: Natural selection describes changes in the traits common in a population over generations, not a planned change in each individual.

Calling a species certain to survive because some individuals vary.

Correction: Variation alone is not enough to predict an outcome. Consider inheritance, reproductive success, the environmental change, and limits in the evidence.

Lesson summary

Environmental change can affect species in different ways.
Natural selection explains how inherited trait differences linked to reproductive success may become more or less common over generations.
Observed patterns and explanations are distinct; evidence must support the links in the model.
Species vulnerability depends on the change and the species’ circumstances. Conclusions should match the evidence.

Check your understanding

Question 1

A survey finds a trait is more common after an environmental change. Which conclusion is best supported by that observation alone?

The trait became more common in the sampled population.
The trait is definitely inherited.
Individuals with the trait definitely had more offspring.
The environmental change caused the trait to appear.

Show answer and explanation

The trait became more common in the sampled population.

The survey supports a pattern in the sampled population. It does not, by itself, establish inheritance or cause.

Question 2

Which evidence would most directly strengthen a natural-selection explanation?

The trait is inherited, and individuals with it leave more offspring under the changed conditions.
The environment changed at about the same time as the trait frequency changed.
Some individuals have the trait, but no other information is available.
The trait is helpful in a different species.

Show answer and explanation

The trait is inherited, and individuals with it leave more offspring under the changed conditions.

This evidence connects inheritance with greater reproductive success under the relevant conditions, key parts of the model.

Question 3

Why might a species that depends on a food source be vulnerable when that food becomes less available?

Its individuals may have more difficulty obtaining food under the changed conditions.
Natural selection must immediately produce a new trait.
Every individual will respond in the same way.
The species is certain to disappear.

Show answer and explanation

Its individuals may have more difficulty obtaining food under the changed conditions.

Dependence on a less available resource can create risk. The evidence given does not determine the species’ final outcome.

Key terms

Environmental change: A change in living or non-living conditions in an organism’s surroundings.
Population: A group of the same species living in an area.
Trait: A feature of an organism, such as body colour or activity timing.
Inherited trait: A trait that can be passed from parents to offspring.
Natural selection: A model explaining how inherited differences linked to reproductive success can change in how common they are in a population over generations.
Reproductive success: How much an organism contributes offspring to later generations.
Vulnerability: The degree to which a species is at risk of harm or difficulty persisting under a particular environmental change.

Continue through SBI3U

View the complete SBI3U Ontario Grade 11 Biology curriculum and lessons

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation C1.2. 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.

Official curriculum reference

Report a correction or ask a question