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C3.1 · Explain biological change over time through natural selection

Learn to explain biological change over time through natural selection through clear examples and targeted practice.

Ontario Grade 11 Biology

Evolution

Ontario Grade 11 Biology — C3.1

In SNC2D, you studied cells, systems, and how scientists use observations to investigate questions. Biology also asks how living things change across many generations. For example, a population of insects may include individuals with different colours. If birds more often eat the insects that stand out, the less noticeable insects may be more likely to survive and reproduce. Natural selection is a model that explains how such differences can become more or less common over time. The individuals do not change their traits because they need to; rather, the population changes as generations pass.

What you will learn

Describe how natural selection can change a population over generations.
Identify variation, inheritance, differences in survival or reproduction, and changes in trait frequency.
Distinguish observations from explanations and recognize limits in evidence.

From an observation to a biological question

A population is a group of organisms of the same species living in the same area. A trait is a feature of an organism, such as its colour or body size. Within a population, individuals can vary: they may have different forms of a trait.

Imagine a population of beetles living on light-coloured tree bark. Some beetles are pale and others are dark. Birds eat beetles. An observer might record that dark beetles are seen less often after birds have fed. That is an observed pattern. It does not, by itself, prove why the pattern happened. Perhaps dark beetles were eaten more often, or perhaps another factor affected how many were observed.

A useful biological question is: could differences in a trait affect which individuals survive and reproduce in this environment? Natural selection is an explanation to consider when a trait is variable, can be passed from parents to offspring, and affects reproductive success. Reproductive success means how much an organism contributes offspring to later generations.

An observation describes what was recorded; an explanation proposes why it happened.
A population contains individuals, and individuals can differ in traits.

The natural selection model

Natural selection is a process in which individuals with some inherited traits leave more offspring than other individuals in a particular environment. Inherited means that a trait can pass from parents to offspring. The environment includes living and non-living conditions that affect organisms, such as predators, food, and surroundings.

The model has connected parts. First, individuals in a population vary. Second, some of that variation is inherited. Third, individuals with certain traits survive or reproduce more successfully in the current environment. Finally, those traits may become more common in the population over generations. A generation is one round of parents producing offspring.

Selection acts on individuals because their traits affect their experiences. But evolution by natural selection is a change in a population across generations. An individual beetle does not become pale during its life because the bark is light. Instead, if pale beetles leave more offspring, the next generations may include a greater proportion of pale beetles.

The outcome depends on the environment. A trait that is helpful in one setting may not be helpful in another. Natural selection does not plan ahead or produce the trait an organism needs. It can only affect the success of existing variation that is inherited.

\text{inherited variation} + \text{unequal reproductive success} \rightarrow \text{population change over generations}

Variation, inheritance, and unequal reproductive success are all needed for this explanation.
Natural selection changes the proportion of traits in a population, not the traits of an individual during its lifetime.
A trait's effect depends on the environment.

Following the change across generations

A simple way to track natural selection is to compare how common a trait is in a population at different times. Trait frequency means how common a particular form of a trait is among the population's individuals. It can be described as a proportion, without needing a complex calculation.

Return to the beetles on pale bark. Suppose pale colour is inherited and makes beetles harder for birds to see. If pale beetles, on average, leave more offspring, pale colour may become more common over generations. This is a prediction of the model, not a guarantee about every beetle or every generation.

Scientists can compare observations with this prediction. They might ask whether the trait varies, whether offspring tend to resemble their parents in that trait, and whether the trait is linked to survival or reproduction in the setting being studied. A pattern that matches the prediction supports the explanation, but observations should be considered carefully. Other causes may also affect which organisms are counted.

The model works at the population level. It does not say that every individual with a helpful trait survives, or that every individual without it dies. Chance and other conditions can affect individual outcomes. It describes differences in success across groups and generations.

A population-level change can be described as a trait becoming more or less common over generations.
A prediction is not a certainty for each individual.
Evidence is stronger when it addresses variation, inheritance, and differences in reproductive success.

What the model can and cannot tell us

Natural selection helps explain biological change over time when inherited differences are linked to different reproductive success. It does not explain every observed change automatically. If a trait does not vary, or if the variation is not inherited, selection on that trait cannot produce the described population change.

A change in what an observer sees is not always a change in the population. For instance, beetles may be harder to spot on one kind of bark, even if the number of each colour has not changed. Good reasoning separates the way organisms are detected from the actual frequency of their traits.

When evaluating an explanation, ask what was observed and what is being inferred. Then check whether the explanation accounts for inherited variation and differences in survival or reproduction. Keep the conclusion matched to the evidence. A limited observation can suggest an explanation without proving that it applies in every environment or over all time.

Do not confuse being less visible to an observer with becoming less common.
A natural selection explanation must connect inherited differences with reproductive success.

Parts of a natural selection explanation

Part	Question to ask	Beetle example
Variation	Do individuals differ in the trait?	Some beetles are pale; others are dark.
Inheritance	Can the trait pass from parents to offspring?	Colour is inherited.
Unequal success	Is survival or reproduction linked to the trait?	Birds catch dark beetles more often on pale bark.
Change over generations	Does the trait become more or less common in the population?	Pale colour may become more common.

Worked example

Beetle colour and bird predation

A population has pale and dark beetles. Colour differences are inherited. On pale bark, birds more often catch dark beetles. Explain how the population could change over generations, and state one condition needed for the explanation.

Identify the variation
The beetles vary in colour: some are pale and some are dark. This gives the population different forms of a trait.
Connect the trait to success
The observation says birds more often catch dark beetles on pale bark. If that difference means pale beetles, on average, leave more offspring, pale colour may become more common.
Describe the population change
Because colour is inherited, offspring can receive the trait from their parents. Across generations, the population may contain a greater proportion of pale beetles. This is a possible population trend, not a promise about every offspring.
Name a needed condition
The explanation depends on colour being inherited, as stated in the problem. It also depends on the difference in being caught leading to a difference in reproductive success.

Answer: Pale colour could become more common over generations if pale beetles leave more offspring on average. The explanation requires inherited colour variation and a link between the difference in being caught and reproductive success.

Check: The answer explains a population-level change and does not claim that individual dark beetles turn pale.

Worked example

A trait that does not affect reproductive success

Two inherited shell colours occur in a population of snails. In the environment described, neither colour affects survival or the number of offspring. Can natural selection explain one colour becoming more common? Explain your reasoning.

Check for variation and inheritance
The problem states that shell colours differ and are inherited. These parts of the model are present.
Check for unequal success
The problem states that neither colour affects survival or the number of offspring. There is no stated difference in reproductive success linked to shell colour.
Match the conclusion to the model
On the information given, natural selection does not explain one colour becoming more common. If a colour-frequency change were observed, more evidence would be needed to identify its cause.

Answer: No. The problem gives inherited variation but no difference in survival or reproduction linked to colour. Natural selection therefore does not explain a change in colour frequency from the information provided.

Check: A trait being inherited is not enough by itself; the model also requires unequal reproductive success linked to the trait.

Worked example

Separate a count from an explanation

A survey finds fewer dark beetles than pale beetles on pale tree bark after birds have visited. What can this observation support, and what does it not prove?

State the observation
The survey recorded fewer dark beetles than pale beetles in the place and time described. This is the observed pattern.
Offer a possible explanation
One possible explanation is that birds catch dark beetles more often, so pale beetles have greater reproductive success. If the colour difference is inherited, the pattern could fit natural selection.
State the limitation
The survey alone does not show that birds caused the difference or that the population's trait frequencies changed over generations. Beetle visibility or other conditions could affect what was counted.

Answer: The observation is consistent with the idea that dark beetles are less common in that survey. It may support a natural selection explanation if further evidence links inherited colour to differences in reproductive success. It does not by itself prove that birds caused a change over generations.

Check: The answer keeps the observed count separate from the proposed cause.

Common mistakes and how to avoid them

Individuals change their traits because they need to survive.

Correction: Natural selection changes trait frequencies in populations over generations. It does not make an individual change a trait because of need.

Every individual with a helpful trait will survive and reproduce.

Correction: The model describes differences in success on average. It does not guarantee the outcome for each individual.

Any trait that is inherited will become more common.

Correction: The trait must be linked to differences in survival or reproduction in that environment for natural selection to explain a change.

One observation proves the cause of a population change.

Correction: An observation records a pattern. More evidence is needed to test whether natural selection explains it and whether the pattern persists across generations.

Lesson summary

Natural selection explains biological change over time through inherited variation and differences in reproductive success.
Selection affects individuals differently, but the change is described in populations across generations.
A trait may become more common when individuals carrying it leave more offspring on average in a particular environment.
Separate observations from explanations, and make conclusions no broader than the evidence supports.

Check your understanding

Question 1

Which set of conditions best supports a natural selection explanation for a population change?

Individuals vary in an inherited trait, and the trait is linked to differences in reproductive success.
All individuals have the same trait, but they need a different one.
A trait is visible, whether or not it is inherited or affects reproduction.
One individual changes its trait during its lifetime.

Show answer and explanation

Individuals vary in an inherited trait, and the trait is linked to differences in reproductive success.

Natural selection requires inherited variation and differences in reproductive success linked to that variation.

Question 2

A survey counts more pale beetles than dark beetles on pale bark. What is the most careful conclusion?

The count is an observation; by itself it does not prove why the numbers differ.
The count proves that every dark beetle was eaten by a bird.
The count proves that dark beetles changed colour during their lives.
The count proves that pale beetles will always be more common.

Show answer and explanation

The count is an observation; by itself it does not prove why the numbers differ.

A count describes a pattern. More evidence is needed to establish its cause and whether it represents change across generations.

Question 3

What changes through natural selection over generations?

The frequency of inherited traits in a population.
The needs of each individual organism.
The colour of every individual at the same moment.
The environment's ability to plan for future conditions.

Show answer and explanation

The frequency of inherited traits in a population.

Natural selection can change how common inherited traits are in a population. It does not involve planning or a guaranteed change in each individual.

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 in traits among individuals in a population.
Inherited: Able to pass from parents to offspring.
Natural selection: A process in which individuals with some inherited traits leave more offspring than others in a particular environment.
Reproductive success: How much an organism contributes offspring to later generations.
Trait frequency: How common a particular form of a trait is among the individuals in a population.
Generation: One round of parents producing offspring.

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