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C3.3 · Explain how new species can arise

Learn to explain how new species can arise through clear examples and targeted practice.

Ontario Grade 11 Biology

Evolution

A Grade 11 model of change, isolation, and reproductive separation

In SNC2D, you learned that living things are made of cells and that systems contain parts that interact. A population is a group of the same kind of organism living in the same area. Individuals in a population can differ. For example, some birds may have slightly different beak shapes. If groups of a population become separated and change over many generations, they may eventually become species that cannot reproduce successfully with one another. This lesson explains that process. It presents a model, not a claim that every population follows the same path.

What you will learn

  • Explain how variation and natural selection can cause populations to change over generations.
  • Describe how isolation can reduce gene flow between populations.
  • Use a model to explain how populations may become separate species.
  • Distinguish evidence of differences between populations from an explanation of how those differences arose.

From variation to different populations

A species is often described as a group of organisms that can reproduce with one another and produce offspring that can also reproduce. This definition is useful, but it does not fit every case neatly. For example, it can be hard to apply to organisms that reproduce without mating, or to groups known only from fossils.
Members of a population are not identical. Some differences can be inherited, meaning they can pass from parents to offspring. A population may contain inherited differences in body size, colour, or feeding structures. Natural selection is a process in which individuals with certain inherited traits are more likely to survive and reproduce in a particular environment. Over generations, those traits may become more common.
Natural selection does not make an individual change because it needs to. Rather, individuals already vary, and some differences affect their chances of leaving offspring. The environment matters: a trait that is helpful in one place may not be helpful in another.
  • Variation means differences among individuals.
  • Natural selection can change how common inherited traits are over generations.
  • A population changes across generations; an individual does not evolve during its lifetime.

Isolation can start the process

Suppose one population of small birds lives across a broad region. A new stretch of dry land forms between two parts of the habitat. The birds on either side no longer meet to reproduce. This is geographic isolation: a physical barrier separates populations and reduces their chances of interbreeding.
Gene flow is the movement of inherited traits between populations through reproduction. When separated groups do not reproduce with each other, gene flow between them becomes limited. The groups may then experience different environments, different inherited variations, or chance changes in which traits are passed on. Over many generations, their traits can become more different.
A physical barrier is not the only route to separation. Groups living in the same broad area may use different habitats or may reproduce at different times. If these differences prevent or greatly reduce mating, the groups can become reproductively isolated. Reproductive isolation means that two groups do not successfully produce offspring together, or that their offspring cannot reproduce. It is a key part of the species-formation model.
  • Geographic isolation separates groups with a physical barrier.
  • Reduced gene flow allows groups to change more independently.
  • Reproductive isolation means the groups no longer exchange inherited traits through successful reproduction.

A model for the formation of species

A simple model begins with one population. A barrier or another form of separation reduces reproduction between parts of that population. Over many generations, the groups accumulate differences. Natural selection may contribute when environments differ. Other changes can also occur, including chance differences in which inherited traits are passed on. If the groups eventually become reproductively isolated, they can be considered separate species under the common biological definition.
This sequence is a model because it helps explain how a process could happen. It is not a direct recording of every past generation. Scientists can compare living populations, fossils, and other evidence for patterns such as shared features or differences. Those observations support or challenge explanations, but a pattern alone does not show every step that caused it.
The time required can vary, and the process is not guaranteed to produce a new species. Separated groups may remain able to reproduce with each other, or they may meet again before becoming reproductively isolated. In that case, the groups might interbreed rather than remain separate.
population→isolation→divergence→reproductive isolation\text{population} \rightarrow \text{isolation} \rightarrow \text{divergence} \rightarrow \text{reproductive isolation}
  • Isolation, independent change, and reproductive isolation are central parts of the model.
  • Different environments can favour different inherited traits.
  • Separation does not always lead to new species.

Evidence, explanation, and limits

Imagine that researchers observe two bird populations on separate islands. The birds have different average beak shapes, and each shape is suited to the food commonly found on its island. The differences are an observed pattern. One possible explanation is that the populations became separated and that different food conditions favoured different inherited beak shapes over many generations.
The observation and the explanation are not the same thing. The beak differences can be measured or described. The proposed history of separation and change is a model that must be judged using evidence. Similar patterns may have more than one possible explanation, so scientists look for several kinds of evidence rather than relying on one difference.
A present-day difference does not by itself prove that the groups are separate species. The species question also involves whether they can reproduce successfully with one another. In many real cases, especially when populations are far apart, that may be difficult to test directly. Scientists may use other evidence and state the limits of their conclusions.
  • An observed difference is evidence; a history explaining it is a model.
  • Differences between populations do not automatically mean they are different species.
  • Evidence may be incomplete, so conclusions should match what has actually been observed.

Worked example

Two bird populations on islands

A bird population is divided when a lasting barrier separates two islands. Over many generations, one island has mostly hard seeds, while the other has mostly soft fruit. Explain a possible route by which the two groups could become separate species.
  1. Identify the starting population
    At first, the birds belong to one population. Individuals may already differ in inherited beak traits.
  2. Describe the separation
    The barrier limits contact and reproduction between the islands. Gene flow between the groups is therefore reduced.
  3. Connect environments to change
    If some inherited beak traits help birds get food on one island, those birds may leave more offspring there. Different food conditions may favour different traits on the other island.
  4. State the condition for separate species
    Over many generations the populations may become increasingly different. They would count as separate species under the common definition if they became reproductively isolated. The scenario makes this possible, but does not prove that it happened.
Answer: The barrier reduces reproduction between the groups. Different food conditions may favour different inherited beak traits. If the groups change enough that they cannot successfully reproduce with one another, they may become separate species.
Check: The example explains a possible process, not a guaranteed outcome. It does not claim that beak differences alone establish separate species.

Worked example

Populations separated by a river

A beetle population lives in a continuous forest. A wide river forms and divides the forest. Generations later, beetles on the two sides differ in colour. Explain what the colour difference tells us and what it does not tell us.
  1. Separate observation from explanation
    The observed pattern is that the beetles on opposite sides differ in colour. That observation alone does not reveal why the difference arose.
  2. Apply the isolation model
    The river may have reduced movement and reproduction between the groups. With less gene flow, inherited traits could become different over generations.
  3. Consider possible causes
    Different surroundings might favour different colours. Chance may also affect which inherited traits are passed on. More evidence would be needed to decide how much each explanation matters.
  4. Limit the conclusion
    Colour difference is not enough to conclude that the beetles are separate species. The groups must also be reproductively isolated, or there must be other suitable evidence when that cannot be tested directly.
Answer: The colour difference is evidence that the populations differ. It is consistent with a history of isolation and independent change, but does not prove the cause or establish that they are separate species.
Check: A careful conclusion distinguishes an observed pattern from the model used to explain it.

Worked example

Different breeding times

Two groups of the same kind of insect live in the same meadow. One group usually reproduces early in the season, and the other usually reproduces later. Explain how this difference could contribute to new species formation.
  1. Name the barrier
    The groups are not separated by a physical barrier. Their different breeding times may reduce the chance that they reproduce with each other. This is a form of reproductive separation.
  2. Explain independent change
    If the groups mostly reproduce within their own group, gene flow between them is reduced. Their inherited traits may then change differently over generations.
  3. Check the outcome
    Different breeding times alone do not prove that new species have formed. The groups would need to become reproductively isolated in a lasting way, so they no longer successfully reproduce together.
Answer: Different breeding times could reduce reproduction between the groups. This may allow them to change independently. If reproductive isolation becomes strong enough, they may become separate species.
Check: This example shows that separation can occur without a geographic barrier, but the result is not certain.

Common mistakes and how to avoid them

Saying an individual evolves because it needs a new trait.
Correction: Individuals do not evolve during their lives. Inherited traits can become more or less common in a population over generations.
Assuming that any visible difference means two groups are separate species.
Correction: Differences are evidence of variation. The common species definition also considers whether groups can reproduce successfully with one another.
Treating isolation as proof that a new species will form.
Correction: Isolation can reduce gene flow, but it does not guarantee that populations will become reproductively isolated.
Presenting a proposed history as if every step had been directly observed.
Correction: Identify observed patterns separately from the model that explains how those patterns may have arisen.

Lesson summary

  • Populations contain inherited variation, and natural selection can change trait frequencies over generations.
  • Geographic barriers or differences in habitat or breeding time can reduce reproduction between groups.
  • Reduced gene flow can allow populations to change independently.
  • If populations become reproductively isolated, they may be considered separate species under the common biological definition.
  • A model explains a possible process; evidence and careful limits are needed to support conclusions.

Check your understanding

Question 1

A mountain divides one population into two groups. What is the most direct effect relevant to species formation?
  1. It guarantees that the groups immediately become different species.
  2. It can reduce reproduction and gene flow between the groups.
  3. It makes every individual in each group change during its lifetime.
  4. It proves that any later visible difference is caused by the mountain.
Show answer and explanation
It can reduce reproduction and gene flow between the groups.
A physical barrier can reduce contact and reproduction. Further change over generations may follow, but separate species are not guaranteed.

Question 2

Two populations have different body colours. What conclusion is best supported by that observation alone?
  1. They are definitely separate species.
  2. They cannot reproduce with each other.
  3. They differ in an observed trait, but the cause and species status need more evidence.
  4. Natural selection is proven to be the only cause of the difference.
Show answer and explanation
They differ in an observed trait, but the cause and species status need more evidence.
A colour difference is an observed pattern. It does not by itself establish the cause or whether the populations are reproductively isolated.

Question 3

Which statement best describes how new species may arise?
  1. Individuals decide to adapt, then pass their acquired changes to their offspring.
  2. A population may become separated, change over generations, and eventually become reproductively isolated.
  3. Any two groups in different places are automatically separate species.
  4. A difference in one trait always prevents reproduction between groups.
Show answer and explanation
A population may become separated, change over generations, and eventually become reproductively isolated.
This describes the model: separation can reduce gene flow, populations can change independently, and reproductive isolation may result.

Key terms

Population
A group of the same kind of organism living in the same area.
Inherited
Passed from parents to offspring.
Natural selection
A process in which individuals with certain inherited traits are more likely to survive and reproduce in a particular environment.
Geographic isolation
Separation of populations by a physical barrier that limits contact and reproduction.
Gene flow
Movement of inherited traits between populations through reproduction.
Reproductive isolation
A condition in which groups do not successfully reproduce with one another, or their offspring cannot reproduce.
Divergence
Increasing differences between populations over generations.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation C3.3. It is a study resource, not an official curriculum publication.

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