Speciation

HSC Biology | Free Study Notes

In this lesson

• what speciation means

• what reproductive isolation is

• how divergent evolution is linked to speciation

• what allopatric speciation is

• what sympatric speciation is

What is speciation?

Speciation is the formation of a new species from an existing population.

This happens when populations of the same species become different enough that they can no longer successfully interbreed and produce fertile offspring.

Key idea

Speciation does not usually happen in one step. It happens gradually as differences build up over many generations.

Reproductive isolation

Reproductive isolation happens when two populations can no longer interbreed successfully.

Why reproductive isolation matters

Reproductive isolation is important because once gene flow between populations stops, the populations can evolve separately.

Over time, differences may build up in:

• allele frequencies

• adaptations

• behaviour

• timing of reproduction

• genetic makeup

Eventually, the two populations may become separate species.

Simple summary

No gene flow between populations means they can diverge over time.

Divergent evolution

Divergent evolution happens when groups from a common ancestor become increasingly different over time.

How this links to speciation

If two populations face different selection pressures, natural selection may favour different traits in each group.

As a result:

• the populations become less similar

• adaptations become different

• reproductive isolation may eventually develop

• speciation can occur

Important point

Divergent evolution helps explain how one ancestral population can give rise to multiple different species.

The Module 3 syllabus explicitly includes divergent evolution as one of the patterns students must explain using Darwin and Wallace’s theory. 

How speciation happens

A basic sequence for speciation is:

1. A population contains variation.

2. Gene flow between groups is reduced or stopped.

3. Different selection pressures act on the groups, or different changes build up within them.

4. The groups become more genetically and phenotypically different.

5. Reproductive isolation develops.

6. A new species forms.

Allopatric speciation

Allopatric speciation happens when populations become geographically separated.

What causes the separation?

Examples of geographic barriers include:

• mountains

• rivers

• oceans

• habitat fragmentation

• migration to a new area

How it leads to speciation

Once the populations are separated:

• they no longer share genes easily

• different environments create different selection pressures

• mutations and genetic drift may also affect them differently

Over time, the populations may become reproductively isolated and form new species.

Key idea

In allopatric speciation, the separation happens first, then divergence builds up.

Sympatric speciation

Sympatric speciation happens when new species form without geographic separation.

How this can happen

Populations living in the same area may become reproductively isolated because of:

• different mating behaviours

• different breeding times

• different food sources or ecological niches

• genetic changes such as chromosome changes in plants

How it leads to speciation

Even though the populations live in the same place, they stop interbreeding. Once gene flow is reduced, they can diverge and eventually become separate species.

Key idea

In sympatric speciation, the populations are in the same geographic area, but reproductive isolation still develops.

Allopatric and sympatric speciation compared

Type of speciation	Main feature	Example idea
Allopatric	Geographic isolation	A river splits a population
Sympatric	No geographic isolation	Populations breed at different times in the same area

Why selection pressures matter

Selection pressures are a major driver of speciation.

If two populations experience different conditions, different traits may be favoured.

Examples of selection pressures include:

• climate

• food type

• predators

• competition

• mating preferences

As different alleles become more common in each population, the populations diverge further.

Speciation and biodiversity

Speciation is one reason biodiversity increases over time.

When new species form:

• the number of species rises

• ecosystems may become more complex

• biological diversity increases

Worked example

Exam-style question

Explain how allopatric speciation can lead to the formation of a new species.

Worked answer

Allopatric speciation occurs when a population is split by a geographic barrier such as a river or mountain. This prevents gene flow between the two groups. Different selection pressures, mutations and genetic drift then cause the populations to diverge over time. Eventually, reproductive isolation develops and the two populations become separate species.

Why this works

This answer:

• defines allopatric speciation

• includes reproductive isolation

• links separation to divergence and new species formation

Common mistakes

• Confusing speciation with adaptation.

• Forgetting that reproductive isolation is the key idea in speciation.

• Saying all speciation requires a geographic barrier.

• Mixing up allopatric and sympatric speciation.

• Describing divergent evolution without linking it to common ancestry and increasing difference over time.

Quick quiz

1. What is speciation?

2. What is reproductive isolation?

3. How does divergent evolution relate to speciation?

4. What is allopatric speciation?

5. What is sympatric speciation?

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