Biology • Year 11 • Module 3 • Lesson 7

Speciation

Lock in the core vocabulary, the biological species concept, the sequence of allopatric speciation, and the distinction between pre-zygotic and post-zygotic isolation before moving to application tasks.

Build · Vocab & Structure

1. Complete the paragraph

Fill each blank with the correct term from the word bank. Use each term once only. 8 marks

Word bank: speciation • reproductive isolation • gene flow • geographic isolation • divergence • allopatric • sympatric • polyploidy

_______________ is the formation of new and distinct species from an ancestral species. It requires _______________ — mechanisms that prevent interbreeding between populations so that _______________ stops. The most common pathway is _______________ speciation, which begins when a barrier creates _______________, physically separating one population into two isolated groups. Over time, the two populations accumulate different mutations and adaptations through _______________. Speciation can also occur without geographic separation: _______________ speciation is common in plants where _______________ — chromosome doubling — suddenly creates reproductive barriers within a population.

Stuck? Revisit the Key Terms panel and Cards 1–3 in the lesson.

2. Term–definition match

Write the matching term from this list in the right-hand column: speciation • allopatric speciation • sympatric speciation • reproductive isolation • geographic isolation • polyploidy • pre-zygotic barrier • post-zygotic barrier. 8 marks

#	Definition	Matching term
2.1	The formation of new and distinct species from an ancestral species.
2.2	Speciation driven by physical separation of populations by a barrier.
2.3	Speciation that occurs in the same geographic area without a physical barrier.
2.4	Mechanisms that prevent interbreeding between different species or diverging populations.
2.5	Physical separation of populations by a mountain range, river, ocean, or other barrier.
2.6	The condition of having more than two complete sets of chromosomes.
2.7	A reproductive barrier that prevents mating or fertilisation from occurring in the first place.
2.8	A reproductive barrier where fertilisation occurs but hybrids die young or are infertile.

Stuck? Revisit the Key Terms panel and Card 3 in the lesson.

3. Sequence allopatric speciation

The following steps describe allopatric speciation. Number them 1–4 in the correct sequence. 4 marks

Order (1–4)	Step
	Accumulated differences mean that, even if the barrier is removed, the populations can no longer produce fertile offspring together — two species now exist.
	A geographic barrier (e.g. river, ocean, mountain range) divides one ancestral population into two isolated groups.
	Each isolated population accumulates different mutations, experiences different selection pressures, and evolves different adaptations.
	Gene flow between the two populations drops sharply because of the physical barrier.

Revisit Card 2 in the lesson — the allopatric speciation diagram and sequence table.

4. Classify reproductive barriers as pre-zygotic or post-zygotic

For each example, write Pre (pre-zygotic) or Post (post-zygotic) in the Type column. 5 marks

Type	Example
	A mule produced from a horse and donkey that is sterile
	Two plant species with different flowering seasons
	Two bird species with incompatible courtship displays that never attempt to mate
	A hybrid fish embryo from two species that dies shortly after fertilisation
	Two insect species with mechanically incompatible reproductive structures

Remember: pre-zygotic barriers act before fertilisation (preventing mating or gamete fusion); post-zygotic barriers act after fertilisation (hybrid fails or is infertile).

5. True or false — with correction

Circle T or F. If false, write the corrected version. 8 marks — 1 for T/F, 1 for correction where needed

5.1 According to the biological species concept, two organisms are the same species if they can mate and produce fertile offspring. T / F

5.2 A mule is a classic example of pre-zygotic isolation because the horse and donkey fail to mate. T / F

5.3 Sympatric speciation can occur without any geographic separation of populations. T / F

5.4 The diversification of marsupials in Australia is an example of allopatric speciation that occurred because Australia became geographically isolated. T / F

Stuck? Revisit Cards 1 and 3 in the lesson, and the misconceptions box.

Answers — Do not peek before attempting

Q1 — Cloze paragraph

In order: Speciation • reproductive isolation • gene flow • allopatric • geographic isolation • divergence • sympatric • polyploidy.

Q2 — Term–definition matches

2.1 speciation • 2.2 allopatric speciation • 2.3 sympatric speciation • 2.4 reproductive isolation • 2.5 geographic isolation • 2.6 polyploidy • 2.7 pre-zygotic barrier • 2.8 post-zygotic barrier.

Q3 — Sequence answers

Correct order: 2 (barrier divides population) → 4 (gene flow drops) → 3 (populations diverge) → 1 (speciation complete).

Q4 — Classify barriers

Post Mule (sterile hybrid) • Pre Different flowering seasons • Pre Incompatible courtship displays • Post Hybrid embryo dies after fertilisation • Pre Incompatible reproductive structures.

Q5 — True/False

5.1 True.

5.2 False. Correction: A mule is a classic example of post-zygotic isolation because a horse and donkey can mate and produce a fertilised egg (a zygote forms), but the resulting mule is sterile — the barrier acts after fertilisation.

5.3 True.

5.4 True.