Speciation
BiologyΒ· Unit 18: Selection and EvolutionΒ· 20 min read
1. Defining a Speciesβ β ββββ± 5 min
The biological species concept is the standard definition expected for CIE A-Level Biology answers, unless a question explicitly asks for an alternative definition.
Biological Species Concept
A group of actually or potentially interbreeding natural populations that produce fertile offspring, and are reproductively isolated from other such groups.
Example:
Horses and donkeys can interbreed to produce mules, but mules are infertile, so they are classed as separate species.
Explain why lions and tigers are classified as separate species even if they can produce fertile ligers in captivity.
- 1
Apply the biological species definition, which requires interbreeding to occur in natural populations.
- 2
In the wild, lions and tigers occupy different habitats and have different mating behaviours, so they never interbreed naturally.
- 3
The requirement for natural interbreeding is not met, so they are reproductively isolated in their natural environment, hence classed as separate species.
2. Allopatric Speciationβ β ββββ± 6 min
Allopatric speciation is the most common form of speciation, driven by geographic isolation that stops gene flow between populations.
Allopatric Speciation
Speciation that occurs when a physical geographic barrier separates a population into two isolated groups, stopping interbreeding and gene flow.
After separation, each population experiences different selection pressures based on their local environment. Allele frequencies change over generations due to natural selection, genetic drift, and new mutations, leading to genetic and phenotypic divergence. Eventually, the populations become so different that they can no longer interbreed to produce fertile offspring even if the barrier is removed.
Explain how geographic isolation led to speciation in squirrels separated by the Grand Canyon.
- 1
Originally, a single interbreeding squirrel population occupied the whole area, sharing a common gene pool.
- 2
The formation of the Grand Canyon created a permanent physical barrier that split the population into two isolated groups that could not cross to interbreed.
- 3
Different environmental conditions (food sources, predators) on each side of the canyon created different selection pressures for each group.
- 4
Over many generations, natural selection changed allele frequencies, leading to large genetic and phenotypic differences between the two populations.
- 5
When the populations came into contact again, they could no longer interbreed to produce fertile offspring, so two new species had formed.
Exam tip:
Always mention changes in allele frequency, not just phenotypic change, as this is a key marking point in CIE exams.
3. Sympatric Speciationβ β β βββ± 6 min
Sympatric speciation occurs without geographic isolation: new species form from a parent population within the same geographic area. The most common mechanism in plants is polyploidy, while it can also occur via disruptive selection for different niches in animals.
Sympatric Speciation
Speciation that occurs within the same geographic area, with no physical barrier stopping gene flow between the new species and parent population.
Explain how polyploidy causes instant sympatric speciation in cultivated wheat.
- 1
A diploid (2n = 14) parent wheat plant has an error in meiosis that produces diploid gametes (n = 14, instead of the normal n = 7).
- 2
Two diploid gametes fuse during fertilisation to produce a tetraploid offspring (4n = 28), with four full sets of chromosomes.
- 3
If the tetraploid mates with a wild diploid plant, the resulting offspring is triploid (3n = 21). Chromosomes cannot pair correctly during meiosis in triploids, so the hybrid is sterile.
- 4
The tetraploid can self-pollinate or mate with other tetraploids to produce fertile tetraploid offspring.
- 5
The new tetraploid population is reproductively isolated from the parent diploid population within the same area, so a new species has formed instantly.
Exam tip:
Remember the key difference: allopatric requires geographic isolation, sympatric does not. This is a common comparison question.
4. Reproductive Isolationβ β β βββ± 3 min
Reproductive isolation is the defining final step of all speciation. It is categorised into pre-zygotic (before zygote formation) and post-zygotic (after zygote formation):
Pre-zygotic isolation: Prevents zygote formation. Examples: different mating seasons, different mating signals, physical incompatibility, gamete incompatibility.
Post-zygotic isolation: Reduces fertility or viability after zygote formation. Examples: hybrid inviability, hybrid sterility, hybrid breakdown.
Test your understanding of isolation types:
Which of the following is an example of post-zygotic isolation?
Two bird species have different mating songs
Hybrid offspring of two fish species are always infertile
Two plant species flower at different times of year
Sperm of one mammal cannot fertilise eggs of another
Reveal answer
1 βCorrect! Hybrid infertility occurs after zygote formation, so it is post-zygotic. All other options prevent zygote formation, so they are pre-zygotic.
5. Common Pitfalls
Wrong move:
Claiming geographic barriers cause adaptive mutations for speciation
Why:
Mutations are random, not induced by the environment or barrier. The barrier only stops gene flow
Correct move:
State that the barrier isolates populations, stopping interbreeding. Random mutations arise in each population, and different selection pressures change allele frequencies over time
Wrong move:
Confusing sympatric and allopatric speciation, stating sympatric requires geographic isolation
Why:
The defining difference between the two modes is the presence/absence of geographic separation, which is a common exam marking point
Correct move:
Allopatric speciation requires geographic isolation; sympatric speciation occurs without geographic isolation in the same area
Wrong move:
Defining a species based only on phenotypic differences
Why:
CIE expects the biological species concept definition, not just appearance. Similar-looking species can be reproductively isolated
Correct move:
Always define species as a group of actually/potentially interbreeding natural populations that produce fertile offspring, reproductively isolated from other groups
Wrong move:
Stating all speciation occurs gradually over many generations
Why:
This is only true for most allopatric speciation. Polyploid sympatric speciation can occur in a single generation
Correct move:
Allopatric speciation occurs gradually over many generations, while sympatric speciation via polyploidy can be instant within one generation
6. Quick Reference Cheatsheet
Concept | Key Feature | Example |
|---|---|---|
Biological Species Concept | Interbreeding natural populations produce fertile offspring | Horses and donkeys are separate species |
Allopatric Speciation | Geographic isolation stops gene flow | Grand Canyon squirrel species |
Sympatric Speciation | No geographic isolation, same area | Polyploidy in cultivated wheat |
Pre-zygotic Isolation | Prevents zygote formation | Different mating seasons |
Post-zygotic Isolation | Reduces hybrid fertility/viability | Infertile mule hybrids |
7. Frequently Asked
What is the difference between evolution and speciation?
Evolution is any change in the allele frequency of a population over generations. Speciation is a specific outcome of evolution where a new distinct reproductively isolated species forms from an ancestral population.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 2
Describe allopatric speciation
- 2023 Β· 4
Compare allopatric and sympatric speciation
- 2021 Β· 1
MCQ on biological species concept
Going deeper
What's Next
Speciation is the core process that generates the biodiversity we see on Earth, and connects concepts of natural selection, genetic variation, and classification. This understanding builds a foundation for studying patterns of evolution over geological time, and is essential for topics on conservation and biodiversity. Speciation also relies on genetic changes in populations that you will revisit when studying inheritance and the genetic basis of variation.