Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.1.7Topic 4 · Biodiversity and Natural Resources

Reproductive Isolation — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 22 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

4.1.7 Reproductive Isolation — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how populations become reproductively isolated and how allopatric and sympatric speciation produce new species.A species is a group of organisms with similar morphological, physiological, biochemi…

  • geographical isolation
  • Ecological isolation
  • Temporal isolation
  • Behavioural isolation
  • Mechanical isolation
  • Polyploidy (especially in plants)

Part 1 of 7What is a species?

A species is a group of organisms with similar morphological, physiological, biochemical and behavioural features, which can interbreed to produce fertile offspring and are reproductively isolated from other species.

What is a species?
The mule, offspring of a horse and donkey, is sterile, so horses and donkeys are different species.
The biological species concept does not apply neatly to asexual organisms such as bacteria, or to fossils, where other features are used.

Part 2 of 7Speciation

Speciation is the formation of new species from an existing one.

Reproductively isolated
It happens when populations of one species become reproductively isolated, so that gene flow between them stops.
Once they are isolated, each population accumulates different mutations and is subject to different selection pressures,
  • so the gene pools diverge, until the populations can no longer interbreed even if brought back together.

Part 3 of 7Allopatric speciation

Allopatric speciation requires geographical isolation. A physical barrier, such as a mountain range, river, ocean or the splitting of a lake, divides one population into two.

1
The two populations are separated, so there is no gene flow.
2
The environment on each side is different, so there are different selection pressures; different alleles are favoured.
  • Random mutations and genetic drift also add to the differences.
3
Allele frequencies diverge over many generations.
4
Eventually the populations are so different that if they meet again they cannot interbreed to produce fertile offspring: they have become separate species.

Darwin's finches on the Galapagos Islands are a famous example: finches from the mainland colonised different islands and evolved different beak shapes for the local food.

Part 4 of 7Sympatric speciation

Sympatric speciation occurs in the same geographical area. It requires some other form of reproductive barrier to arise within the population. Examples are:

Sympatric speciation
Ecological isolation
Ecological isolation groups use different parts of the same habitat, for example different host plants, and so rarely meet.
Temporal isolation
Temporal isolation groups breed or flower at different times of the year or day.
Behavioural isolation
Behavioural isolation differences in courtship songs, displays or pheromones mean individuals do not recognise others as mates.
Mechanical isolation
Mechanical isolation differences in structure mean that mating cannot occur, as with flower shapes that fit different pollinators.
Polyploidy (especially in plants)
Polyploidy (especially in plants) a failure of meiosis produces gametes with the full diploid set; fusion with another gives a tetraploid,
  • which cannot breed successfully with the diploid parent but can breed with other tetraploids.
  • Many crops, including wheat, are polyploid.

Part 5 of 7Reproductive isolating mechanisms

TypeMechanismExample
PrezygoticPrevent fertilisation: ecological, temporal, behavioural, mechanical, gameticFireflies with different flash patterns
PostzygoticFertilisation occurs but the hybrid is non-viable or sterileMule (sterile hybrid)

Different chromosome numbers in the parents can make meiosis fail in a hybrid because chromosomes cannot pair, leading to sterility.

Part 6 of 7Evidence and importance

Evidence for speciation comes from comparing DNA and protein sequences, from the pattern of related species on islands and across barriers, from fossils and from the observation of new polyploid species in plants. Speciation, together with natural selection, accounts for the diversity of life on Earth, and a better understanding of it helps conservationists to decide which populations should be protected as distinct species or units.

Isolation barriers can act together.

1
Two populations of a plant may flower at different times (temporal isolation) and also be pollinated by different insects (mechanical or ecological isolation).
2
In animals, behavioural differences such as courtship songs often evolve early because females prefer males with the local pattern,
  • and this is a strong barrier even though the animals are physically capable of mating.
3
The speed of speciation depends on the strength of selection, the generation time and the size of the populations.
4
Small isolated populations change faster because of genetic drift.
5
Island groups are especially good places for study: for example the Hawaiian honeycreepers radiated from one ancestral finch into species with many different beak shapes that suit different foods, an example of adaptive radiation, in which one species diversifies rapidly to fill many niches.

Exam answers should state clearly that isolation stops gene flow, that different selection pressures favour different alleles in each population, that allele frequencies change and that eventually interbreeding cannot produce fertile offspring. Evidence for speciation includes comparisons of DNA, ring species where neighbouring populations interbreed but the ends do not, and the observation that polyploid hybrids can form new plant species within a few generations.

Part 7 of 7Exam-style questions

  1. Describe how geographical isolation can lead to the formation of a new species. [4]4 marks

  2. Explain why a mule is not classified as a species. [2]2 marks

  3. Explain how polyploidy can lead to sympatric speciation in plants. [3]3 marks

Key terms

TermMeaning
SpeciesA group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from others.
SpeciationFormation of new species.
Allopatric speciationSpeciation resulting from geographical isolation.
Sympatric speciationSpeciation in the same geographical area.
PolyploidyHaving more than two sets of chromosomes.
Gene flowTransfer of alleles between populations by interbreeding.
Reproductive isolationInability of populations to interbreed successfully.

Avoid these mistakes

1
Saying that populations separated by a barrier are automatically different species; they must become unable to interbreed.
2
Confusing allopatric (different place) with sympatric (same place).
3
Stating that individuals evolve into new species; populations evolve.
4
Forgetting to mention mutation and selection pressures as the reasons for divergence.
5
Saying hybrids are always sterile; some are fertile.

Last updated: