17.3.217-18 · Selection, Evolution, Classification and Conservation
Allopatric and Sympatric Speciation — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 16 of 36 1 quiz
Lesson summary
17.3.2 Allopatric and Sympatric Speciation — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Compare allopatric and sympatric speciation and explain how each leads to reproductive isolation.Speciation — formation of new species from pre-existing species as gene pools …
- Allopatric speciation
- Darwin's finches
- Ecological separation
- Behavioural separation
- Polyploidy
- Prezygotic isolation
Section 1 of 10Speciation and isolation
Speciation
- Speciation formation of new species from pre-existing species as gene pools change from generation to generation.
Genetic isolation between the new population and the original species is essential.
Two situations allow speciation:
- Two groups of a species are separated by a geographical barrier.
- Two groups are reproductively isolated while living in the same area, with similar selection pressures.
Isolating mechanisms may be prezygotic (stop fertilisation: different seasons, courtship, habitat) or postzygotic (hybrids not viable or sterile).
Section 2 of 10Allopatric speciation
Allopatric speciation
- Allopatric speciation speciation caused by geographical isolation; it is the most common type.
A population splits into groups separated by a natural barrier (water body, mountain range) or a man-made one (motorway).
- No gene exchange can occur between the separated populations.
If selection pressure or genetic drift is strong enough, the gene pools diverge and form separate species.
- Alleles change in each population, so phenotypes change.
- The populations differ in physiology, behaviour and morphology over time.
Section 3 of 10Allopatric speciation in trees (steps)
1
A population of one tree species is continuous.
2
A mountain range forms and divides it in two.
3
The barrier prevents interbreeding, so there is no gene flow.
4
The two populations experience different selection pressures and genetic drift.
5
Over thousands of years they become two species that can no longer interbreed.
Section 4 of 10Allopatric examples
Darwin's finches
- Darwin's finches descendants of one mainland ancestor diversified on the Galapagos islands, beak shape matching the food on each island.
Squirrels on opposite sides of the Grand Canyon.
Cichlid fish in African lakes.
Section 5 of 10Sympatric speciation
Sympatric speciation
- Sympatric speciation speciation with no geographical barrier.
- A group living in one place must form two populations with no gene flow between them.
Ecological separation
- Ecological separation populations live in different environments within the same area, e.g. different soil pH affects plant growth and flowering.
Behavioural separation
- Behavioural separation populations differ in feeding, communication or social behaviour.
Polyploidy
- Polyploidy a failure of meiosis produces diploid gametes, and fusion gives a tetraploid.
- Tetraploids interbreed with each other, but offspring with diploids are triploid and sterile.
- A new species forms in one generation, which is common in plants (bread wheat, cotton).
Section 6 of 10Sympatric speciation in fish (steps)
1
A fish species lives in a lake; some feed on the bottom and others in open water.
2
Behavioural separation leads to ecological separation.
3
The groups face different selection pressures: long jaws suit bottom feeding, short jaws suit mid-water feeding.
4
Natural selection makes the populations diverge and evolve different courtship displays.
5
They can no longer interbreed and are separate species.
Section 7 of 10Allopatric and sympatric compared
| Feature | Allopatric | Sympatric |
|---|---|---|
| Location | Geographically separated | Same area |
| Initial barrier | Geographical | Ecological, behavioural, genetic or chromosomal |
| Speed | Slow, thousands of generations | Can be rapid (polyploidy in one generation) |
| Frequency | Most common | Less common, more so in plants |
Section 8 of 10More from the specification
1
Allopatric speciation via geographical isolation
- Physical barriers divide populations, preventing gene flow; differing selection pressures and genetic drift lead to reproductive isolation and separate species.
- (mountains, rivers)
2
Sympatric speciation mechanisms
- Speciation within the same geographical territory arises through behavioural isolation, temporal differences in breeding seasons,
- or polyploidy (in plants).
3
Phylogenetic trees and molecular clock dating
- Comparing amino acid or nucleotide sequence divergences across taxa allows evolutionary biologists to construct phylogenetic trees and estimate divergence times.
4
Role of gene mutation in evolution
- Role of gene mutation in evolution spontaneous mutation generates novel alleles and genes within organisms, establishing the raw variation required for evolutionary divergence.
5
DNA sequence analysis in phylogenetics
- DNA sequence analysis in phylogenetics comparing nucleotide sequences of shared genes across organisms enables biologists to construct evolutionary phylogenetic trees.
6
Speciation theory and reproductive isolation
- Speciation theory and reproductive isolation geographical or behavioural isolation prevents interbreeding between members of separated populations, culminating in distinct descendant species.
7
Molecular clock
- Use of sequence differences, assuming a constant mutation rate, to date divergence.
8
Evolution
- Change in adaptive features of a population over time through natural selection.
9
Organism
- A living thing, such as a plant or an animal.
10
Sequence
- The order of bases in DNA or amino acids in a protein.
11
Mutation
- A random change in genetic material.
12
Theory
- A well-tested explanation supported by a large body of evidence.
13
Genome
- All the genetic information in an organism.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Speciation | Formation of new species from existing ones. |
| Allopatric speciation | Speciation following geographical isolation. |
| Sympatric speciation | Speciation within the same area without a geographical barrier. |
| Gene flow | Movement of alleles between populations. |
| Polyploidy | Having more than two sets of chromosomes. |
| Prezygotic isolation | Isolation that prevents fertilisation. |
| Postzygotic isolation | Isolation in which hybrids are non-viable or infertile. |
Section 10 of 10Exam technique
1
Mixing the cause of separation with the barrier to breeding — in the fish, feeding behaviour separates the populations and courtship differences prevent interbreeding.
2
Forgetting mutation — without new alleles selection cannot produce the differences between species.
3
Saying sympatric speciation needs a geographical barrier — it occurs in the same area.
4
Saying isolated populations become separate species immediately — they diverge over generations until they cannot interbreed.
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