17.317-18 · Selection, Evolution, Classification and Conservation
Evolution — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 10 parts Lesson 14 of 36 2 quizzes
Lesson summary
17.3 Evolution — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how gene pools change, how new species form by allopatric and sympatric speciation, and how DNA shows evolutionary relationships.Species — a group of organisms that can interbreed to produce fert…
- Ecological separation
- Behavioural separation
- Molecular clock
- Allopatric speciation
- Phylogenetic tree
- Mules (from a male donkey and female horse) are infertile, so horses and donkeys are separate species
Part 1 of 8Species and the gene pool
Species
- Species a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other species.
- Members share similar behavioural, morphological and physiological features.
- Mules (from a male donkey and female horse) are infertile, so horses and donkeys are separate species.
- In practice species are hard to define and must be investigated.
Gene pool
- Gene pool the sum of all alleles at all loci in an interbreeding population.
Natural selection, genetic drift and the founder effect change the gene pool over time.
Evolution
- Evolution change in allele frequencies in a population over generations.
Part 2 of 8Reproductive isolation and speciation
A new species needs genetic and reproductive isolation from the original population.
- Isolation may arise from mutations causing incompatible gametes or sex organs, or from different breeding behaviour.
- Isolated populations do not exchange genes, so changes in allele frequency are not shared.
Speciation
- Speciation formation of new species when populations become unable to interbreed.
Speciation relies on mutation, which supplies the new alleles for selection to act on.
Speciation takes many generations, but is seen far faster in organisms with short generation times such as bacteria.
Part 3 of 8Allopatric speciation
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A species population is split into groups by a geographical barrier (mountain range, water body or motorway).
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Gene flow between the groups stops.
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Different selection pressures and genetic drift act on each population, and mutations arise independently.
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Allele frequencies diverge, so the populations differ in physiology, behaviour and structure.
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Eventually they cannot interbreed to produce fertile offspring, and are separate species.
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Allopatric speciation is the most common type, e.g. a rising mountain range splitting a tree population.
Part 4 of 8Sympatric speciation
Sympatric speciation
- Sympatric speciation speciation with no geographical barrier, within the same area.
- Two populations must form within the group with no gene flow between them.
Ecological separation
- Ecological separation populations live in different environments in the same area, e.g. soil pH differences affecting plant flowering.
Behavioural separation
- Behavioural separation populations differ in feeding, communication or social behaviour.
Fish in a lake: bottom-feeders and open-water feeders are separated by feeding behaviour.
- Long jaws suit bottom feeding and short jaws suit mid-water feeding.
- They evolve different courtship displays and can no longer interbreed.
Polyploidy in plants can give instant speciation: a tetraploid cannot produce fertile offspring with diploid parents.
Disruptive selection can drive the split, e.g. a population exploiting two niches.
Part 5 of 8Isolating mechanisms
| Type | Acts | Examples |
|---|---|---|
| Prezygotic | Before fertilisation | Habitat isolation, different breeding seasons, different courtship, incompatible genitalia or gametes |
| Postzygotic | After fertilisation | Hybrid is non-viable or sterile, e.g. the mule with 63 chromosomes that cannot pair in meiosis |
Part 6 of 8DNA and evolutionary relationships
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DNA from the nucleus, mitochondria and chloroplasts can be sequenced to show evolutionary relationships.
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The more similar the base sequences, the more closely related the species and the more recently they separated.
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DNA comparison is used to build phylogenetic trees.
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DNA can be taken from blood or skin of living organisms and from fossils.
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Humans and chimpanzees share almost 99% of their DNA and humans and bonobos 98%.
Part 7 of 8Mitochondrial DNA and the molecular clock
Zygotes contain only maternal mtDNA, and mtDNA does not undergo crossing over, so its sequence changes only by mutation.
Molecular clock
- Molecular clock assumes a constant mutation rate, so more sequence differences means a more distant common ancestor.
- It is calibrated using fossils dated by carbon dating.
Mitochondrial Eve is thought to have lived in Africa about 200 000 years ago.
Comparison of sequences of amino acids is less sensitive than DNA because the genetic code is degenerate.
Part 8 of 8More from the specification
Theory
- A well-tested explanation supported by a large body of evidence.
Genome
- All the genetic information in an organism.
Key terms
| Term | Meaning |
|---|---|
| Species | Group of organisms that interbreed to produce fertile offspring and are reproductively isolated from other groups. |
| Gene pool | All alleles at all loci in an interbreeding population. |
| Speciation | The formation of new species. |
| Allopatric speciation | Speciation caused by geographical isolation. |
| Sympatric speciation | Speciation in the same area without a geographical barrier. |
| Molecular clock | Use of sequence differences, assuming a constant mutation rate, to date divergence. |
| Phylogenetic tree | Diagram showing evolutionary relationships between species. |
Avoid these mistakes
1
Saying a species is any group that can mate — offspring must be fertile and the group reproductively isolated.
2
Confusing the original cause of separation with the barrier to breeding — in sympatric speciation, feeding differences separate and courtship differences prevent breeding.
3
Forgetting that speciation depends on mutation — without new alleles there is nothing for selection to act on.
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Saying allopatric speciation needs no barrier — it needs a geographical barrier; sympatric does not.
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Assuming the molecular clock is exact — it assumes a constant mutation rate.
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