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17-18 · Selection, Evolution, Classification and Conservation

Cambridge A Level Biology — Full F-drive source: 17-18 · Selection, Evolution, Classification and Conservation — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 32 of 36 Unit quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

Cambridge A Level Biology — Chapters 17-18 Overview: Selection, Evolution, Classification and Conservation — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Summarise how natural selection, speciation, classification and conservation connect in Cambridge A Level Biolo…

  • Allopatric speciation
  • Sympatric speciation
  • Biochemical evidence
  • Observed examples
  • Stabilising selection
  • Directional selection

Part 1 of 7Variation and natural selection

Individuals differ because of genetic variation (mutation, meiosis, random fertilisation) and the environment.
Populations produce more offspring than the environment supports, so individuals compete for limited resources.
Individuals with advantageous alleles (greater fitness) are more likely to survive and reproduce.
They pass the advantageous alleles on, so allele frequencies change over generations.
Fitness
  • Fitness the ability to survive and reproduce; adaptations may be anatomical, physiological or behavioural.

Part 2 of 7Types of selection

TypeEffect
StabilisingKeeps the mean and reduces variation, e.g. human birth weight
DirectionalMoves the mean in one direction when the environment changes, e.g. antibiotic resistance
DisruptiveFavours both extremes and maintains two phenotypes, e.g. beak size

Part 3 of 7Speciation

Species
  • Species a group that can interbreed to produce fertile offspring.
Allopatric speciation
  • Allopatric speciation populations separated by a geographical barrier no longer share a gene pool and diverge.
Sympatric speciation
  • Sympatric speciation within the same area, e.g. polyploidy in plants or behavioural isolation.
Reproductive isolation involves different breeding season, courtship behaviour or gamete incompatibility.

Part 4 of 7Evidence for evolution

Fossil record and comparative anatomy (homologous structures).
Biochemical evidence
  • Biochemical evidence similar DNA and protein sequences in related species.
Observed examples
  • Observed examples antibiotic resistance in bacteria and pesticide resistance in insects.
Greater similarity in DNA or proteins suggests a more recent common ancestor.

Part 5 of 7Describing selection in answers

1
Variation exists, due to mutation.
2
A selection pressure acts (predation, disease, competition, climate).
3
Individuals with the advantageous allele survive and reproduce more.
4
The allele is passed on.
5
Allele frequency increases over generations.
6
Use alleles rather than genes, and say populations evolve, not individuals.

Part 6 of 7Classification and the three domains

1
Evolution produces a branching pattern, so organisms are placed in a hierarchy: domain, kingdom, phylum, class, order, family, genus, species.
2
Organisms are named with the binomial system (genus and species).
3
Closely related species share recent ancestors and have more similar DNA and protein sequences.
4
Domains: Archaea, Bacteria and Eukarya.

Part 7 of 7Biodiversity and conservation

Biodiversity and conservation
Biodiversity is conserved in situ (nature reserves and protected areas) or ex situ (zoos, captive breeding, seed banks).
Agreements such as CITES control trade in endangered species.
Conservation aims to maintain the genetic diversity on which future natural selection depends.

Key terms

TermMeaning
FitnessAbility to survive and reproduce successfully.
Stabilising selectionSelection favouring the intermediate phenotype.
Directional selectionSelection favouring one extreme phenotype.
Disruptive selectionSelection favouring both extremes.
Allopatric speciationSpeciation after geographical isolation.
Sympatric speciationSpeciation within the same area.
In situ conservationProtecting species in their natural habitat.
Ex situ conservationProtecting species outside their natural habitat.

Avoid these mistakes

1
Saying individuals evolve — populations evolve over generations.
2
Writing "genes" for "alleles" when describing selection.
3
Saying disruptive selection favours the mean — it favours both extremes.
4
Forgetting that conservation protects the genetic diversity needed for adaptation.

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