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Genetics · Chapter 4: Population genetics and natural selection

Year 11 Science · About 20 minutes with activities · Turn your sound on

Chapter 4 · Population genetics
Kākāpō1995: only 51 birds left
Low genetic diversityafter a population bottleneck
Gene poolall the alleles in the population
B: 12 ÷ 20 = 60%b: 8 ÷ 20 = 40%
Population A
Population B
Immigrationa new allele arrives
Emigrationalleles may be lost
Competition for food or lightbiotic
Diseasebiotic
Predationbiotic
Droughtabiotic
High windabiotic
Black
Pepperedwhite with black speckles
Industrial Revolutionsoot-covered bark
After the Clean Air Actclean bark
Variation must already existbeneficial phenotypes don’t arise out of need
Small population + little variation + strong selection pressure→ risk of extinction
Small population
Large population
Fixed at 100% by chance
Original population
New, isolated populationsmaller gene pool
Different allele frequenciesnot representative of the original
Beforelarge, varied gene pool
Bottleneck evente.g. habitat loss, hunting, disease
Afterfewer alleles, even if numbers recover
Genetic drift → less genetic diversity → lower future survival
Less variation for natural selection to work on
Smaller populations → even more genetic drift
More inbreeding → reduced biological fitness

Chapter 4 recap

  • Gene pool: all the alleles in a population. Allele frequency: the percentage of each allele. Gene flow: alleles moving between populations by migration (immigration may add alleles, emigration may remove them).
  • Natural selection: variation → survival advantage → survive to reproduce and pass on alleles → favourable allele frequency increases. Selection pressures (biotic or abiotic) decide which phenotype is favourable.
  • Genetic drift: random change in allele frequencies, strongest in small populations. The founder and bottleneck effects shrink the gene pool, reducing diversity and future survival.
  • Next: Chapter 5 · Inheritance and Punnett squares

What is the kākāpō’s hidden problem?

Chapter 4 · About 20 minutes · sound on

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