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4.9.1Additional lessons

Natural Selection and Evolution — IB Biology SL Revision Notes & Topical Questions

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Part 1 of 5Evolution and Darwin's Argument

4.9.1 Natural Selection and Evolution

Natural selection is the main mechanism proposed for adaptive evolution.

1
Evolution is the cumulative change in the heritable characteristics of a population over successive generations.
2
Organisms of a species vary, and some of the variation is heritable because it is determined by alleles.
3
Populations produce more offspring than the environment can support, so individuals compete for limited resources.
4
Individuals with characteristics better suited to the environment are more likely to survive and reproduce.
5
They pass on advantageous alleles in greater numbers, so over generations the frequency of those alleles rises.

Part 2 of 5Sources of Variation

Mutation
  • Random changes in DNA create new alleles, and are the original source of all genetic variation.
Crossing over
  • Exchange of segments between non-sister chromatids in meiosis I creates new combinations of alleles on chromosomes.
Random orientation
  • Homologous pairs line up independently at metaphase I, so gametes receive different mixtures of maternal and paternal chromosomes.
Random fertilisation
  • Any sperm may fuse with any egg, so offspring genotypes are very diverse.
SourceStageEffect
MutationDNA replicationNew alleles
Crossing overProphase INew allele combinations
Random orientationMetaphase INew chromosome combinations
Random fertilisationFertilisationNew genotypes

Part 3 of 5Example: Rabbit Fur

Example: Rabbit Fur
Variation
  • In a rabbit population one allele codes for brown fur and another for white fur.
Selection pressure
  • Predators hunt in a snowless woodland, and white rabbits are easier to see against dark ground.
Differential survival
  • More brown rabbits survive to breed than white rabbits, producing more offspring.
Result
  • The frequency of the brown allele rises across generations, and the white allele becomes less common.

Part 4 of 5Darwin and Lamarck

Lamarckism
  • It claimed that characteristics acquired during life, such as stretched necks, were passed on to offspring.
Why rejected
  • Acquired characteristics do not change DNA in gametes, so they are not heritable.
Darwin's theory
  • Natural selection explained adaptation by differential survival of heritable variation, with strong supporting evidence.
Paradigm shift
  • Replacing Lamarck's ideas with natural selection changed the whole framework used by biologists,
    • which is why it counts as a paradigm shift.

Part 5 of 5Exam Checklist & Tips

4.9.1 – Key Points for the Exam

  • Evolution Cumulative change in the heritable characteristics of a population.
  • Natural selection Differential survival and reproduction of individuals with heritable advantageous traits.
  • Heritable Able to be passed to offspring through genes.
  • Mutation A change in the base sequence of DNA.
  • Paradigm shift A fundamental change in the accepted ideas of a scientific field.

Common Mistakes

  • Writing that individuals evolve; populations evolve over generations.
  • Implying that organisms change on purpose to survive.
  • Saying well-adapted organisms are certain to survive; they are more likely to.
  • Including acquired traits such as scars as examples of heritable variation.

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