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3.6Unit 3 — Life on Earth

Evolution of Species — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 2 parts Lesson 6 of 6

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Section 1 of 2Mutations and adaptations

  • Mutations:
    • A mutation is a random, spontaneous change in the structure or quantity of an organism's genetic material (DNA).
    • Mutations are the only source of completely new alleles in an interbreeding population.
    • Types of mutations:
      • Neutral: Have no significant effect on survival (e.g. tongue rolling).
      • Disadvantageous: Decrease survival chances (e.g. cystic fibrosis, albinism in prey animals).
      • Advantageous: Impart a survival benefit in a specific environment (e.g. antibiotic resistance in bacteria).
    • Mutagenic agents: Environmental factors that increase the spontaneous mutation rate:
      • Radiation: Ultraviolet (UV) rays, X-rays, gamma radiation.
      • Chemicals: Mustard gas, polycyclic hydrocarbons in cigarette tar.
  • Adaptation: An inherited structural, physiological, or behavioural characteristic that enhances an organism's survival and reproductive success in its natural environment.

Section 2 of 2Natural selection and speciation

1Natural selection (Survival of the Fittest)3 points
:
  1. 1. Overproduction & Struggle — Species produce far more offspring than the environment can support; competition for finite resources ensues.
  2. 2. Variation & Selection — Natural phenotypic variation exists within the population due to new mutations. Individuals possessing advantageous alleles best suited to prevailing environmental selection pressures survive.
  3. 3. Reproduction & Transmission — Surviving individuals reproduce, passing their advantageous alleles on to the next generation; over time, these alleles increase in frequency.
2Speciation (Evolution of a new species)6 points
:
  1. Stage 1: Isolation — An initial population is split into two or more sub-populations by an isolating barrier preventing gene flow:
    • GeographicalMountains, rivers, oceans, deserts.
    • EcologicalDifferences in pH, soil moisture, temperature, breeding sites.
    • BehaviouralDistinct courtship displays, mating rituals, nocturnal habits.
  2. Stage 2: Independent Mutation & Selection — Different random mutations occur independently in each isolated sub-population. Different selection pressures act on each group, selecting for different advantageous alleles.
  3. Stage 3: Speciation Complete — Over many generations, the gene pools diverge so substantially that even if the barrier is removed, individuals from the two groups can no longer interbreed to produce fertile offspring.
    • They are now officially two distinct separate species.

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