3.6Unit 3 — Life on Earth
Evolution of Species — SQA National 5 Biology Revision Notes & Topical Questions
5 min read 4 parts Lesson 28 of 31
Section 1 of 4Mutations 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:
Mutation Description 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 agentsEnvironmental factors that increase the spontaneous mutation rate:
- RadiationUltraviolet (UV) rays, X-rays, gamma radiation.
- ChemicalsMustard 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 4Natural selection and speciation
1Natural selection (Survival of the Fittest)
- 1. Overproduction & StruggleSpecies produce far more offspring than the environment can support;
- competition for finite resources ensues.
- 1. Overproduction & StruggleSpecies produce far more offspring than the environment can support;
- 2. Variation & SelectionNatural phenotypic variation exists within the population due to new mutations.
- Individuals possessing advantageous alleles best suited to prevailing environmental selection pressures survive.
- 3. Reproduction & TransmissionSurviving individuals reproduce, passing their advantageous alleles on to the next generation;
- over time, these alleles increase in frequency.
- 3. Reproduction & TransmissionSurviving individuals reproduce, passing their advantageous alleles on to the next generation;
2Speciation (Evolution of a new species)
- Stage 1: IsolationAn 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.
- Stage 2: Independent Mutation & SelectionDifferent random mutations occur independently in each isolated sub-population.
- Different selection pressures act on each group, selecting for different advantageous alleles.
- Stage 3: Speciation CompleteOver 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.
Section 3 of 4Key terms
| Term | Meaning |
|---|---|
| Mutation | A random, spontaneous change in an organism's genomic DNA sequence; the sole origin of novel alleles. |
| Natural selection | The evolutionary process where individuals with adaptive traits survive and reproduce more successfully. |
| Speciation | The evolutionary formation of two or more distinct species from a common ancestral population. |
| Isolating barrier | A physical, ecological, or behavioural obstacle preventing genetic exchange between sub-populations. |
Section 4 of 4Exam technique
Avoid these mistakes
Forgetting the word "fertile" when defining speciation — speciation is complete only when individuals cannot interbreed to produce fertile offspring.
Saying organisms "choose to mutate to survive" — mutations are completely random events; natural selection only acts on variations that already happen to exist.
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