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3.6.2Unit 3 — Life on Earth

Natural Selection — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 4 parts Lesson 30 of 31

📚 SQA National 5 Biology Hub / Topical Revision
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Part 1 of 5Key Ideas

3.6.2 Natural Selection
Adaptation
  • An adaptation is an inherited characteristic that makes an organism well suited to survive in its environment or niche.
Natural selection
  • This is the process by which species change and become better adapted over many generations.
Selection pressure
  • A factor such as predation, disease, competition or climate that affects which individuals survive.
Fitness
  • The better adapted an organism, the more likely it is to survive and reproduce.

Part 2 of 5The Stages of Natural Selection

1
More offspring are produced than the environment can support.
2
There is variation in the population, and some individuals have advantageous alleles while others do not.
3
Selection pressures act on the individuals in the population.
4
The best adapted individuals are more likely to survive and reproduce, which is survival of the fittest.
5
Advantageous alleles are passed to offspring, so their frequency in the population increases.

Part 3 of 5Example: Rabbit Fur Colour

Example: Rabbit Fur Colour
Variation
  • One allele codes for brown fur and another for white fur, so the rabbit population includes both colours.
Selection pressure
  • Foxes hunt the rabbits, and rabbits that are easier to see are more likely to be caught.
Survival
  • Brown rabbits have better camouflage in this habitat, so more of them survive and breed.
Result
  • The allele for brown fur is passed on more often, so its frequency rises in the population.

Part 4 of 5Answering Exam Questions

Use the same stages
  • Apply the steps in order to whatever organism is given in the question.
Mention alleles
  • Say that the advantageous allele is passed on and increases in frequency, not that an individual changes.
Name the pressure
  • Identify the selection pressure in the question, such as predators, disease or food supply.
Over time
  • Natural selection acts on a population over many generations, not on an individual in its lifetime.

Part 5 of 5Exam Checklist & Tips

3.6.2 – Key Points for the Exam

  • Natural selection The process in which better adapted individuals survive and pass on advantageous alleles.
  • Adaptation An inherited feature that helps an organism survive in its environment.
  • Selection pressure A factor that affects the survival of individuals in a population.
  • Fitness How well an organism survives and reproduces in its environment.
  • Allele frequency How common an allele is within a population.

Common Mistakes

  • Writing that individuals change to suit the environment;
    • the change happens across a population over generations.
  • Leaving out variation, which natural selection depends on.
  • Saying brown rabbits want to survive; selection acts without any intention.
  • Forgetting that advantageous alleles are passed on to offspring and increase in frequency.

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