5.3.8Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
Mutation, Natural Selection and Evolution — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 32 of 36 Unit quiz
Section 1 of 5Evolution and Inheritance
Evolution
- Evolution is a change in the heritable characteristics of a population over generations.
Heritable
- Only characteristics controlled by alleles can be passed on, so a damaged leaf or a gain in weight is not inherited.
Mutation
- Random changes in alleles produce new versions of genes, some of which are more or less advantageous.
Natural selection
- Better adapted individuals are more likely to survive and reproduce, so advantageous alleles become more frequent.
Section 2 of 5Sources of Variation
Sources of Variation
Mutation
- Changes in the base sequence of DNA create new alleles and are the original source of variation.
Meiosis
- Independent assortment and crossing over in meiosis produce gametes with different combinations of alleles.
Random fertilisation
- Any male gamete can fuse with any female gamete, so offspring have many possible combinations.
Importance
- Selection can act only when variation exists within a population.
Source of DNA differences
- All of these sources work by producing small differences in DNA base sequences between individuals in the same population.
Section 3 of 5Stages of Natural Selection
1
Variation exists in the population because individuals carry different alleles.
2
Environmental factors such as predation, disease and competition for food act as selection pressures.
3
Individuals with advantageous alleles are more likely to survive and reproduce, so they leave more offspring.
4
These individuals pass on the advantageous alleles to their offspring.
5
The frequency of the advantageous allele increases over many generations, and the frequency of unfavourable alleles falls.
Section 4 of 5Applying the Model
Any exam scenario can be answered by working through the same five stages in order, using the details given in the question.
Rabbit example
- Brown fur camouflages rabbits better against foxes than white fur, so more brown rabbits survive and breed.
Allele frequency
- Over generations the allele for brown fur becomes more common and the allele for white fur becomes rarer.
No purpose
- Organisms do not change in order to survive; random variation exists first and the environment then selects.
Wording
- Describe evolution as occurring by natural selection acting on random variation, not as a response the organism chooses.
Losing alleles
- Unfavourable alleles fall in frequency because their carriers leave fewer offspring,
- and they may eventually disappear from the population.
Section 5 of 5Exam Checklist & Tips
5.3.8 – Key Points for the Exam
- Evolution A change in the heritable characteristics of a population over successive generations.
- Natural selection The process by which better adapted organisms survive, reproduce and pass on advantageous alleles.
- Selection pressure An environmental factor, such as predation, that affects survival and reproduction.
- Allele frequency How common an allele is in the gene pool of a population.
Common Mistakes
- Writing that organisms evolve in order to survive; selection acts on variation that already exists.
- Saying individuals evolve; populations evolve, and individuals only survive or die.
- Treating acquired characteristics, such as an injury, as if they can be inherited.
- Leaving out one of the stages, especially the passing on of advantageous alleles to offspring.
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