3.2.19Reproduction & Inheritance
Darwin’s Theory of Evolution by Natural Selection — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions
5 min read 4 parts Lesson 4 of 4 Unit quiz
Part 1 of 4The Theory
Evolution is the change in the inherited characteristics of a population over many generations.
Variation
- Individuals in a species differ from each other, and many of these differences are caused by differences in their genes.
Competition
- More offspring are produced than the environment can support, so individuals compete for food, space and mates.
Survival
- Those with characteristics that suit the environment are more likely to survive;
- this is sometimes called survival of the fittest.
Inheritance
- Survivors reproduce and pass on the alleles for their useful characteristics to their offspring.
Change over time
- Over many generations the advantageous alleles become more common, so the population changes.
Part 2 of 4Writing an Explanation
Use the same sequence in any exam question, whatever the species.
1
State that the population shows genetic variation, which originally arose from mutation.
2
Say that individuals with the advantageous characteristic are better adapted and have a higher chance of surviving.
3
Say that these individuals reproduce more successfully than others.
4
State that they pass the advantageous allele to their offspring.
5
Conclude that over many generations the frequency of that allele increases in the population.
Part 3 of 4Worked Examples
Worked Examples
Peppered moth
- Dark moths were better camouflaged on soot-darkened trees, so more of them survived bird predation and their allele became common.
Snail shells
- Shell colours that blend with the background are eaten less by predators, so they become more frequent in that habitat.
Antibiotic resistance
- A mutation in a bacterium may allow it to survive an antibiotic, and its offspring then form a resistant population.
Not by need
- Individuals do not change because they need to; the useful variation must already exist in the population.
Part 4 of 4Exam Checklist & Tips
3.2.19 – Key Points for the Exam
- Natural selection The process by which better-adapted individuals survive and pass on their alleles.
- Evolution The gradual change in the inherited features of a population over many generations.
- Variation Differences between individuals of the same species.
- Adaptation A feature that helps an organism survive in its environment.
- Allele A different version of the same gene.
Common Mistakes
- Saying individuals evolve; populations evolve over generations, individuals do not.
- Writing that organisms change because they want or need to;
- variation already exists and the environment selects.
- Forgetting to say the advantageous characteristic is inherited, so the allele is passed on.
- Using the phrase survival of the fittest to mean strongest, when it really means best suited to the environment.
Last updated:
\n