Variation and selection explains why individuals in a species differ, where those differences come from, and how the environment makes some forms more common over generations. In IGCSE Biology it follows cell division and inheritance and leads into ecology and energy flow.
This page gives you a route through the five lessons, one orienting example and the traps that cost marks. All organisms and numbers in the lessons are invented for practice.
What should you know before starting?
- Genes and alleles. Alleles are different forms of a gene.
- Gametes and fertilisation. Gametes carry half the chromosomes.
- Reading a table and finding a percentage. For example, 12 out of 40 is 30%.
- Mean, range and mode from a short list of values.
If alleles still feel unclear, the cell division and inheritance module covers them first.
One orienting example
Question: In a fictional beetle population, some beetles are green and some are brown. Birds eat more green beetles on brown soil. Explain how the proportion of brown beetles changes.
The beetles vary in colour because they carry different alleles. Birds find green beetles more easily on brown soil, so more brown beetles survive. The brown beetles reproduce and pass on the allele for brown colour, so the proportion of brown beetles rises in the next generation.
Notice that the answer names variation, a selection pressure, survival, reproduction and alleles. The beetles never “need” or “try”. The five lessons train each step separately.
In what order should you study the lessons?
- Distinguish continuous and discontinuous variation: a classification skill with a data table and a graph choice.
- Explain a source of genetic variation: mutation, meiosis and random fertilisation, with a counting model.
- Trace natural selection across generations: the full chain, with survival numbers.
- Compare selection with an organism choosing to adapt: the most common wrong explanation and how to replace it.
- Interpret a distribution without biological stereotypes: what group averages do and do not tell you.
Finish with the mixed practice set. The inheritance model board and the probability tree and counting board let you test simple models with invented alleles.
What traps catch students?
- Calling any numerical data continuous. Counts in whole steps are discontinuous.
- Saying sexual reproduction makes new alleles. Only mutation does.
- Ending a selection answer at survival. Add reproduction and passing on alleles.
- Writing that organisms needed or wanted to change. Use variation and selection instead.
- Treating a group mean as a fact about each individual. Check the overlap.
How should you use the practice set?
Attempt each question on paper before opening the answer. Mark yourself on the working, not only the final number. Then use the routing section at the end to choose which lesson to reread.
Some students can calculate survival percentages correctly but lose marks on the explanation that follows. A teacher in online one-to-one Biology tuition can look at that gap with you, and the full list of modules sits in the Biology learning guide.