Written for this site from the class slides, with textbook page references for each statement. The chapter is on pages 779–789 of the course textbook. Image credits are on slide 15.
Natural selection operates all the time and has done so for billions of years, producing the biodiversity of life on Earth. Individuals vary, more are born than can survive, and those better adapted leave more offspring.
Mutation makes new alleles, and sexual reproduction makes new combinations of alleles, through crossing over, random orientation and random fertilization.
More offspring are produced than resources can support, so members of a species compete. Food, water, space and nesting sites can limit a population's carrying capacity.
Density-independent factors, such as high or low temperatures, reduce survival at any population density. Individuals with heritable traits suited to the conditions are more likely to survive.
This is the basis of selection. Fitness is how likely a genotype is to survive and leave offspring, and it depends on the environment. Competition within a species favours those with higher fitness.
Characteristics acquired in life because of the environment are not encoded in base sequences, so they are not inherited and cannot evolve.
Physical and behavioural differences can affect success in attracting a mate, and they can signal fitness. Birds-of-paradise are well-known examples, in which males have exaggerated plumage and displays.
Bright spots attract females, as well as predators. Endler kept guppies under different levels of predation. Spots increased without predators, through sexual selection, and fell with dangerous predators, through natural selection. Selection pressures in the experiment were controlled.
Select a video to play it here. Select it again to close it.
Print this guide and complete it as you work through the slides and videos. It asks for your own explanations, so close the notes while you try each section.
The self-check needs JavaScript to run.
A printable worksheet covering all eight statements, from quick recall to data questions and a longer answer. It has 36 marks and takes about 35 minutes, or you can do one section at a time.
Your teacher has the mark scheme.
Questions built on realistic experimental data, in the style of Paper 1B and Paper 2. Work them on paper and show your working. 14 marks in total. Your teacher has the mark scheme.
Male guppies (Poecilia reticulata) in Trinidad vary in colour and in behaviour. Males were sorted as bright or dull, and as bold (approaching a predator fish) or timid (swimming away). Forty females were each allowed to choose between males, with and without a predator fish visible in a nearby tank.
| Males chosen | No predator | Predator present |
|---|---|---|
| Bright and bold | 14 | 15 |
| Bright and timid | 13 | 5 |
| Dull and bold | 7 | 14 |
| Dull and timid | 6 | 6 |
Marine iguanas feed on algae on rocky shores of the Galapagos Islands. Males defend territories where females come to mate. The body length (snout to vent) of adults was measured on two islands. On Genovesa there is much less algae than on Santa Fe. Error bars show ± 1 SD.
You will need: Forceps, spoons and clothes pegs as beaks, seeds or beads of different sizes, cups, timer
Safety: Keep beads and seeds off the floor to avoid slips.
Carry this out with your teacher's supervision and follow your school's lab rules.
Peter and Rosemary Grant measured Galapagos finches for decades, and directly observed beak size changing after droughts.
Evolution usually acts over long periods. Consider how far short-term observations can support claims about deep time.
Summarised in our own words. Use each code to find the exact wording in your textbook or the IB subject guide.
© 2026 Akash Parthi Bendran. Please ask before using or sharing any resources from this site: see permission to use.