Evolution and Biological Diversity
What you must understand
Understand evolution as change in allele frequencies in populations over time. Know Darwin's mechanism: variation, inheritance, differential survival and reproduction (natural selection). Distinguish natural vs artificial selection and know the sources of variation (mutation, recombination, gene flow). Understand adaptation, speciation (isolation leading to reproductive barriers), and the difference between homologous structures (same origin, different function) and analogous structures (different origin, same function). Know the evidence for evolution: fossils, comparative anatomy, embryology, molecular (DNA/protein) similarities. Learn the basis of classification (taxonomy: domain, kingdom, phylum, class, order, family, genus, species; binomial nomenclature) and the concept of biodiversity at genetic, species and ecosystem levels.
Traps candidates fall into
- Saying evolution "tries" to improve organisms - it is not goal-directed; selection acts on existing variation.
- Confusing homologous (same ancestry) with analogous (same function, different ancestry) - e.g. bird wing and insect wing are analogous.
- Thinking individuals evolve - populations evolve, not individuals.
- Mixing "survival of the fittest" with "strongest" - fittest means best reproductively successful in that environment.
- Getting the taxonomic hierarchy order wrong or forgetting binomial naming (Genus species, italicised).
Worked example / rule of thumb
Bacteria exposed to an antibiotic: some already carry resistance mutations, survive and reproduce, so the population becomes resistant. This is natural selection, not the antibiotic "creating" resistance. Rule of thumb: variation exists first → selection filters it → population changes. For structures: same origin = homologous; same job = analogous.
Laboratory Work and Practical Skills
What you must understand
Know how to use a light microscope: carry it correctly, start on low power, focus with the coarse knob then fine knob, calculate magnification (eyepiece × objective). Understand preparing a slide (wet mount, coverslip lowered to avoid bubbles, staining). Learn safety rules (goggles, no eating, handling chemicals and glassware, disposal) and how to read lab equipment (measuring cylinder, balance, thermometer, pipette). Know the basics of experimental design: variables (independent, dependent, controlled), a control group, repeat trials, and recording data in tables/graphs. Understand simple field and lab techniques: sampling, using a quadrat, simple tests (e.g. starch with iodine, protein with biuret), and the importance of accuracy and repeatability.
Traps candidates fall into
- Calculating magnification as eyepiece + objective - it is multiplied.
- Forgetting to lower the coverslip at an angle, causing air bubbles that look like cells.
- Confusing independent (what you change) with dependent (what you measure) variables.
- Omitting a control and then being unable to attribute results to the variable.
- Reading a measuring cylinder at the wrong level - read at the bottom of the meniscus at eye level.
Worked example / rule of thumb
An eyepiece of 10× with a 40× objective gives total magnification 400×. If a cell appears 20 mm across at 400×, its real size is 20 mm ÷ 400 = 0.05 mm (50 µm). Rule of thumb: total magnification = eyepiece × objective; real size = measured size ÷ magnification. For experiments: change one variable, measure one outcome, keep everything else constant, repeat.