Genetics, Evolution and Chemical Cycles Revision Notes | GCSE AQA Higher Biology

1. The Cycling Story - Why Nothing Gets Lost

Every carbon atom in your body has been part of countless other organisms before you. The same nitrogen that builds your proteins once helped a dinosaur grow. Nothing in nature gets wasted - chemicals constantly cycle through ecosystems in an endless loop of life, death, and renewal.

Picture the carbon cycle as nature's recycling system. Plants absorb carbon dioxide from the air during photosynthesis, locking carbon into glucose molecules. When animals eat plants, that carbon becomes part of their tissues. When organisms die and decay, or when they respire, carbon returns to the atmosphere as CO₂, ready to start the cycle again.

The nitrogen cycle works similarly but involves more steps. Nitrogen gas from the atmosphere gets 'fixed' by bacteria into compounds plants can use. These compounds pass through food chains, and when organisms die, decomposers break down proteins and return nitrogen compounds to the soil or atmosphere.

2. Your Genetic Blueprint - Genome vs Environment

You are not just your genes. Your characteristics result from a constant interaction between your genome (your genetic instructions) and your environment. Think of your genome as a recipe book - it contains all the instructions, but the final dish depends on the ingredients available and cooking conditions.

Consider height in humans. Your genes set the potential range - you might have genes for 'tall' or 'short'. But your final height also depends on nutrition during childhood, exercise, and health. Someone with 'tall' genes who lacks proper nutrition might end up shorter than someone with 'average' genes who eats well.

Trap: Many students think characteristics are either 'genetic' or 'environmental'. Actually, most traits result from both factors working together. Even seemingly simple traits like eye colour can be influenced by environmental factors during development.

3. Natural Selection - The Engine of Evolution

Evolution isn't random - it's driven by natural selection, a process so logical it seems obvious once you understand it. Here's how it works: organisms with helpful characteristics survive better and reproduce more. Their offspring inherit these useful traits. Over many generations, the helpful traits become more common in the population.

Worked example: The Four Steps of Natural Selection

Explain how antibiotic resistance evolves in bacteria

1. In any bacterial population, some individuals have random mutations that make them slightly resistant to antibiotics — Genetic variation is the raw material for natural selection
2. When antibiotics are used, most bacteria die, but the resistant ones survive — The antibiotic acts as the selection pressure
3. Resistant bacteria reproduce rapidly in the absence of competition from dead bacteria — Survivors have more offspring than non-survivors
4. The offspring inherit the resistance genes, so the next generation is more resistant — Advantageous traits become more common over time

Answer: The bacterial population evolves to become antibiotic-resistant through natural selection

This process explains both the incredible diversity of life and why all organisms share common features. Different environments select for different traits, leading to the amazing variety we see. But the underlying mechanisms - DNA, proteins, cellular respiration - remain similar because they work so well.

4. The Evidence Web - How We Know Evolution Happened

Evolution isn't just a theory - it's supported by evidence from multiple sources that all point to the same conclusion. Like detectives solving a case, scientists have gathered clues from fossils, DNA, anatomy, and direct observation.

Fossil evidence shows a clear progression from simple to complex life forms over geological time. DNA evidence reveals that organisms with similar DNA are more closely related - humans share 99% of their DNA with chimpanzees, but only 50% with bananas. Anatomical evidence shows that many different animals have similar bone structures, suggesting common ancestry.

We can even observe evolution happening today. Darwin's finches on the Galápagos Islands have been studied for decades. During droughts, birds with larger beaks survive better because they can crack tough seeds. The average beak size in the population increases within just a few generations.

5. Exam Mastery - Getting Full Marks on Evolution Questions

Evolution questions often ask you to explain how a characteristic evolved. Follow this structure: describe the variation, identify the selection pressure, explain why some variants survive better, and describe how the trait becomes more common over generations.

Common mistakes include saying organisms 'need' to evolve or that they evolve 'in order to' do something. Evolution has no purpose or direction - it's the result of natural selection acting on random variation. Organisms don't choose to evolve; those with helpful traits just happen to survive better.

When describing inheritance patterns, always distinguish between genotype and phenotype. Use genetic diagrams to show crosses, and remember that environmental factors can influence how genes are expressed. For characteristics influenced by multiple genes, expect continuous variation rather than distinct categories.