Use the pathway to build a clear mental model. Open each node and answer the check question.
Every population has variation - individuals are slightly different from each other. Some rabbits are faster runners, some birds have longer beaks, some moths are darker in colour. This variation is crucial because it provides the "raw material" for natural selection to work on.
Check: Why is variation important for natural selection?
In nature, organisms compete for limited resources - food, water, shelter, and mates. Not all individuals can survive and reproduce. There are always more offspring born than can survive to adulthood. This creates competition - a "struggle for survival."
Check: What do organisms compete for in their environment?
Individuals with helpful variations are more likely to survive and reproduce - this is called being "fit" or having "fitness." A faster rabbit escapes predators better; a bird with the right beak shape finds more food. These individuals are more likely to have offspring and pass on their helpful characteristics.
Check: What does "survival of the fittest" actually mean?
When individuals with helpful variations survive and reproduce, they pass these variations to their offspring. Over many generations, helpful variations become more common in the population. This is natural selection - nature "selecting" which characteristics become more common.
Check: How does natural selection change populations over time?
Natural selection is the process by which organisms with helpful variations are more likely to survive, reproduce, and pass those variations to their offspring. Over many generations, this causes populations to change and become better suited to their environment. It's the main mechanism driving evolution.
Natural selection needs four things to occur:
Imagine a population of mice living in a field. Most mice are brown, but a few are darker. When a hawk hunts, it spots the lighter brown mice more easily against dark soil. The darker mice survive better and have more offspring. Those offspring inherit the dark colour. Over many generations, darker mice become more common. The population has changed through natural selection!
This famous phrase doesn't mean the strongest or biggest. "Fitness" in biology means how well suited an organism is to its environment. A "fit" organism is one that survives well and produces many offspring. In the mouse example, darker mice are "fitter" because they survive and reproduce better - not because they're stronger!
Before the Industrial Revolution, most peppered moths in Britain were pale with dark speckles - great camouflage on lichen-covered tree bark. During industrialisation, soot blackened the trees. Now dark moths survived better (birds couldn't see them), while pale moths were eaten. Dark moths became common through natural selection. After the Clean Air Act reduced pollution, trees became lighter again - and pale moths are now more common once more!
Natural selection happens today with bacteria. When you take antibiotics, most bacteria are killed. But a few might have a variation making them resistant. These survive, reproduce, and their offspring are also resistant. This is why doctors say to finish your antibiotic course - to kill ALL bacteria and prevent resistant ones from taking over. Antibiotic resistance is natural selection in action!
In the 1950s, the disease myxomatosis was introduced to control rabbit populations in Britain. At first, it killed 99% of rabbits. But some rabbits had variations making them slightly more resistant. These survived and reproduced. Today, British rabbits are much more resistant to myxomatosis - natural selection has changed the population over just a few decades!
On the Galápagos Islands, Charles Darwin observed finches with different beak shapes. On islands with large seeds, finches with larger, stronger beaks survived better - they could crack the seeds. On islands with small seeds or insects, smaller or thinner beaks were more useful. Over generations, natural selection produced different beak shapes for different food sources. This was key evidence for Darwin's theory!
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Wrong thinking: "Giraffes stretched their necks to reach leaves, so their necks got longer." Correct: Organisms don't choose to change. Giraffes with slightly longer necks (by chance variation) survived better and had more offspring. Over millions of years, longer necks became more common. The change happened to the population over generations, not to individuals during their lifetime.
Wrong thinking: "Survival of the fittest means the strongest animals survive." Correct: "Fitness" means how well suited an organism is to its environment. Sometimes being small, slow, or camouflaged is "fitter" than being big and strong. A camouflaged moth is "fitter" than a bright one if it avoids being eaten!
Wrong thinking: "Natural selection is just random chance." Correct: While the variations that appear ARE random, natural selection itself is NOT random. The environment consistently favours certain variations over others. A dark moth is always more likely to survive on a dark tree than a pale moth - that's not random!
Wrong thinking: "If the environment changes, animals will quickly evolve new adaptations." Correct: Natural selection usually takes many generations (often thousands of years). This is why rapid environmental changes (like climate change) are dangerous - species may not have time to adapt. Bacteria can evolve quickly because they reproduce so fast!