Natural selection — Science, 11–13
Natural selection is a process in which inherited differences affect which organisms survive and reproduce. Across many generations, helpful traits can become more common in a population; individuals do not change because they simply need to.
How natural selection works
A population contains differences, and some are inherited. If the environment makes one inherited difference helpful, organisms with it may survive longer or leave more offspring. Their offspring may inherit the difference, so it becomes more common over generations. The population changes; an individual does not transform during its lifetime.
What problem it explains
Nature has many different organisms, and each species can change over time, but species do not need a designer to adjust every generation. Darwin and Wallace sought an explanation for adaptation using observations of variation, competition and reproduction. Natural selection links these facts: individuals vary, resources are limited, and inherited advantages can spread.
Moths on dark trees
Imagine 100 moths: 70 light and 30 dark, with colour partly inherited. Step 1: soot darkens tree trunks, making light moths easier for birds to see. Step 2: birds eat more light moths, so fewer survive to reproduce. Step 3: dark moths leave more offspring. Step 4: after many generations, the dark form may become common. The birds did not create the colour; they changed which form reproduced more.
Need does not create a trait
A tempting explanation says giraffes stretched their necks to reach leaves, then passed the longer neck to their young. It feels logical because effort can strengthen a person's muscles. Natural selection is different: inherited variation already exists, and individuals with useful differences leave more offspring. Their descendants make the difference more common over time.
Natural selection today
Natural selection helps explain antibiotic resistance. A few bacteria may already have a heritable change that makes a medicine less effective; the medicine kills more sensitive bacteria, leaving resistant ones to multiply. The medicine did not train each bacterium to resist it. This is why doctors use antibiotics carefully and why completing prescribed courses matters.
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