EVOLUTION is the change in the inherited characteristics of a population over many generations.
It is driven by natural selection — the process by which individuals with characteristics better suited to their environment are more likely to survive and reproduce.
Evolution explains:
Why species change over time.
Why there are so many different species on Earth.
Why species share features with each other (common ancestry).
Why organisms are so well adapted to their environments.
Evolution is the central organising principle of modern biology — it is one of the most well-supported theories in all of science, supported by evidence from fossils, genetics, anatomy and direct observation.
Darwin's Theory of Natural Selection
Charles DARWIN (1809–1882) developed the theory of natural selection after extensive observation of wildlife, particularly during his voyage on HMS Beagle (1831–1836).
Darwin's theory in five steps:
1. VARIATION — individuals within a population show differences in their characteristics.
2. OVERPRODUCTION — populations produce more offspring than the environment can support.
3. STRUGGLE FOR SURVIVAL — competition for resources (food, water, mates, territory). Not all offspring survive.
4. SURVIVAL OF THE FITTEST — individuals with characteristics best adapted to the environment are more likely to SURVIVE and REPRODUCE.
5. INHERITANCE — surviving individuals pass on their ALLELES to offspring. The next generation has more individuals with the advantageous characteristics.
Over many generations → advantageous alleles become MORE COMMON in the population → the population changes → EVOLUTION has occurred.
Why Darwin's Theory Took Time to be Accepted
Darwin published his theory in 'On the Origin of Species' in 1859.
It was NOT immediately accepted — reasons include:
Religious opposition — contradicted the biblical account of creation and the idea that species were fixed and created by God.
Lack of a mechanism — Darwin did not know HOW inheritance worked (DNA and genetics were not yet understood). He could not fully explain HOW traits were passed on.
Insufficient evidence at the time — the fossil record had many gaps.
Difficulty of the concept — evolution over millions of years is hard to observe directly.
Over time, as genetics was discovered (first Mendel, then Watson and Crick with DNA structure), and as more fossil evidence accumulated, Darwin's theory gained overwhelming scientific support.
Today, evolution by natural selection is the scientific consensus — one of the most strongly evidenced theories in science.
Alfred Russel Wallace
ALFRED RUSSEL WALLACE (1823–1913) independently developed a theory of evolution by natural selection at approximately the same time as Darwin.
Wallace sent his ideas to Darwin in 1858 — this prompted Darwin to publish his own work.
Both men's papers were presented to the Linnean Society of London in 1858.
Wallace made important contributions to biogeography — the study of how species are distributed across the Earth — and the patterns of species distribution supported the theory of evolution.
His work in Southeast Asia led to him identifying what is now called 'Wallace's line' — a boundary between Asian and Australian species distributions.
⚠️ Common Mistake
Natural selection does NOT cause mutations — mutations happen randomly. Natural selection SELECTS from variation that already exists. Also: individuals do NOT evolve — POPULATIONS evolve over many generations. And: 'survival of the fittest' does not mean 'strongest' — it means best adapted to the environment.
📌 Key Note
Evolution = change in inherited characteristics over generations. Darwin's steps: variation → overproduction → struggle for survival → survival of the fittest → inheritance. Natural selection acts on existing variation — it does not cause mutations.
🎯 Matching Activity — Match the Step of Natural Selection
Match each step of natural selection to its correct description. — drag the symbols on the right to match the component names on the left.
Variation
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Overproduction
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Struggle for survival
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Survival of the fittest
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Inheritance
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Individuals best adapted to the environment are more likely to survive and reproduce
More offspring are produced than the environment can support
Individuals in a population show differences in their characteristics
Surviving individuals pass on advantageous alleles to offspring
Competition for resources — not all offspring survive
🎯 Test Yourself
Question 1 of 3
1. A population of mice lives on dark soil. Over many generations, the population becomes predominantly dark-coloured. What explains this?
2. Why was Darwin's theory of natural selection initially controversial?
3. What does 'survival of the fittest' mean in evolution?
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