Darwin, Wallace and Natural Selection

The theory of evolution by natural selection is one of the most important ideas in biology. It was developed independently by two naturalists: Charles Darwin and Alfred Russel Wallace. In this lesson we explore how the theory was developed, what it explains, and why it was controversial.

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Charles Darwin (1809–1882)

The Voyage of HMS Beagle

In 1831, Charles Darwin set sail on HMS Beagle as the ship's naturalist on a five-year survey voyage around the world. During the voyage, Darwin made careful observations of the natural world, particularly when the ship visited South America and the Galapagos Islands in the Pacific Ocean.

Key Observations on the Galapagos Islands

Darwin noticed that the finches on different islands of the Galapagos had different beak shapes:

Island Environment	Beak Shape	Food Source
Islands with large, hard seeds	Large, strong beak	Cracking seeds
Islands with small seeds	Small, pointed beak	Picking up tiny seeds
Islands with insects in bark	Long, thin beak	Probing bark for insects
Islands with cacti	Medium, curved beak	Feeding on cactus flowers

Darwin reasoned that all the finches had descended from a single ancestral species that had colonised the islands. Over time, the populations on each island had adapted to their local environment through natural selection. This is known as adaptive radiation.

He also observed that the giant tortoises on different islands had differently shaped shells, and that wildlife on the islands resembled — but was distinct from — species on the South American mainland.

Developing the Theory

After returning to England in 1836, Darwin spent over 20 years gathering evidence, conducting experiments (including on pigeons and barnacles), and refining his ideas. He was reluctant to publish because he knew the theory would be controversial.

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Alfred Russel Wallace (1823–1913)

Wallace's Contribution

Alfred Russel Wallace was a Welsh naturalist and explorer who spent years collecting specimens in South America and the Malay Archipelago (modern-day Malaysia and Indonesia). Independently of Darwin, Wallace developed a very similar theory of natural selection.

In 1858, Wallace wrote a paper outlining his ideas and sent it to Darwin, asking for his opinion. Darwin was shocked to find that Wallace had independently reached the same conclusions.

The Joint Presentation

Rather than one taking credit over the other, Darwin and Wallace's ideas were presented together at a meeting of the Linnean Society of London on 1 July 1858. This was arranged by the geologist Charles Lyell and the botanist Joseph Hooker.

The following year, in 1859, Darwin published his famous book On the Origin of Species, which laid out the theory in full detail with extensive evidence.

Wallace's Legacy

Wallace made additional important contributions to biology:

• He identified the Wallace Line — a boundary in the Malay Archipelago separating Asian and Australian animal species
• He developed ideas about warning colouration in animals (now called aposematism)
• He is considered the father of biogeography — the study of the distribution of species

Despite these achievements, Wallace's contribution to the theory of natural selection has often been underappreciated compared to Darwin's. This is partly because Darwin published the more comprehensive book and had a higher social standing in Victorian society.

Exam tip: Edexcel specifically mentions both Darwin AND Wallace. Make sure you can describe Wallace's role and explain that they developed the theory independently.

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The Theory of Natural Selection

The Key Steps

Natural selection can be summarised in a clear sequence:

1. Variation — Within a species, individuals show genetic variation in their characteristics (e.g., beak length, fur colour, running speed).
2. Competition — Organisms compete for limited resources such as food, water, territory and mates. There is also pressure from predators and disease.
3. Survival of the fittest — Individuals with characteristics best suited to the environment are more likely to survive. ("Fittest" means best adapted, NOT strongest.)
4. Reproduction — Those that survive are more likely to reproduce and pass on their genes.
5. Inheritance — The advantageous alleles are passed on to the next generation.
6. Accumulation — Over many generations, the frequency of the advantageous allele increases in the population, and the species gradually changes.

Exam tip: When writing about natural selection, always mention variation, a selection pressure, differential survival, and inheritance. These are the four elements examiners look for.

Important Clarifications

• Natural selection acts on phenotypes (observable characteristics), but it is the genotype (alleles) that is inherited.
• Natural selection does NOT have a "goal" — it does not plan ahead or drive organisms toward perfection.
• The variation must be genetic (inherited), not acquired during an organism's lifetime.

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Lamarck's Theory — An Alternative (Now Disproved)

Jean-Baptiste Lamarck (1744–1829)

Before Darwin, the French naturalist Lamarck proposed a different mechanism for evolution, known as the inheritance of acquired characteristics:

1. Organisms change during their lifetime in response to their environment.
2. These acquired changes are passed on to their offspring.

Lamarck's Famous Example

Lamarck proposed that giraffes originally had short necks. Individual giraffes stretched their necks to reach high leaves, and this stretching made their necks slightly longer. They then passed this longer neck to their offspring, and so over generations, giraffe necks became long.

Why Lamarck Was Wrong

Lamarck's Idea	Why It Is Incorrect
Organisms change during their lifetime and pass on changes	Changes acquired during an organism's lifetime do not alter its DNA and therefore cannot be inherited
Use of an organ causes it to develop; disuse causes it to shrink	Physical changes to the body (e.g., muscles from exercise) are not passed to offspring
Evolution is driven by an organism's "need"	Evolution has no direction or purpose; it results from random variation and selection pressures

Darwin's Explanation for Giraffes

Within the ancestral giraffe population, there was natural genetic variation in neck length. Giraffes with slightly longer necks could reach more food and were more likely to survive and reproduce. Over many generations, the alleles for longer necks became more common in the population.