Charles Darwin proposed that species change over time through natural selection. Set a pollution level and watch a simulated peppered moth population shift over generations.
During his voyage on HMS Beagle, Charles Darwin observed that species on the Galápagos Islands (notably the finches) showed remarkable variation between islands — different beak shapes suited to different food sources (e.g. cracking seeds vs. catching insects vs. feeding on cacti). He also noted that individuals within a species are never identical — there is always natural variation in a population.
Darwin (and independently, Alfred Russel Wallace) proposed natural selection as the mechanism driving evolutionary change. The theory rests on several key observations and inferences:
Before the Industrial Revolution in Britain, most peppered moths were light-coloured with dark speckles, camouflaging them against lichen-covered tree bark. A rare dark (melanic) form also existed. As industrial pollution killed lichen and blackened tree trunks with soot, the light form became easier for birds to spot and eat, while the dark form was now better camouflaged.
Over subsequent generations, the frequency of the dark form increased dramatically in polluted industrial areas — a phenomenon called industrial melanism. When pollution was later reduced, the light form again became more common. This is a well-documented, real-world example of natural selection acting on an existing variation (moth colour) within a population.
Before Darwin, Jean-Baptiste Lamarck proposed the inheritance of acquired characteristics — the idea that traits an organism develops or strengthens during its lifetime (through use) can be passed directly to its offspring (e.g. a giraffe stretching its neck to reach leaves, then passing on a longer neck to its young). This theory has since been shown to be incorrect — acquired characteristics are not encoded in an organism's genes and so cannot be inherited.
| Darwin — Natural Selection | Lamarck — Acquired Characteristics | |
|---|---|---|
| Source of variation | Pre-existing heritable variation in the population | Traits acquired/changed during an individual's lifetime |
| Mechanism | Differential survival and reproduction of already-varying individuals | Direct transmission of a lifetime's changes to offspring |
| Scientific status | Supported by extensive evidence; accepted theory | Not supported — acquired traits are not inherited |
Evolutionary biologists have debated the pace at which evolutionary change occurs, giving two models:
| Gradualism | Punctuated equilibrium | |
|---|---|---|
| Pattern of change | Slow, steady, continuous accumulation of small changes over long periods | Long periods of little or no change (stasis), interrupted by relatively short, rapid bursts of change |
| When rapid change occurs | N/A — change is assumed constant | Typically associated with a speciation event, environmental upheaval, or a small isolated population under strong selection pressure |
| Fossil record prediction | Expect a smooth, gradual series of intermediate fossils | Expect long unchanged fossil sequences with sudden gaps where new forms appear abruptly |
Set the pollution level, then press Play. Birds preferentially eat whichever moth colour stands out most against the tree bark, shifting the population's colour ratio over generations.
| Year | 1850 | 1900 | 1950 | 2000 |
|---|---|---|---|---|
| Dark (melanic) moths (%) | 5% | 95% | 90% | 15% |