Life changes over time
Earth’s living things haven’t always looked the way they do today. Evolution is the change in inherited traits of a population over many generations. It explains both how living things are related and why they are so well suited to their environments.
Charles Darwin
In the 1830s, Charles Darwin sailed on HMS Beagle. On the Galápagos Islands, he noticed that finches on different islands had different beaks suited to different foods. In 1859, he published On the Origin of Species, explaining evolution by natural selection. Alfred Russel Wallace developed the same idea independently.
How natural selection works
Natural selection has four main parts:
| Step | What happens | Example: peppered moths |
|---|---|---|
| 1. Variation | Individuals differ in traits | Some moths are light, some dark |
| 2. Inheritance | Traits are passed to offspring | Colour is inherited |
| 3. Overproduction and competition | More offspring are born than can survive | Many moths, limited survival |
| 4. Selection | Individuals with helpful traits survive and reproduce more | On soot-darkened trees in industrial England, dark moths were better hidden from birds |
Over generations, the helpful trait becomes more common. In industrial England during the 1800s, dark moths became more common. After clean-air laws reduced soot, light moths became common again.
Adaptations
An adaptation is an inherited trait that helps an organism survive and reproduce. Examples:
- the long tongue of a hummingbird moth for reaching nectar
- the thick fur of an Arctic fox
- the waxy leaves of desert plants
Individual organisms do not evolve. Populations evolve, as the frequency of traits changes over generations.
Evidence for evolution
| Evidence | What it shows |
|---|---|
| Fossils | Older rock layers contain simpler and different life forms; transitional fossils like Tiktaalik (fish to land animal) and Archaeopteryx (dinosaur to bird) show intermediate traits |
| Comparative anatomy | Similar bone structures (homologous structures) in a human arm, bat wing and whale flipper suggest a common ancestor |
| Vestigial structures | Reduced parts with little use, such as whale hip bones |
| Embryology | Early embryos of vertebrates look very similar |
| DNA | More closely related species share more DNA; humans and chimpanzees share about 98–99% |
| Direct observation | Bacteria evolve antibiotic resistance; insects evolve pesticide resistance |
Speciation
When populations of one species become separated, by a mountain range, river or ocean, they may evolve differently. Over time they can become so different that they can no longer breed together. They are now separate species.
Extinction
Extinction happens when every member of a species dies. Species go extinct if they can’t adapt to changes fast enough. More than 99% of all species that ever lived are now extinct. Five mass extinctions have occurred, including the one 66 million years ago that wiped out the non-bird dinosaurs.
Evolution happening now
- Antibiotic resistance: When antibiotics are used, resistant bacteria survive and multiply. This is why doctors say to finish prescriptions and avoid unnecessary antibiotics.
- Darwin’s finches: Researchers Peter and Rosemary Grant measured beak sizes changing within a few generations after droughts.
Common mistakes
- Thinking individuals evolve during their lifetime.
- Thinking organisms try to adapt. Variation is random; selection is not.
- Saying humans evolved from chimpanzees. Humans and chimpanzees share a common ancestor.
Try it yourself
Simulate natural selection: scatter 50 green and 50 brown paper dots on green grass. Have a "bird" pick up as many as possible in 30 seconds. Count the survivors. Which colour survived better? Why?
Quick recap
- Evolution is change in inherited traits of populations over generations.
- Natural selection needs variation, inheritance, overproduction and selection.
- Adaptations are inherited traits that help survival and reproduction.
- Evidence includes fossils, anatomy, DNA and direct observation.
- New species can form; species that can’t adapt may go extinct.