Evidence for evolution, variation and natural selection

What the fossil record shows

• Fossils show that living things have taken a huge range of forms over Earth's history.
• Some ancient forms look a lot like living ones. Fossil crocodiles from many millions of years ago have the same long snout and armoured back as crocodiles today.
• Others belong to whole groups that have died out. Trilobites, sea animals with jointed legs and a three-part body, were common for hundreds of millions of years and are now extinct. So are ammonites, coiled-shell sea animals often found as fossils in New Zealand rock.
• The tuatara is the only living member of a reptile group known mostly from fossils, so it is a living link to that ancient group.

Three kinds of evidence

• The theory of evolution says that populations and species change over time. In science, a theory is a big explanation backed by a large body of evidence, not a guess.
• Fossils: deeper rock layers are older, and the kinds of fossils change from layer to layer, so you can see life changing through time.
• Shared features: a human arm, a bat's wing and a whale's flipper have the same set of bones arranged in the same order, which makes sense if they came from a common ancestor.
• DNA: species that look closely related, such as different parrots, also have very similar DNA, and the similarity matches what the fossils and features suggest.

Variation within a species

• Individuals of the same species are never exactly alike. These differences are called variation.
• Male Wellington tree wētā show striking variation: some have much bigger heads and jaws than others, which they use when fighting other males over a hole in a tree.
• In people, height, hair colour and the shape of ears all vary.
• Variation is the raw material evolution works on. Without differences, there would be nothing to select between.

Which differences are passed on?

• Some variation is heritable: it is coded in an organism's genes and can be passed from parents to their offspring.
• Other variation comes from the environment or from a lifetime of experience, and is not passed on.
• Only heritable variation matters for evolution, because only it can show up in the next generation.

Try it: heritable or not?

Selection pressures

• A selection pressure is any feature of the environment that affects which individuals survive or how many offspring they have.
• Examples include a shortage of food, predators, disease, cold spells, strong sunlight and competition for mates.
• Each pressure favours some variations over others. When birds hunt beetles on dark soil, dark beetles are harder to spot than pale ones.

How natural selection works

• Natural selection is the process that links variation and selection pressures.
• 1: Individuals in a population vary, and some of that variation is heritable.
• 2: A selection pressure means some variations give a better chance of surviving and breeding.
• 3: Individuals with those helpful variations, on average, leave more offspring.
• 4: Those offspring inherit the helpful variations, so the next generation has more of them.

Try it: toothpicks the birds missed

When the environment changes too fast

• If the environment changes, a species may survive if some individuals already have variations that suit the new conditions.
• If none do, or the change is too fast for the population to keep up, the species can become extinct: every last individual dies.
• Habitat destruction is a major cause. The huia, a New Zealand bird of the North Island forests, became extinct in the early 1900s after much of its forest was cleared and it was hunted for its feathers.
• Changes in temperature, new predators, new diseases and the loss of a food supply can all push a species to extinction.

How the theory developed

• For a long time many people thought species never changed and never died out.
• Around 1800 the French scientist Georges Cuvier studied fossil bones such as those of mammoths and showed they belonged to animals no longer alive, so extinction is real.
• Jean-Baptiste Lamarck suggested that animals change by using body parts more, and pass on those changes. Later evidence did not support this, because features gained in a lifetime are not inherited.
• As more fossils, more careful studies of variation and finally DNA evidence came in, natural selection explained the evidence far better, which is how a scientific theory grows.