Darwin, Wallace, new species and common ancestry

Two naturalists, one idea

• Charles Darwin sailed round the world on HMS Beagle from 1831 to 1836, collecting animals, plants and fossils. He even stopped in the Bay of Islands in December 1835.
• Back in England he spent over twenty years gathering evidence for natural selection, but did not publish.
• Alfred Russel Wallace, collecting animals in the islands of South-East Asia, came to the same idea on his own and in 1858 sent Darwin an essay describing it.
• Their work was presented together at a scientific meeting in London in July 1858, and Darwin published his book On the Origin of Species the next year.
• Two people reaching the same explanation from different evidence made the idea much harder to dismiss.

Helpful traits spread over generations

• Natural selection works over many generations, not overnight.
• Individuals with a heritable trait that helps them survive or breed are a little more likely to leave offspring.
• Their offspring inherit the trait, so in the next generation a slightly bigger share of the population has it.
• Repeat that over many generations and a rare trait can become the common one.

Populations evolve, individuals do not

• A single moth cannot change its colour to survive. It is born pale or dark and stays that way.
• What changes is the population: the share of dark and pale moths shifts from one generation to the next.
• Evolution means a change in the inherited traits of a population over generations.
• So it is wrong to say an animal evolves during its life. It grows and learns, but it does not evolve.

From one ancestor to new species

• If groups of one species live in different places with different selection pressures, each group changes in its own way.
• Over very long periods the changes add up until the groups are so different that they can no longer breed together. They are now separate species.
• New Zealand's kea, kākā and kākāpō are all descended from one ancestral parrot.
• The kākāpō became a heavy, flightless parrot that feeds at night on the ground. The kea adapted to cold South Island mountains, and the kākā to life in native forest.
• Their shared ancestry shows in their features and their DNA.

Try it: kea, kākā or kākāpō?

Some species cannot keep up

• Natural selection can only work on variation a population already has, and it needs many generations.
• If the environment changes faster than that, the species may die out.
• New Zealand had nine species of moa, huge flightless birds. After people arrived, moa were hunted and much of their habitat was burned, and within a few centuries every species was extinct.
• Haast's eagle, which hunted moa, then died out too, because it lost its main food.

Try it: why did it die out?

All life is related

• All vertebrates (animals with a backbone) share a body plan: a backbone, a skull, and limbs built from the same set of bones.
• The same upper-arm bone, two forearm bones, wrist bones and finger bones appear in your arm, a bat's wing, a whale's flipper and a bird's wing, shaped for different jobs.
• All living things use DNA in the same way to carry their genetic information.
• The evidence collected so far supports the idea that all living organisms are related, descended from shared ancestors over billions of years.