Species can’t change, but they evolve over time
Through the process of evolution, how can new species arise? How can one species turn into another? The simple answer is, of course, it can’t. Species can’t change; it is populations of species that change or evolve over very long periods of time. Take the following hypothetical example.
Say there are a few families of Wood Mice living in the nooks and crannies of a big old tree growing on the bank of a large river. A catastrophic flood happens, the riverbank is eroded, the tree is undermined causing it to topple into the river and get washed out to sea carrying its cargo of mice.
Far out at sea, the tree gets washed up on the extensive sandy beaches on an offshore island that happens to be free of mice. The hungry mice are lucky to make a landfall alive and they scramble ashore to feed. The best feeding happens to be on the extensive sandy beaches that surround the island, so the mice make themselves at home there.
Food is plentiful, the climate is favourable, and life is good. The mice breed rapidly, and the population expands. Then birds of prey discover the abundance of fat mice and move in the take advantage of the newly-discovered food source.
When the mice are threatened by an overhead predatory bird some bolt and leg away as fast as they can to avoid being eaten. Others freeze and hope they won’t be spotted. The poor runners get caught and both the pale and the dark mice that freeze are easily picked off.
The good runners escape to live another day. The sandy-coloured ones that freeze also survive because of their camouflage. As time goes by only the very best runners and those with the best combination of freezing and sandy-coloured coats survive to breed. Nature selects from the population those that are fittest to survive. No individual mouse is changing but the population is very slowly diverging into two distinct groups: great runners and great sandy-coloured freezers.
Charles Darwin was the first to describe this kind of natural selection when he observed 15 species of finch on the Galapagos. They were all superficially alike but differed in body size, beak shape, song and feeding behaviour. Darwin concluded that they all must have evolved over about two million years from a common ancestor, probably a small party of finches that got accidentally wind-blown to the remote islands during a storm event.