Natural selection and speciation

1The moth that changed colour

The peppered moth, Biston betularia, rests by day on tree trunks, where birds hunt it. Its usual form is pale, speckled with black, and is well hidden on trunks covered with pale lichens. In 1848 a dark form was recorded near Manchester, in England. Factories there burned coal, and the soot killed the lichens and blackened the bark. By 1895 almost all the peppered moths around Manchester were dark. Then, after clean-air laws in the 1950s cut the smoke, the lichens came back and the pale form became common again.

No moth chose its colour, and no moth changed colour in its lifetime. What changed was which moths survived to breed. This is natural selection, and when it acts on dark colouring in polluted places it is called industrial melanism.

2How natural selection works

Charles Darwin and Alfred Russel Wallace set out the theory together in 1858, and Darwin explained it fully in On the Origin of Species (1859). It follows from a few observations:

1. Overproduction. Organisms produce more offspring than can survive. A female peppered moth lays hundreds of eggs.
2. Variation. The offspring vary, and some of the variation is inherited (Lesson 1).
3. Struggle for existence. Offspring compete for food, space and mates, and face predators and disease; many die young.
4. Survival of the fittest. Individuals whose variations suit the environment best are more likely to survive and reproduce. "Fittest" means best suited, not strongest.
5. Inheritance. The survivors pass their alleles to their offspring, so the helpful alleles become more common, generation after generation.

In the moth, the dark colour is caused by a dominant allele. On sooty bark, birds found and ate more pale moths, so dark moths left more offspring and the dark allele became more common. On clean, lichen-covered bark the advantage reversed.

3Sickle-cell and malaria: a balance

In Lesson 2 you met the sickle-cell allele, $\mathrm{H}{\mathrm{b}}^{\mathrm{S}}\backslash mathrm\{Hb^S\}$. People with two copies ($\mathrm{H}{\mathrm{b}}^{\mathrm{S}}\mathrm{H}{\mathrm{b}}^{\mathrm{S}}\backslash mathrm\{Hb^S\; Hb^S\}$) have sickle-cell disease. Without treatment many die young, so you might expect natural selection to remove the allele. Yet in regions where malaria is common, including much of tropical Africa, the allele stays relatively common.

The reason is that carriers ($\mathrm{H}{\mathrm{b}}^{\mathrm{A}}\mathrm{H}{\mathrm{b}}^{\mathrm{S}}\backslash mathrm\{Hb^A\; Hb^S\}$), who have the sickle-cell trait, are usually healthy and are more likely to survive severe malaria than people with $\mathrm{H}{\mathrm{b}}^{\mathrm{A}}\mathrm{H}{\mathrm{b}}^{\mathrm{A}}\backslash mathrm\{Hb^A\; Hb^A\}$. Where malaria kills many children, carriers survive and reproduce more, which keeps the $\mathrm{H}{\mathrm{b}}^{\mathrm{S}}\backslash mathrm\{Hb^S\}$ allele in the population. Where there is no malaria, carriers have no advantage, and the allele is rarer.

The pattern follows malaria, not any group of people: it is natural selection by a disease in a place.

4How new species form

Figure 1. How a new species can form after a population is split by a barrier (geographical isolation).

If two groups of one population are kept apart, no alleles pass between them. Each group gets its own new mutations, and natural selection acts differently on each if their environments differ. After many generations the two groups may differ so much, in body, behaviour, breeding season or chromosomes, that they can no longer interbreed even if they meet. They are then separate species.

The finches of the Galápagos Islands, which Darwin collected, are a classic example: more than a dozen species with different beaks, suited to different foods, are thought to have come from one ancestral species that reached the islands.