Natural Selection & Evolution

So far we have looked at change within a species. But evolution can also produce new species: a process called speciation.

What a species is

A species is a group of organisms that can interbreed to produce fertile offspring. A horse and a donkey can mate to produce a mule, but the mule is sterile, so horses and donkeys are different species. A Labrador and a Poodle can mate to produce fertile labradoodles, so they are the same species (Canis familiaris).

How a new species can form: geographic isolation

The most common way new species evolve is allopatric speciation, which means "in different places":

1. A single population of one species is split into two by a physical barrier. The barrier might be a new river, a rising mountain range, a sea level change, or just a piece of land becoming too dry for the species to cross.
2. The two halves of the population can no longer interbreed with each other.
3. The environments on either side of the barrier are slightly different.
4. Different mutations arise in each half of the population (mutations are random).
5. Natural selection favours different traits in each environment.
6. Over many generations, the two populations diverge genetically.
7. Eventually the differences are so great that even if the barrier is removed, members of the two populations can no longer interbreed to produce fertile offspring. They are now separate species.

A classic example: Darwin's finches

When Charles Darwin visited the Galápagos Islands in 1835, he noticed that each island had its own slightly different species of finch. The finches differed mainly in beak shape:

• Some islands had finches with large strong beaks for cracking hard seeds.
• Other islands had finches with slender pointed beaks for picking up insects.
• Others had medium beaks for cactus flowers.

Darwin worked out that all the finches must have descended from a single ancestral species that arrived on the islands long ago. Once they were spread across different islands, the populations were geographically isolated from each other. The food sources on each island were different, so natural selection favoured different beak shapes on each island. Over many generations, the different populations evolved into distinct species, each adapted to its own island's food.

A modern example: ring species and salamanders

Some species are caught in the act of speciating, where the process is incomplete. The Ensatina salamanders of California form a ring around the Central Valley. Neighbouring populations can interbreed, but populations at the two ends of the ring (where they meet again at the southern end) cannot. They behave as the same species at every step, yet act as separate species at the ends.

Why speciation usually takes a long time

For two populations to diverge into separate species, they need enough generations for genetic differences to build up. For most large animals (with generation times of years to decades), this takes tens or hundreds of thousands of years. For bacteria with generation times of 20 minutes, speciation can happen in human-scale time.