Reproduction and Inheritance · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 14% of your exam marks.
Natural selection explanations and antibiotic resistance as an application are tested in almost every series.
So far we have looked at change within a species. But evolution can also produce new species: a process called speciation.
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).
The most common way new species evolve is allopatric speciation, which means "in different places":
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:
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.
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.
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.