Evolution & Speciation IGCSE Biology 0610

Evolution and Speciation: How New Species Form

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

Evolution is the change in the inherited characteristics of a population over many generations through natural selection, which may result in the formation of new species. Speciation occurs when populations are isolated and evolve differently until they can no longer interbreed.

Evolution and speciation questions combine natural selection with the concept of reproductive isolation. Students must describe a sequence of events from isolation through divergent evolution to the formation of a new species. The explanation must reference allele frequency changes.

Defining evolution

Evolution is the change in the inherited characteristics of a population over time through the process of natural selection. It may eventually result in the formation of a new species. Evolution does not happen to individuals; it happens to populations over many generations.

The evidence for evolution includes the fossil record (showing changes in organisms over geological time), similarities in DNA and protein sequences between related species, and the existence of homologous structures (similar body structures in different species that suggest a common ancestor).

How speciation occurs

Speciation is the formation of a new species. It typically occurs through the following sequence:

1. A population is separated into two groups by a geographical barrier such as a mountain range, river, or sea (this is called geographical isolation).
2. The two groups experience different environmental conditions and different selection pressures.
3. Natural selection acts differently on each group, favouring different alleles in each population.
4. Over many generations, the allele frequencies diverge so much that the two populations become genetically distinct.
5. Eventually, even if the barrier is removed, the two populations can no longer interbreed to produce fertile offspring. They are now separate species.

What defines a species

A species is a group of organisms that can interbreed to produce fertile offspring. This definition means that organisms that look similar but cannot breed together (or whose offspring are infertile) are different species.

For example, horses and donkeys can breed to produce mules, but mules are infertile, so horses and donkeys are classified as different species. This definition has limitations for organisms that reproduce asexually, but it is the standard definition used at IGCSE level.

Try a focused question (4 marks)

Explain how two populations of the same species could evolve into two separate species.

Show the answer and marking guidance

The populations become geographically isolated by a barrier such as a river or mountain range.
Different environmental conditions in each area produce different selection pressures.
Natural selection favours different alleles in each population over many generations.
Eventually the populations are so genetically different that they can no longer interbreed to produce fertile offspring, so they are separate species.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Describing evolution as something that happens to an individual rather than a population.
Forgetting geographical isolation as the starting point for speciation.
Not mentioning that the populations must be unable to interbreed to be separate species.
Using "stronger" instead of "better adapted" or "with a survival advantage".