For SL and HL, speciation is the evolutionary process by which a new species develops from an existing population. It occurs when populations diverge and become reproductively isolated, so they can no longer interbreed successfully.
This belongs to Subtopic 3.1: Biodiversity and evolution. At SL, know the definition and its relationship to natural selection. The detailed mechanisms of reproductive isolation are HL only.
How Speciation Occurs
- A population contains inherited variation.
- A barrier separates groups and reduces gene flow, the exchange of alleles through reproduction.
- The groups experience different selection pressures, such as food availability, climate or competition.
- Individuals with advantageous traits survive and reproduce more successfully. Across generations, allele frequencies change and the populations diverge.
- Reproductive isolation prevents successful interbreeding, producing distinct species.
For HL students, isolation can occur in several ways:
| Type | Mechanism |
|---|---|
| Geographical | A physical barrier, such as an ocean or mountain range, separates populations. |
| Ecological | Populations occupy different niches or reproduce in different habitats. |
| Behavioural | Different courtship displays, calls or mating behaviour prevent reproduction. |
Darwin's finches on the Galápagos Islands provide a named example. Island populations faced different food sources and selection pressures; divergence in beak traits contributed to the formation of multiple species.
A common misconception is that individuals evolve into new species. They do not: populations evolve over many generations. Geographical separation alone also does not guarantee speciation; reproductive isolation must develop.
Speciation increases biodiversity, but habitat fragmentation is not automatically beneficial. Small isolated populations may lose genetic diversity and become extinct before speciation occurs. Conservation must therefore consider habitat connectivity and local communities.