Speciation
Speciation is the process by which one species evolves into two or more separate species. It is the fundamental mechanism by which biodiversity increases. Understanding the different modes of speciation and the barriers that prevent interbreeding is essential for A-Level Biology.
What Is a Species?
Key Definition: A species is a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other such groups. This is the biological species concept.
Limitations of the Biological Species Concept
- It cannot be applied to asexually reproducing organisms (e.g., bacteria, many plants).
- It is difficult to apply to fossils (we cannot test whether extinct organisms could interbreed).
- Some distinct species can produce viable hybrids (e.g., horse × donkey → mule, but mules are infertile; wolf × dog → fertile hybrids).
- Ring species (discussed below) challenge the concept because neighbouring populations can interbreed but populations at the ends of the ring cannot.
Despite these limitations, the biological species concept remains the most widely used definition for sexually reproducing organisms.
How Does Speciation Occur?
Speciation requires two key processes:
- Reproductive isolation — a barrier prevents gene flow between two populations.
- Genetic divergence — the isolated populations accumulate different genetic changes (through mutation, natural selection, and genetic drift) until they can no longer interbreed, even if the barrier is removed.
The two main modes of speciation are allopatric and sympatric.
Allopatric Speciation
Key Definition: Allopatric speciation occurs when a population is divided by a geographical barrier (e.g., mountain range, river, ocean, glacier, road), preventing gene flow between the separated groups.
Steps of Allopatric Speciation
- A population of one species occupies a continuous range.
- A geographical barrier arises (or some individuals disperse across an existing barrier), physically separating the population into two or more groups.
- Gene flow between the groups ceases.
- Each group experiences different environmental conditions and therefore different selection pressures.
- Different mutations arise in each group. Natural selection and genetic drift act independently on each group.
- Over many generations, the groups diverge genetically — allele frequencies change in different ways.
- Eventually, the groups become so genetically different that they can no longer interbreed to produce fertile offspring, even if the barrier is removed.
- The two groups are now separate species.
Example — Darwin's Finches
- The ancestral finch population colonised the Galapagos Islands from the South American mainland.
- Different islands presented different environments (different food sources, different habitats).
- Populations on different islands were geographically isolated from each other.
- Natural selection favoured different beak shapes and sizes on different islands (e.g., large beaks for cracking hard seeds, slender beaks for probing cacti).
- Over time, the populations diverged to the point where they could no longer interbreed — approximately 14 distinct species evolved.
- This is an example of adaptive radiation — the rapid diversification of a single ancestral species into many species, each adapted to a different ecological niche.
Sympatric Speciation
Key Definition: Sympatric speciation occurs when a new species evolves from a population without geographical separation. The populations occupy the same area but become reproductively isolated through other mechanisms.
Mechanisms of Sympatric Speciation
Polyploidy is the most common and best-documented mechanism of sympatric speciation, particularly in plants:
- Polyploidy occurs when an organism has more than two complete sets of chromosomes (e.g., 3n, 4n, 6n).
- Autopolyploidy: An error during meiosis or mitosis produces cells with extra chromosome sets from the same species. A tetraploid (4n) individual cannot produce fertile offspring with diploid (2n) individuals because meiosis produces unbalanced gametes → reproductive isolation.
- Allopolyploidy: Hybridisation between two different species produces a sterile hybrid. If the hybrid undergoes chromosome doubling, it becomes a fertile allopolyploid that can reproduce with other allopolyploids but is reproductively isolated from both parent species.