explain how speciation may occur as a result of genetic isolation by: geographical separation (allopatric speciation), ecological and behavioural separation (sympatric speciation)

Evolution – Speciation through Genetic Isolation

Speciation is the process by which one species splits into two or more genetically distinct lineages that no longer inter‑breed in nature. Genetic isolation prevents gene flow between populations, allowing independent evolutionary trajectories.

1. Types of Genetic Isolation

• Geographical (allopatric) isolation – physical barriers separate populations.
• Ecological and behavioural (sympatric) isolation – populations occupy the same area but diverge because of niche differentiation, mating preferences, or polyploidy.

2. Allopatric Speciation (Geographical Separation)

Allopatric speciation occurs when a population is divided by a physical barrier such as a mountain range, river, or ocean. The separated groups evolve independently.

2.1 Mechanism

1. Formation of a barrier that prevents inter‑breeding.
2. Genetic drift, mutation, and natural selection act on each isolated population.
3. Accumulation of genetic differences leads to reproductive incompatibility.

2.2 Example – The Galápagos Finches

When a storm carried finches to different islands, each island’s population adapted to local food sources. Over many generations, beak shapes diverged, and inter‑island mating became rare.

2.3 Key Genetic Concepts

The probability that two alleles are identical by descent in a small isolated population can be expressed as:

\$\$

F{t+1}= \frac{1}{2N} + \left(1-\frac{1}{2N}\right)Ft

\$\$

where \$N\$ is the effective population size and \$F_t\$ is the inbreeding coefficient in generation \$t\$.

3. Sympatric Speciation (Ecological & Behavioural Separation)

Sympatric speciation occurs without physical separation. Divergence is driven by ecological niche exploitation, sexual selection, or genetic mechanisms such as polyploidy.

3.1 Mechanism

1. Individuals exploit different resources or habitats within the same area (resource partitioning).
2. Assortative mating develops – individuals preferentially mate with those using the same resource or displaying similar traits.
3. Reproductive barriers (pre‑zygotic or post‑zygotic) evolve, leading to genetic isolation.

3.2 Example – Apple Maggot Fly (Rhagoletis pomonella)

Originally a hawthorn fruit specialist, a subset shifted to the introduced apple tree. Flies that emerged on apples mate on the apple tree, while hawthorn‑origin flies mate on hawthorn, creating temporal and behavioural isolation.

3.3 Polyploidy in Plants

Whole‑genome duplication can instantly create reproductive isolation. A tetraploid individual (\$2n=4x\$) cannot produce fertile offspring with the diploid parent (\$2n=2x\$) because of meiotic mismatches.

\$\$

\text{Gamete chromosome number: } n{\text{diploid}} = x,\; n{\text{tetraploid}} = 2x

\$\$

4. Comparison of Allopatric and Sympatric Speciation

5. Summary of Key Points

• Genetic isolation stops gene flow, allowing independent evolution.
• Allopatric speciation relies on physical separation; drift and selection act on isolated gene pools.
• Sympatric speciation occurs within a shared area; ecological or behavioural divergence creates reproductive barriers.
• Polyploidy is a common mechanism of sympatric speciation in plants.
• Understanding speciation mechanisms helps explain biodiversity patterns and evolutionary history.

6. Sample Examination Questions

1. Explain how a river can lead to allopatric speciation in a population of freshwater fish. Include the roles of genetic drift and natural selection.
2. Describe the process by which the apple maggot fly (Rhagoletis pomonella) illustrates sympatric speciation. Highlight the importance of host preference and temporal isolation.
3. Compare and contrast the genetic consequences of a small isolated population (founder effect) with those of a polyploid event in a plant species.