Allopatric speciation

Essence and start of the process

Any allopatric speciation starts when a single group of individuals (the initial population) inhabiting a common range encounters a physical barrier. The original geographic space is divided into isolated areas—so-called isolates.

Key factors of such separation can include:

• The emergence of massive natural barriers. These include new water barriers (canals, rivers, seas), relief features (mountains), or impassable landscapes (deserts).
• A sharp change in population size, which can fragment a single range.

At this point, the physical barrier makes free movement of individuals impossible. Spatial isolation serves as the primary basis for all subsequent evolutionary changes.

Stage 1. Geographic separation and subspecies

As soon as the original range is divided into parts, gene flow between the isolated populations ceases completely. The process of deepening genetic differentiation begins in each new isolate.

Since environmental conditions in different parts may vary, populations begin to evolve along their own independent paths. Significant genetic differences gradually accumulate, and the initial mechanisms of reproductive isolation are established.

The direct result of this first stage is the formation of subspecies—groups of individuals that are already genetically different from each other, but are still at an intermediate stage of evolutionary divergence.

Stage 2. Reproductive isolation

For the process of allopatric speciation to be considered fully complete, a geographic barrier alone is not enough. An insuperable reproductive isolation must arise.

At the second stage, genetic differences between isolated subspecies reach a critical level. The dynamics of the process lie in the fact that each subspecies continues to evolve completely independently.

The main characteristic of completed speciation: when attempting to crossbreed individuals from different isolates, hybrids either fail to form at all or prove completely unviable. As a result, the former subspecies finally turn into two independent, autonomous species.

Classic examples and evolution rate

The rate of these described evolutionary processes varies greatly among different taxa. It is known that hominids evolved into new species very quickly compared to many other groups.

Classic examples from nature perfectly illustrate trait divergence under geographic isolation:

1. Hares (genus Lepus): Depending on the territory, the original group diverged. The mountain hare adapted and occupied almost the entire territory of the country. The European hare settled exclusively in areas with snowless winters, while the tolai hare (desert hare) successfully adapted to life in mountainous regions.
2. Galapagos finches: A common ancestor of these birds once migrated from the mainland to the islands. Due to spatial isolation on different islands, pronounced divergence occurred in feeding habits and, consequently, in beak shape. As a result, ground finches (feeding on seeds), tree finches (eating insects), and warbler finches (specializing in small insects) formed.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of reproductive isolation: why hybrids turn out to be unviable
• The impact of sudden population size spikes on range fragmentation
• Causes of the anomalously high rate of speciation in hominids
• Morphological divergence using the beak shape of Galapagos finches as an example
• Ecological adaptation of Lepus species to various climatic zones

Other topics in the "Speciation" section

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