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Evolution (1 questions)
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Allopatric speciation occurs when a population is divided into two or more geographically isolated populations. This separation prevents gene flow between the populations, allowing them to evolve independently. The process typically unfolds as follows:
- Initial Geographic Separation: A physical barrier (e.g., mountain range, river, ocean) arises, dividing a previously interbreeding population.
- Independent Evolution: Each isolated population experiences different selective pressures due to varying environmental conditions. This leads to the accumulation of different mutations and genetic variations within each population.
- Reproductive Isolation: Over time, the genetic differences between the isolated populations become significant. This can result in the evolution of reproductive isolating mechanisms, preventing successful gene flow even if the geographic barrier is removed. These mechanisms can be:
- Prezygotic Isolation: Mechanisms that prevent mating or fertilization from occurring. Examples include:
- Habitat Isolation: Populations occupy different habitats and rarely encounter each other.
- Temporal Isolation: Populations breed during different times of day or year.
- Behavioral Isolation: Populations have different courtship rituals or signals.
- Mechanical Isolation: Physical incompatibility prevents mating.
- Postzygotic Isolation: Mechanisms that occur after the formation of a hybrid zygote. Examples include:
- Reduced Hybrid Viability: Hybrids are unable to develop or survive.
- Reduced Hybrid Fertility: Hybrids are infertile (e.g., mules).
- Hybrid Breakdown: First-generation hybrids are viable and fertile, but subsequent generations become infertile or weak.
- Prezygotic Isolation: Mechanisms that prevent mating or fertilization from occurring. Examples include:
Example: The Darwin's finches on the Galapagos Islands provide a classic example. A single ancestral finch population arrived on the islands. Geographic isolation on different islands led to different food sources (e.g., different sized seeds). This resulted in the evolution of different beak shapes adapted to those specific food sources. Eventually, these populations became reproductively isolated, leading to the evolution of distinct finch species.