Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 24.2 Speciation Can Take Place With or Without Geographic Separation
Speciation—the formation of new species—can occur via two main modes: allopatric speciation, which requires geographic separation, and sympatric speciation, which occurs without geographic separation.
Allopatric ("Other Country") Speciation
- Allopatric speciation occurs when populations are geographically isolated, preventing gene flow.
- Isolation happens through geological changes (e.g., rivers forming, lakes splitting) or colonization of isolated areas (e.g., flightless cormorants on Galápagos).
- After isolation, populations diverge genetically due to mutations, natural selection, and genetic drift.
- Example: Mosquitofish populations in the Bahamas adapted to predator presence or absence, leading to different body shapes and behavioral reproductive isolation.
Evidence for Allopatric Speciation
- Laboratory experiments: Fruit flies (Drosophila pseudoobscura) raised on starch vs. maltose media evolved mating preferences after just 40 generations, forming partial reproductive isolation.
- Field studies: Snapping shrimp species separated by the formation of the Isthmus of Panama evolved into sister species, demonstrating allopatric speciation.
Sympatric ("Same Country") Speciation
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Sympatric speciation happens without geographic separation; reproductive barriers form within a single geographic area through:
- Polyploidy
- Sexual selection
- Habitat differentiation
Polyploidy
- Polyploidy involves errors during cell division, creating extra chromosome sets.
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Two types:
- Autopolyploidy: Chromosome duplication within a single species produces reproductive isolation (common in plants).
- Allopolyploidy: Hybrids between different species can become fertile after chromosome doubling, forming new species.
- Example: Goatsbeard plant (Tragopogon) species arose by hybridization and polyploidy.
Sexual Selection
- Mate choice based on physical traits can create reproductive barriers.
- Example: Female cichlid fish in Lake Victoria select mates based on coloration, leading to reproductive isolation despite physical proximity.
Habitat Differentiation
- Subpopulations utilize different resources or habitats, forming reproductive barriers.
- Example: North American apple maggot flies (Rhagoletis pomonella) diverged into populations feeding on hawthorns vs. apples, creating temporal and habitat isolation.
Comparing Allopatric and Sympatric Speciation
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Allopatric speciation:
- More common due to clear geographic barriers reducing gene flow.
- Genetic divergence primarily through natural selection, genetic drift, or mutation.
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Sympatric speciation:
- Less common, requires strong reproductive isolation mechanisms.
- Driven by polyploidy, sexual selection, or habitat specialization.
In a Nutshell
Speciation can occur via geographic isolation (allopatric speciation) or without geographic separation (sympatric speciation). Both processes create reproductive isolation, allowing populations to diverge into distinct species over time.