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
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.
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
Allopatric speciation:
More common due to clear geographic barriers reducing gene flow.
Genetic divergence primarily through natural selection, genetic drift, or mutation.
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.