Rucete ✏ Campbell Biology In a Nutshell
Unit 4 MECHANISMS OF EVOLUTION — Concept 24.1 The Biological Species Concept Emphasizes Reproductive Isolation
Speciation—the process by which new species arise—is crucial for generating life's immense diversity. The biological species concept defines species based on reproductive isolation, emphasizing that species are groups of populations capable of interbreeding and producing viable, fertile offspring.
The Biological Species Concept
Species: Groups whose members can interbreed and produce viable, fertile offspring, but cannot do so with other groups.
Reproductive compatibility unites species genetically and maintains their distinction from other species.
Example: Humans worldwide form one species because they can interbreed despite geographic distances, whereas humans and chimpanzees remain separate due to reproductive barriers.
Reproductive Isolation: Key to Speciation
Reproductive isolation prevents different species from interbreeding, maintaining distinct species boundaries.
It can be categorized into:
Prezygotic barriers (before fertilization): block mating attempts or prevent fertilization.
Postzygotic barriers (after fertilization): prevent hybrids from developing into viable, fertile adults.
Prezygotic Barriers
Habitat isolation: Different habitats within the same area limit interactions (e.g., apple maggot flies vs. blueberry maggot flies).
Temporal isolation: Species breed at different times (e.g., different mating seasons for eastern and western spotted skunks).
Behavioral isolation: Unique courtship rituals prevent interbreeding (e.g., blue-footed boobies' mating dances).
Mechanical isolation: Physical incompatibilities prevent successful mating (e.g., snails with differently spiraled shells).
Gametic isolation: Gametes cannot fuse due to biochemical incompatibility (e.g., different sea urchin species).
Postzygotic Barriers
Reduced hybrid viability: Hybrid offspring have reduced survival (e.g., salamander hybrids).
Reduced hybrid fertility: Hybrid offspring are sterile (e.g., mules from horses and donkeys).
Hybrid breakdown: First-generation hybrids fertile, but subsequent generations are weak or sterile (e.g., certain rice strains).
Limitations and Alternative Species Concepts
Biological species concept limitations:
Doesn't apply to fossils or asexually reproducing organisms.
Species boundaries may blur due to occasional gene flow (e.g., polar bears and grizzly bears hybridize occasionally).
Alternatives include:
Morphological species concept: Defines species by body structure, useful in practice but subjective.
Ecological species concept: Defines species by ecological niches, suitable for both sexual and asexual species, emphasizing adaptation to environments.
In a Nutshell
The biological species concept centers on reproductive isolation, crucial in forming and maintaining distinct species. Prezygotic and postzygotic barriers prevent interbreeding, ensuring genetic distinctiveness. Though the concept has limitations, it effectively highlights reproductive barriers' role in species formation and diversity.