The three-spined stickleback, Gasterosteus aculeatus, is a small, widespread fish used to study rapid evolution and speciation. Ancestrally a marine species, populations have repeatedly colonized and adapted to freshwater environments across the Northern Hemisphere. The stickleback’s prominence in research stems from the dramatic physical differences seen between ancestral marine and derived freshwater forms. These differences, which manifest in body shape, armor plating, and feeding structures, often evolve within just a few thousand years. The physical isolation of these populations from their ocean-dwelling relatives was the first step in this repeatable evolutionary divergence.
Glacial Melt and the Creation of Isolated Habitats
The primary mechanism for the physical isolation of stickleback populations was the last Ice Age, which concluded approximately 10,000 to 20,000 years ago. As continental ice sheets retreated, they exposed new land and drastically altered global sea levels and drainage patterns. Ancestral marine sticklebacks are anadromous, meaning they live primarily in saltwater but migrate into freshwater rivers and streams to breed. This life cycle positioned them perfectly to colonize the changing landscape.
The melting glaciers created numerous temporary connections between coastal marine waters and newly formed lakes across the Northern Hemisphere. Sticklebacks, tolerant of both fresh and saltwater, swam up these corridors to colonize the freshwater bodies. Once the ice fully receded, two major processes permanently severed these connections, trapping the fish in their new homes.
One process was the continued retreat of the water, as river mouths closed or became impassable due to sediment buildup. The second process was isostatic uplift, where the land, previously compressed by the immense weight of the ice sheets, began to rebound. This uplift raised the land around the lakes, effectively cutting them off from their marine outlets. This created thousands of independent, landlocked freshwater populations descended from the same marine ancestor.
Selection Pressures Driving Morphological Divergence
Once isolated in freshwater habitats, sticklebacks faced new selective pressures leading to rapid evolutionary changes. The marine ancestor possesses heavy armor, including bony lateral plates and prominent pelvic spines, which defend against large ocean predators. In freshwater, however, many large marine predators are absent, and the cost of producing this heavy armor outweighs the benefit.
Freshwater environments often lack the calcium or nutrients required to build and maintain the extensive armor plating. Consequently, natural selection rapidly favored individuals with reduced armor, which saved significant energy and resources that could be allocated to growth or reproduction. This has led to the widespread evolution of “low-plated” or unplated morphs, a change often driven by regulatory gene changes, such as in the Eda gene.
The freshwater environment also imposes different feeding challenges, particularly in lakes where the primary food source is small zooplankton in the open water. In the open-water (limnetic) zone, fish evolved a streamlined body and longer, more numerous gill rakers to sieve tiny prey. Conversely, populations adapted to bottom-feeding (littoral zone) developed a stockier body and wider mouth for consuming larger invertebrates. These distinct adaptations demonstrate how the isolated environment drove divergence in body morphology.
Mechanisms Preventing Interbreeding
Rapid physical divergence is accompanied by reproductive isolation, which prevents distinct populations from merging even if they come into renewed contact. One strong barrier to gene flow is pre-mating isolation, driven by behavioral changes in mate choice. During breeding, male sticklebacks develop nuptial coloration, often displaying bright red throats and bellies, to attract females.
Freshwater females exhibit specific preferences for males based on body shape, size, or intensity of coloration, which differs between populations. This assortative mating, where fish choose mates of their own type, acts as a powerful barrier to interbreeding. Additionally, females in small populations may avoid mating with close relatives, minimizing the negative effects of inbreeding depression.
Post-zygotic isolation also plays a significant role in maintaining the separation of these populations. Hybrid offspring resulting from crosses between different ecotypes often have reduced ecological fitness. This reduced fitness is not typically due to intrinsic genetic incompatibilities, but rather to an inability to thrive in either parental environment. A hybrid body shape, for example, may be poorly suited for both open-water feeding and bottom-dwelling, making it an ineffective forager or easy target for predators.
Specific Examples of Isolated Populations
A well-studied example of rapid isolation and divergence is found in lakes of British Columbia, Canada, which host benthic and limnetic species pairs. These pairs exist in lakes like Paxton Lake on Texada Island, where two distinct forms evolved from a single anadromous ancestor that colonized the lake after glacial recession. The limnetic stickleback is a slender, open-water specialist that feeds primarily on zooplankton floating in the water column.
The benthic stickleback is a stockier, deeper-bodied fish with a wider mouth adapted to consuming larger invertebrate prey found on the lake bottom. Although they share the same lake, these two forms are considered separate, reproductively isolated species utilizing distinct ecological niches. The repeated, independent evolution of these specialized forms in multiple lakes provides compelling evidence of parallel evolution driven by strong environmental selection.