Stickleback fish show how neighboring species can ignite evolution

A team working in western Canada found that a single neighboring fish can steer another fish down a different evolutionary path. The key result is simple: when prickly sculpin are present, threespine stickleback begin to split apart into groups that avoid mating with each other.

The scale is clear. Parentage tests on hundreds of young fish showed consistent mate choice within the same lake type. This is a sign that reproductive barriers are forming between sticklebacks from sculpin lakes and those from sculpin free lakes.

The evolution of sticklebacks

The new work is led by Dr. Marius Roesti at the Institute of Ecology and Evolution at the University of Bern (UB). His team asked whether a single ecological difference, living with or without sculpin, can set speciation in motion.

These lakes formed after the last ice age, less than 12,000 years ago, and were colonized by marine sticklebacks. That timescale is short, yet it is long enough for speciation to begin.

In sculpin lakes, sticklebacks evolve slimmer, open water bodies. In lakes without sculpin, they become stockier bottom feeders, a pattern that hints at ecological speciation, evolution of reproductive barriers driven by natural selection across habitats.

The idea that selection can drive reproductive isolation has deep roots in modern evolutionary biology. A 2022 review outlined core challenges and connections between selection and barriers to mating.

Testing mate separation

To move beyond hints, the researchers mixed fish from multiple lake types in large outdoor ponds and tracked who mated with whom. In this new study, the team genotyped 411 offspring to reconstruct pairs.

The researchers found clear assortative mating, a form of assortative mating, choosing mates that are similar in key traits, that kept fish aligned with their lake type. The pattern ranged from weak to complete separation across populations.

“Assortative mating between populations of the same ecological type was moderate on average but ranged from weak to complete,” wrote Roesti.

This was not random. Populations that had diverged more in body shape and across the genome, the full set of an organism’s DNA, showed stronger mate separation in the ponds.

Silent ecological pressure

Sculpin and sticklebacks barely interact face to face. Instead, they eat similar prey and face the same predators, an indirect interaction, influence that flows through shared food or enemies, that can shift how each species uses the lake.

When sculpin share the lake, sticklebacks are drawn into open water and away from the bottom. That shift changes body form and behavior, which then feeds back into mate choice, making premating barriers more likely to grow.

“The strength of premating isolation increased with increasing morphological and genomic population divergence,” wrote Roesti.

The logic aligns with classic results from stickleback research. Decades ago, field experiments showed that environmental differences can raise or lower hybrid success, tightening or loosening barriers.

Rapid shifts in stickleback evolution

These lakes sit in separate valleys, so gene flow among them is limited. That makes them a natural test of allopatry with no gene flow, where selection can still push mating barriers to arise.

The new results argue that ecological differences do not need to be dramatic. A single competitor can be enough to redirect evolution if it nudges feeding, habitat use, and risk in consistent ways across lakes.

Premating isolation appears early, before full speciation is complete. That is important because premating isolation, barriers that stop mating before fertilization, can snowball by reducing hybrid formation and reinforcing mate choice.

The work also reminds us that selection often acts through food webs. Shared predators and shared prey can connect species whose only conflict is silent, playing out in the background of daily survival.

Why this matters now

The lesson for conservation is pragmatic. Small changes in local species lists can reshape evolutionary paths, either accelerating splits or erasing unique lineages when new predators are introduced.

The team paired field experiments with high resolution genetic work, then made their data openly available for others to test and re-analyze. That openness will help researchers probe mechanisms with fresh eyes.

Similar stories are unfolding in Central Europe. In the Lake Constance region, sticklebacks have rapidly diversified in size, diet, and habitat use.

This rapid divergence hints that ecological pressures can reshape species even in recently colonized lakes, echoing the same selective forces seen in western Canada.

Across systems, a common thread appears. Natural selection can link ecology to mate choice by changing which traits matter for survival – a connection echoed by this experiment.

The research shows that even quiet, indirect pressures in shared ecosystems can shape the very boundaries of what we call a species.

The study is published in the journal Proceedings of the National Academy of Sciences.

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