Reinforcement Process Promotes Diversity and Population Particularity

By on April 10, 2023
Lucania goodei (Bluefin Killifish). (Credit: Brian Gratwicke via Flickr CC BY 2.0)

When it comes time to find a mate, some fish are perfectly happy to look for love beyond the bounds of their own species. However, those that stick to expected mating trends are often particularly picky among their own kind as well. This pickiness, or lack thereof, is commonplace across the animal kingdom as species search for their, sometimes seasonal, other half.

Much like humans, fish like killifish tend to have a type. A new paper from the University of Illinois shows that female killifish who refuse to mate with males of a related species are also more discerning when choosing mates among unfamiliar populations of their own species, according to a university press release. The study is the first of its kind to observe cascade reinforcement in fish.

Reinforcement within Species

Simply put, reinforcement describes the evolution of mating behaviors and reproductive traits in two closely related species that reduce the likelihood of hybridization. This process occurs so that the species avoid producing nonviable or weak offspring. When this phenomenon occurs within similar populations of common, but still different, species, it is called cascade reinforcement.

Reinforcement is an important evolutionary process that preserves species and increases recruitment rates. In general, when two species of fish mate, their offspring often possess inferior traits and are unlikely to reach adulthood. Recruitment rates are dependent on fish reaching spawning age, and so hybrids harm the potential of larger, healthier future generations of fish. The behavior can also cause problems within the species as female killifish that reject males of a similar species may also reject unfamiliar males within the same species.

The study from the University of Illinois focused on rainwater killifish and bluefin killifish. Both species are similar in size but vary in appearance as well as habitat preferences. According to the Smithsonian, rainwater killifish tend to prefer brackish waters with salinity ranging from 0 to approximately 50 PSU. In contrast, the Smithsonian reports that bluefin killifish reside in habitats with 0 to about 35 PSU. Due to overlap in salinity conditions, both killifish can live in the same waters.


The University of Illinois study found that when both species live in close proximity, female rainwater killifish will actively avoid males of their species that belong to another population. While reinforcement is known to occasionally cause a rift between males and females in the same species, the process is still closely connected to both short-term benefits to survival and long-term genetic benefits.

The author of the study, Rebecca Fuller stated that the pickiness demonstrated as a result of cascade reinforcement that leads to further isolations of populations may be the beginning of the long process of speciation.

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