Understanding how species evolve is one of the most fundamental questions in biology. In a new study, researchers have investigated different populations of the distinctive mosquitofish (Gambusia hubbsi). It is a species that spends its time in underwater caves in the Bahamas and is known for its opportunistic feeding habits and its tendency to cannibalise its own fry.
These isolated caves, known as blue holes, act as natural laboratories where fish populations evolve independently. Some of these habitats contain larger predatory fish, whilst others are completely free of predators. The difference in predator pressure has led the mosquitofish to adapt to their respective environments – not least in terms of their behaviour.
“When fish adapt to different predator environments, the females develop a strong aggression to males from other populations, often attacking them. In practice, this could mark the beginning of the formation of new species – fish of the same species, due to sexual hostility on the part of the females, eventually evolving into two separate species,” says Kaj Hulthén, a biology researcher at Lund University.
In the study, researchers examined fish originating from wild populations. Some of the fish had adapted to environments with a high risk of predation, whilst others came from populations that had never had to fear being eaten by larger fish. Mating experiments were then carried out, in which fish from the different populations were brought together. A total of 30 different combinations were tested. The males’ behaviour differed markedly between the environments. They made more mating attempts when they encountered females from a different predation regime, presumably in an attempt to compensate for the greater difficulty in attracting them.
“The males were more persistent when encountering an foreign female, but with low mating success when trying to mate with a female from a different predation regime. Their persistence rarely helped. The resistance from the females was often far too strong,” says Varpu Pärssinen, a biology researcher at Lund University at the time the study was conducted.
The females’ behaviour created an obvious barrier. They reacted more aggressively towards males from other populations, particularly if the males came from an environment with a different level of predator pressure to their own. The females’ aggression had clear consequences since increased aggression led to a decline in fertilisation success and
pairs of fish from different environments had significantly lower fertilisation success than pairs from the same environment.
“We were surprised at how aggressive the females could be. In some cases, the attacks were so severe that they resulted in the death of the male. It shows just how effective such repellent behaviour can be as a barrier between populations,” says Kaj Hulthén.
Aggression moves life forward
The research complements “female choice” in the evolutionary process and instead highlights “female resistance” as a powerful mechanism underlying reproductive isolation. The study also shows how rapid environmental changes can have far-reaching consequences for biodiversity.
“Our study shows that evolution takes surprising forms: aggression and resistance can be just as important driving forces for the development of life as cooperation and attraction,” says Kaj Hulthén.
