Roads Help Fish Please the Ladies

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Human activities are altering landscapes, dividing habitats and changing ecosystems. We rarely hear of any good coming from this, mostly because there isn’t. However, recent research has revealed an interesting evolutionary consequence of road construction for a small fish in the Bahamas. Over just 50 years of evolution, females in fragmented populations have exercised their preference to produce males that are better equipped to please them and more considerate of their feelings.

Humans are constantly modifying our landscape to make it more connected. We build roads, tunnels and bridges, we design boats and planes, all in an attempt to maximise our ability to move around. At the same time, these human structures are dividing ecological landscapes – our roads create barriers that keep many species from crossing and can have a significant impact on ecosystems. This is known as habitat fragmentation and it is one of the biggest concerns for biodiversity. Habitat fragmentation can split groups and isolate small, vulnerable or unviable populations. This can be a huge problem, reducing genetic diversity and leading to local or even global species extinction.

RoadFish003But recent research in the Bahamas has found one example of how habitat fragmentation might have actually worked out quite well, for now at least. In the tidal creeks of Bahaman islands, mosquito fish live, feed and reproduce. They get their name from their diet – mosquito fish, unsurprisingly, eat mosquito larvae. Once a year they mate and the females give birth to several broods of live young. They are often eaten by larger fish, and predation is most severe closer to the sea. For the mosquito fish, mating is a very risky behaviour – the chances of getting eaten whilst attracting a female or doing the deed, is very high. So evolution has opted for the no frills approach, and male mosquito fish behaviour and genitalia is aimed at fast, efficient and inconspicuous mating. Often, this means the female mosquito fish don’t get a chance to offer consent.

In the middle of the 20th century, a number of roads were built across the island, and where these crossed rivers they fragmented mosquito fish populations and isolated some fish with no access to the sea. Suddenly, these mosquito fish found themselves in a much safer world, relatively predator-free. Over the 40 or 50 years since those roads were built, evolution took an interesting turn. With mating no longer a particularly risky activity, the mosquito fish started to evolve genitalia that was focussed not only on speed and efficiency, but also on pleasing the female.

RoadFish002In this new environment, female mosquito fish were able to exercise their choice of when to mate and who to mate with, and this sexual selection led to males with more female-friendly genitalia. In particular, males in isolated fragments had longer and more rounded gonopodia (a modified fin used to transfer sperm – the fish penis, if you will). Females have previously been shown to prefer larger gonopodia (*ehem*), but natural selection limits their size because they increase drag and slow the males down when escaping predators. The male gonopodia also showed a reduction in bony hooks which are thought to anchor the female in place during forced copulation, suggesting that females in the fragmented populations get more choice in their matings. So, for female mosquito fish in the Bahamas, habitat fragmentation seems to have worked out rather well.

The authors of the study point out that human alteration of habitats could lead to speciation events, and the evolutionary responses of animals to habitat fragmentation might be predictable. If the genitals of the two mosquito fish populations change enough, they won’t be able to interbreed anymore even if they meet – the very definition of speciation. However, although this all sounds great, evolutionary adaptations to habitat fragmentation are much harder for species with longer generation times. Not only that, but creating two new species each with half the original population size may not be such a great plan – this is simply reducing the availability of mates and the habitat available and potentially producing two unviable populations.

However, evolutionary change on this timescale is an exciting and interesting discovery, showing the power of natural and sexual selection to shape populations and create new species over just a few generations.

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