Guppies, Guppies Everywhere!

Guppies (Poecilia reticulata) are one of those species of fish that are all over the place. Walk into just about any pet store with live fish for sale. You’ll probably find a display of guppies. Go snorkeling in a freshwater stream somewhere near the Equator. You may find guppies there, too. Take a close look at one of Rio de Janeiro’s sewers. You’ll find that guppies even live there quite happily!

But even though these are the same species of fish, they may have very different traits. For example, they might have different breeding patterns or different diets. Let’s look at how and why this happens.

What are some examples of local adaptation in guppies?

Guppies are abundant and widespread. There’s a lot of them, and they live in many different places and environments. Guppies in different environments need specific characteristics, or traits, to live in those environments.

The development of these differences is a process that ecologists call local adaptation. In this process, populations adapt to their environment in a way that will let them survive. These adaptations may eventually limit breeding between populations. In other words, these adaptations may lead to speciation, the formation of new species.

For example, some guppies coexist with (live in the same streams as) their predators. Others do not. Ecologists (scientists who study interactions between organisms and their environment) have found that these two categories of guppies have different traits.

One guppy predator is a type of fish called the cichlid. Some cichlids that live alongside guppies have features that make them fierce predators. For example, they have long bodies that let them swim quickly. They also have strong, fast-moving jaws that can help them catch more prey.

With cichlids around, individual guppies are less likely to survive to adulthood. Guppies that coexist with cichlids have strategies to make sure at least some of their offspring survive. They start reproducing earlier, produce many offspring and reproduce often. Ecologists call animals like this r-strategists. This reproductive strategy often occurs in environments with heavy predation.  

How do different guppies impact their environment?

Guppies that coexist with cichlids also impact their environment differently than those that do not. That’s because these two categories of guppy have different diets. Guppies that coexist with cichlids usually do not live near many other guppies. This way, they do not have to compete with other guppies for food. These guppies get access to the best quality food, like midges and other invertebrates. 

Meanwhile, guppies that don't coexist with cichlids live at much higher densities. They have to compete for food with many other guppies, so they cannot be picky! They have a much broader diet that includes microorganisms and decomposing (rotting) organisms.

This is really important. It means that predator-prey interactions can change the traits of a population, and vice versa. In other words, ecological processes (the flow of matter and energy) and evolutionary processes affect each other. Biologists used to think that evolutionary processes took thousands of years and ecological processes happened much more quickly. Discovering that the two processes affect each other was an exciting breakthrough!