#bioPGH Blog: Pumpkins and Zucchini—What’s in a Species?

By Dr. Maria Wheeler-Dubas, Science Education and Research Outreach Coordinator

This time of year, how many of us have gone on a quest to our local farmer’s markets and grocery stores in search of the perfect pumpkin for a jack-o’-lantern? Or picked out the most colorful assortment of ornamental gourds for a centerpiece cornucopia display? Or maybe cooked up a hearty bowl of acorn squash soup with a side of warm zucchini bread? With all of these varied autumn activities, it’s strange to think they all have something very specific in common: the exact same species of squash, Cucurbita pepo!

The same species? we might ask. The same species! Granted, they are different cultivars—varieties that were selectively bred for different traits such as taste or growing conditions. Yet from the long green of a zucchini to the round orange of (most) pumpkins, and numerous squash in between, they are all technically the same species.

This level of harvest time variety hints at a daunting question in biology: how do you even define a species? How do you decide when these living things are different enough from those living things to be considered a new species? It turns out that is quite tricky to answer. Life on earth is incredibly diverse, and defining strict “rules” that apply to all living things is nearly impossible. Thus, biologists have developed what are called “species concepts” to aid in the definition of a species. None of the concepts are perfect (and cultivated plants create a whole extra level of trickiness), but they are interesting topics to explore.

To understand our pumpkins and zucchini as the same species, let’s start with something familiar. If you think of tigers and lions, they look so different that is easy to call them two different species. That assessment would be based on the morphological species concept—if organisms are morphologically similar, they can be called the same species, but morphologically different means a different species. This concept doesn’t always work perfectly, though. Alligators and crocodiles seem similar at first but are ultimately different species; and our pumpkins and zucchini fruits surely don’t look alike but are the same species. Their leaves and flowers are much more similar, but let's try another species concept to be sure. The ecological species concept suggests that a species are a group of organisms that occupy same ecological niche—meaning they serve a particular role in their ecosystem or community. Well, that’s a bit tricky for our cultivated crops, so we can’t really use that concept either.

There are still roughly 24 other species concepts, but let’s jump ahead to the biological species concept. This concept suggests that a species is defined by populations’ ability to interbreed and produce viable (fertile) offspring. Can our Cucurbita pepo squash cross-pollinate? Why yes, they can! You may have heard traditional lore advising you not to grow these squash next to each other or you will have an odd-tasting acorn squash or a zucchini-shaped pumpkin. That’s not quite the case. Rather, what you would find if your garden’s squash plants cross-pollinated is that their seeds (the next generation) would have some odd fruit. This does show, though, that our squash can interbreed—thus fulfilling the criteria of the biological species concept. It also turns out that all of these C. pepo squashes (some pumpkins, zucchini, gourds, pattypan squash, etc.) were cultivated from an ancestral squash most likely in Mexico. A bit of debate over some ancestral details remain, but it is safe to say that this known lineage can also satisfy the evolutionary species concept.

Well, who knew that our favorite fall produce could be so complicated? We barely scratched the surface on defining a species, especially in a cultivated plant, but now we can carve those jack-o’-lanterns with a little extra appreciation of our pumpkin's biological and historic complexity. 

Connecting to the Outdoors Tip: Now is the perfect time to investigate a bit of plant anatomy at home! Even though the various C. pepo squashes look very different on the outside, what traits do they actually share? If you cut open a pumpkin, acorn squash, and zucchini, what do the insides look like? How are they the same? How are they different? How many seeds do they have? A few or many? How are the seeds shaped? What if you compared a zucchini to something completely unrelated like an apple or a peach? How are the fruits all similar? How are they different? Can you guess what traits would help group your pumpkin and acorn squash together as a species compared to an apple or peach?

Resources:
North Carolina State University Cooperative Extension – McDowell County Center 
Missouri Botanical Garden – Plant Finder 
Purdue University Horticulture – Cucurbits Page 
Texas AgriLife Extension Service, Texas A&M System – Squash Named from an Indian Word 
UC Berkeley – Evolution 101 – Biological Species Concept 
UC Berkeley – Understanding Evolution – Other Species Concepts 
UC Berkely – Defining a Species 
National Center for Science Education – Species Concepts in Modern Literature  

Select photos © Wikimedia Users: Tim Sackton CC-BY-SA-2.0 and Danielle Scott CC-BY-SA-2.0