A Double-Edged Sword

by James Morris
Vegetables from Russo’s in Watertown, Massachusetts

What do cabbage, broccoli, cauliflower, kale, Brussels sprouts, and kohlrabi all have in common?

Brussels SproutSure, they are all vegetables. Yes, your parents probably made you eat them when you were a child. But there’s something more.

What they share is something that might surprise you – they are all the same species. This means they are not just closely related, or all in the same family. They are all Brassica oleracea.

KaleWhy then do they look so different from one another?

It’s because they have been bred over time to be that way. Farmers selected different traits from wild cabbage – the common ancestor – and the results today are as different as a Great Dane is from a Chihuahua. They are in fact the plant equivalents of dogs – many different breeds, wildly different from another, but all one species.

BroccoliHow can they all be one species if they look so different from one another? Aren’t members of a species supposed to look the same?

Not necessarily. We have many examples of individuals of one species appearing quite different from one another (think dogs), as well as individuals of different species looking similar, sometimes indistinguishable from one another (many species of birds and butterflies, for example).

KohlrabiWhat criterion, then, do we use when deciding whether two individuals belong to the same species? Not appearance. Not habitat. Not even DNA sequence. Today, we usually define a species on the basis of reproduction: members of the same species can interbreed and produce fertile offspring; members of different species generally can’t.

CabbageAlthough cabbage and kale may be unfamiliar, other examples of artificial selection are very well known, such as dogs and cats, horses and cows, apples and corn.

RomenescaEven in Darwin’s time, artificial selection was familiar to the public. But it wasn’t cats and dogs that came to mind (or even kale). Instead, it was pigeons. Pigeon breeding was popular in the mid-19th century, and there were literally hundreds of different breeds of pigeons, each one different from the next.

So, when Darwin sat down to write On the Origin of Species, he didn’t start off with examples from the far away Galápagos, or South America, or Asia. Instead, he began with something familiar and right at home – pigeons: “Believing that it is always best to study some special group,” Darwin writes, “I have, after deliberation, taken up domestic pigeons.”

He describes how different one breed is from the next, including the English carrier, short-faced tumbler, runt, barb, pouter, turbit, Jacobin, trumpeter, and laugher. “The diversity of the breeds is something astonishing,” he writes with characteristic understatement.

He notes that all of the various breeds can be traced to a common ancestor – the rock pigeon. Then, he walks us through the process by which pigeon breeders are able to shape pigeons to various ends to create all kinds of diverse forms. Pigeons vary one from the next. A breeder might notice ones with slightly longer tail feathers and breed these. Over time, a new breed of pigeon with conspicuous tail feathers is formed.

Why did Darwin begin this way? Pigeons were familiar, and pigeon breeding was popular. Instead of starting with complex or unfamiliar topics, he started with something that everyone knew, that everyone accepted, that was not controversial even at the time.

But even more importantly, the process by which all of the various breeds were created is analogous to the process of natural selection. Chapter 1 is essentially an analogy – his entire theory in a nutshell.

But, like all analogies, there are limitations. In artificial selection, there is a breeder – a human directing the process. In natural selection, there is no “selector” guiding or overseeing the process. That’s one of the fundamental ways his theory breaks with the past.

In addition, in artificial selection, the breeder has a goal in mind – leafier cabbage, a faster greyhound. But one of the central ideas of evolution by natural selection is that there is no goal, no end in mind. Birds don’t evolve wings “in order to fly.” There is no forethought or intentionality in the process.

So, analogies are useful, but only up to a point.

In science teaching, we use analogies too. We call mitochondria the “powerhouse” of the cell. In this way, we describe a complicated structure involved in energy metabolism in a way that is readily understood. Or, we call ATP the energy “currency” of a cell.

In physics, we have a ball deforming a rubber mat to represent the effect of gravity on space-time. Or “strings” to represent fundamental elements.

These analogies are helpful, but they have limits too. Consider ATP as energy currency. It’s useful because it calls our attention to the fact that ATP is a packet of energy used in all kinds of daily transactions in the cell. But currency, like a dollar bill, doesn’t actually hold any real value – it merely represents value that we all accept. By contrast, ATP doesn’t represent energy; it actually holds energy in its chemical bonds.

We also use visual analogies. In many textbooks, ATP is shown as a burst of light, a lightning bolt, or the sun. In this way, the authors hope that students will connect ATP with energy. But these visual short-hands can also lead students down the wrong path. Students sometimes ask why ATP is solar power, and they point to the starburst as the source of their question.

In physics, analogies are similarly a great place to start, but eventually you have to do the math.

Emily Martin, an anthropologist at New York University, brings up other issues with analogies. She argues that analogies can not only reveal, but also reinforce stereotypes. The egg and sperm are often described in terms that reflect gender stereotypes: The passive, nurturing egg and active, powerful sperm; the immune system uses language of warfare – attack and defense, self and non-self.

These kinds of stories not only provide a window on our biases, but also limit the kinds of questions we ask. We might ignore the active role that the egg plays in fertilization, or miss beneficial aspects of bacteria that live in our guts.

Analogies like these are essentially stories. And everyone loves a good story. We are, after all, storytellers. Stories help us understand and make sense of the world around us. From Greek myths to fairy tales to our own personal journeys, we often frame what we see and experience in the form of a narrative.

Stories can be very powerful. They engage and involve us in very deep and meaningful ways. They help us connect with others. Who, for example, doesn’t get choked up by the ending of It’s a Wonderful Life or the beginning of Up?

We might convey the process of discovery in the same way that a mystery novel unfolds, complete with false starts, dead ends, and that moment (or moments) of insight. But, we need to take care that we don’t turn science into fairy tales: Newton wasn’t hit in the head by an apple when he “discovered” gravity, and Darwin didn’t arrive on the Galápagos and exclaim “Eureka!”

Stories can be compelling, but we have to be careful that they don’t trump the truth. In effect, stories can cut both ways, which perhaps we should reserve for vegetables.


Photo credit: Dan Perlman

© James Morris and Science Whys, 2016

1 thought on “A Double-Edged Sword

  1. Sam Richardson

    Thanks for the delightful post. I never realized that all of those vegetables are the same species, molded in different ways to bring out different tastes and textures. And I love the way you connect that simple observation to the dangers and pitfalls of analogies in science. Nice job!


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