Spea bombifrons, Spadefoot toad
How do scientists know if an organism is one species or another, and how do they know whether a critter is a sub-species? In short, what defines a species? I started asking scientists at the Florida Museum of Natural History about this when I worked there as a staff science writer. Many researchers would come back from survey trips with what they said were species new to science, or paleontologists would come back from digs with bits of fossil bone that they said also represented new species. In the beginning, I passively accepted their assertions, making careful notes of what features distinguished this newly discovered organism from its close relatives. Then I began to wonder about the boundaries that separate species.
My questions lingered for a time, and were piqued again by two events: writing an article about research showing that two species of spadefoot toads will hybridize under specific environmental and nutritional conditions (pg. 12 in this linked magazine); and researching evidence of hybridization events between eastern Canadian wolves and coyotes. When I peppered him with questions about what defines a species, a scientist friend suggested that I read “Genes, Categories and Species: the evolutionary and cognitive causes of the species problem,” by Jody Hey, an evolutionary geneticist at Rutgers University. {1} My friend said it was the clearest and most thorough treatment of the species problem he’d ever read. It took me about 18 months to follow up on his advice, but I recently finished Hey’s book. Before I go further, I should remind readers that I myself am not a scientist. I am simply a writer who chooses to focus on science topics (though I do have some scientific background), and I have a particular affinity for learning about the diversity of life — past and present.
Hey’s discussion is an elaborate one. He aims the book to both knowledgeable lay readers and academics, so it spans a wide audience. The purpose is not, as he states on pg. 103, to define what species are so much as to explore why we have such “difficulty with the word.” He introduces the species problem by stating,”The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word ’species.’ ” In practice, most species definitions do not work when applied across all the taxonomic kingdoms. What describes accurately species in one group can fail to describe species of another group. Oddly, for a book on species, the text lacks many concrete examples. It is, for the most part, a book about theory, abstraction and cognition. As someone who loves learning about the natural world, I was often left straining to apply Hey’s concepts to a real example. How does this work in frogs, or ants, or bacteria, or leopards, or fossils, I wanted to know.
But nevertheless, I learned a lot from this text. In fact, I’m still going back and reading sections to fully digest them. It is human nature to want to draw tight little boundaries around organisms to delineate where one species ends and another begins, but this often does not work in practice. SPECIES are the most basal grouping of organisms in a taxonomic ranking. (Hey capitalizes SPECIES when discussing the concept of what species are, to differentiate from times where he is discussing an actual organismal category or a specific species.) As much clarity as SPECIES bring for classifying organisms on their evolutionary journey, SPECIES also bring many points of consternation. The “species problem” rears its head when evidence is found that what was thought to be one species are really two closely-related but different species, when a new organism is found that scientists believe to be a new species (why exactly is it new? what exactly separates it from its relatives?) or when two species that were thought to be separate are found to be able to interbreed and have viable offspring. The act of breeding itself turns out to be a quite fallible idea for categorizing species — what to do with organisms that don’t breed, but swap bits of genetic code instead, as some bacteria do, often times crossing that elusive barrier that we thought separated different “species”? Or what to do with wide-ranging species, such as wolves, that have extreme regional variations at opposing ends of their ranges? Is the Arctic wolf in Canada truly a separate species than the Mexican wolf that once ranged through the Sonora desert in Mexico — we call them separate species, but technically they could interbreed. Reproductive capability forms the core of the Biological Species Concept, which is one of the most widely accepted species concepts; and yet despite its failure to explain all species, and particularly when to identify new species and how to address hybridizations that occur in wild nature, it may be the closest thing we have to an all-encompassing definition. Still, it is full of holes and is not entirely operational.
One of the more intriguing aspects of Hey’s book is his idea that our language, and the way we think of “categories,” has contributed to the problem of SPECIES. He explore this idea in Part II of the book, “Species Within Nature and Within the Mind.” At the beginning of this section he writes:
Now we turn to focus more explicitly on language and the way that language corresponds to reality. People use language to further a seemingly endless variety of goals, but is it an inherently reliable tool for building a model of the larger world?
It is an intriguing question, and he probes it by looking at the history of how people have devised and applied categories to describe reality. This leads to a rather philosophical examination of nominalist and realist views. But Hey drives home the relevance by stating that it is necessary for biologists to consider categories of organisms as philosophical categories, and species as the quintessential category. He asserts that categories exist primarily in people’s minds as an organizational tool, and that our minds “draw upon idealized representatives” to form and hold categories. As such, categories are necessary for our own conception of how to organize and classify organisms, but even though they are necessary we have to also recognize that they limit our understanding of what species are in nature, and how they change across time as they are exposed to evolutionary pressures. As taxing as this chapter was to read, it had the effect of freeing up my thinking about how scientists think about species. If that’s not Meta, I don’t know what is.
Hey does a good job of sorting through different species concepts and presenting the logic that underpins each, or where the theory described in one concept falls short of observed reality. For example, there are all sorts of “fuzzy species,” as Hey terms them, that defy one given definition or another. Even more frustrating for those who would have a single definition, simple classification takes on a whole new twist when the dimension of geologic time is added. Now, it’s not just a matter of trying to delineate lines between the species we can collect and observe today, it’s also a matter of drawing lines in the past as to when one species became another or branched off into a different lineage. In short, getting at the evolutionary processes that create and maintain species adds a complicating layer to trying to fit species into tightly bounded categories, or within single definitions. The act of conceiving of SPECIES as organisms that can be described by their physical characteristics, their diet, behavior and range — even their genetic code — is quite a different concept than conceiving of species as the basic unit of an evolving population. In the first concept, SPECIES are somewhat static and discrete; in the second, SPECIES are continuous and dynamic. If species are viewed as continuous, it becomes much more difficult to draw a line in the sand between “fuzzy species” to delineate a parent species from a newer, different species.This really gets at the crux of the species problem, as Hey sees it: the dual conflict between biologists to describe and identify species versus the role of systematists to get at the evolutionary relationship of categorized species. In the first, there is implied stasis; but the second demands a dynamic perspective.
One of the more difficult chapters for me to read came in Part II and dealt with species as evolutionary groups. Whereas most discussions about evolutionary groups are cast in terms of species and species diversity, Hey frames his discussion in term of molecular replicators (DNA specifically). When I read, my mind works cinematically — words on the page are converted to pictures on a movie screen — and so it was difficult to get a good mental picture of how molecular replicators related to an evolutionary group. One way of thinking about this is that you can look at evolutionary groups at a molecular level (an organism’s DNA), or at the phenotypic level (the species). I found myself struggling a bit to keep up with the abstraction of “DNA replicators” in lieu of “species” but after a chapter or so it came more naturally. Hey wrote that the potential for species to change occurs at the molecular tips of phylogenetic trees based upon selections for and against DNA replicators under limited conditions. I’m familiar enough with evolutionary trees, but I’ve never before conceived of them at the fine-resolution scale of the DNA at their very tips. One inference you can make from this is that phylogenetic trees are not static, in the sense of speciation, and that if change is going to happen, the mechanism of natural selection occurs at an organism’s molecular level. In this way, species may be the smallest unit within an evolving population, but natural selection works at the molecular level within individuals of that group. He discusses the roles of genetic drift, geographic isolation, recombination and the emergence of barriers between species to get at how different mechanisms can lead to phylogenetic branching.
One of the more interesting abstract ideas in Hey’s book dealt with the role of nested hierarchies within taxon, and the idea that patterns in biodiversity may be fractal. He wrote:
One common view is that the SPECIES category should hold only basal taxa. Another common and closely related view is that species are terminal taxa, meaning that they lie at the tips of evolutionary trees that are used to connect species. But if the hierarchical patterns of biodiversity are actually fractal in nature, then there may not exist patterns that could be used to devise a truly basal taxon, or a truly terminal tree tip. If biodiversity is fractal, then we would expect that one could always find a finer pattern within a pattern, a smaller group within a group, except in the not very useful limit wherein basal taxa include just one organism.
This statement perplexed me to the point that I read it and re-read it. I couldn’t help but think about regional variations in species, haplotypes, hybrids, virus strains, and other sorts of classifying characteristics that parse species groups (evolutionary groups) into smaller and smaller sub-categories. What if biodiversity is fractal? How do you ever come to a point where you can quantify what species are? This is really important, because quantitative science is about counting and measuring: recording how many of something exists, how frequently something occurs, and how much something eats or excretes. And so it is with species, biologists like to be able to count them too. But herein lies another perplexing characteristic of species in that they can be really hard to count. Not because they are hard to find, but because they are hard to classify with crisp edges. And one of the things that makes those edges less crisp, according to Hey, is recombination. He explores the idea of recombination acting together with isolation by distance, competition and genetic drift to reinforce hierarchically nested evolutionary groups that often have fuzzy boundaries.
In Part III, Hey explores how sub-divisions within biology deal with the species problem in their profession, whether it be people who study phylogenetics, sytematics or evolutionary biology. He concludes on a hopeful note, remarking that while there is still much confusion over what SPECIES are, there has also been a lot of improvement in how biologists think of SPECIES. Hey writes:
I think the reason for our monism* is that the bulk of our usage of SPECIES is not motivated by an understanding of the processes that cause species. Rather, it is caused by the way we construct categories or organisms, and thus it is ultimately motivated by recurrence among organism. [*Monism being the idea that there is, or should be, just one kind of species.]
This is a nicely crafted way of getting at the idea of species as organisms that we recognize with similar attributes (something that fits nicely with how we conceive of categories) versus understanding the evolutionary processes that create and maintain species across time and space. And this also gets to the problem of why systematists have problems matching species up with categories of evolutionary groups.
Scientists like Hey have expended a tremendous amount of energy trying to understand what species are. Hey concludes that one definition is not be tenable, in part because our minds are somewhat limited in how we conceive of species, versus what they are in reality, and also because we not only want species to describe similar groups or organisms, we want species to explain evolutionary groups and how they change through time. he asserts that they can not do both. It’s not a conclusion that he came to easily. In fact, it took him 193 pages to argue why a single definition is impractical, and why a definition can not serve both ends. I learned a lot from this book, and while I would not recommend it for the faint of heart, I would recommend it for those with a scientific background or a serious commitment to learning about how species are defined and why it is so difficult to draw firm lines around their fuzzy edges.
If any biologists or systematists read this post and have additional thoughts on the species problem, or species concepts, please share them. Your comments are welcome here, especially if they increase my knowledge on this subject.
NOTES:
{1}Hey, Jody. 2001. Genes, Categories and Species: the evolutionary and cognitive causes of the species problem. Oxford University Press, Inc. New York, N.Y.
{2} Visit Dr. Hey’s online resource: The Species Problem which includes links to relevant peer-reviewed publications, some more recent than his book.
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