In October of last year David Hillis, a well known evolutionary and systematic biologist at UT Austin whom I admire, published what I saw as a good post about speciation—on Facebook, of all places! I asked him at the time if he minded if I put it in a post here, but I got no response. It turns out that he saw my request only the other day after he was reviewing his correspondence. And he said it was fine if I put it up.
So it’s below, indented. I wanted to post it for two reasons. At the time, I wanted people to understand that the definition of species is different from the features we use to recognize species. The “Biological Species Concept” (BSC), which I use (but not Hillis), defines species as groups of populations that are unable to exchange genes (i.e., produce fertile hybrids in nature) with other such groups. But we recognize species, in general, by seeing groups of animals and plants that maintain their phenotypic distinctness from other groups when they live in the same area (in “sympatry”). As Allen Orr and I explain in our book Speciation, the definition is more fundamental than the recognition because the definition ultimately explains why there are species—why nature is lumpy rather than continuous. Why isn’t there just one kind of bird that is a spectrum ranging from hummingbirds to condors, instead of what we really see: a series of distinct groups of birds? (If there weren’t, field guides would be useless.) The recognition just reiterates that nature is lumpy, but the origin of reproductive barriers between these groups, which involves the definition of species, explains why the lumps exist. (This, by the way, is the real “problem of speciation” that Darwin didn’t solve. It was solved in the 1930s and 1940s by people like Ernst Mayr and Theodosius Dobzhansky). The definition thus leads directly to a research program that explains the lumpiness: what are the genetic barriers between groups and how do they arise?
(By the way, you needn’t tell me that there are gray areas in species delimitation: some species that can produce fertile hybrids. We discuss all the possible exceptions in our book.)
It’s important to note that Hillis uses a species definition different from mine, but as I argue in my book with Orr, the reason that lineages remain distinct from each other when sympatric is due to barriers to gene exchange. (David may not agree, but there you go.)
The second reason I’m posting this discussion is because in many ways the distinction between definition and recognition we see in speciation also applies to biological sex. Biological sex is defined as whether an organism has the equipment to make either large and immobile gametes (females) or small and mobile gametes (males). But in general we recognize the sexes by sex-related criteria that are not quite as binary as the definition itself: genitalia, chromosome constitution, and so on. But I want to make it clear, since some people maintain this claim, that biologists did NOT define the sexes as binary because somehow they wanted —for reasons seen as bigotry—sex to be a binary when it’s really a spectrum. No, sex is defined as a binary because it is a binary in nature: in all plants and animals. (In humans there are a few exceptions, about 0.018%, but that’s close enough to a binary for me). The binary nature of sex is a result of natural selection, but I won’t go into the messy mathematical details about why the condition of two sexes with different gametes is an “evolutionary stable strategy,” and that a third sex, or more sexes, simply cannot invade the system.
And for reasons similar to why the definition of species is more fundamental than the recognition of species, the definition of biological sex is more fundamental than other characters we use to recognize sex. The differential investment of males and female animals in gametes, with females investing far more than males (an investment that continues with pregnancy or laying eggs) is what leads to many of the fundamental differences we see between males and females. Those include, in particular, differences in behavior, size, ornamentation, and other features produced by sexual selection, all of which begin with that differential investment in gametes. (See my paper with Luana Maroja in the Skeptical Inquirer.)
But this second reason arose after I’d already decided I wanted to publish David’s thoughts on species definition (i.e., species “concepts’) vs. species recognition. So here it is:
I see a lot of people, including many experienced biologists, confusing “species concepts” with “methods of species delimitation,” as well as with the study of “speciation mechanisms.” These three areas of study are very different from one another.
Species concepts provide a conceptual explanation of what we mean when we use the word “species.” They are a description of what species are, not how we can recognize them in nature, nor why they exist. For example, I would describe a species as “An ancestor–descendant lineage of populations that maintains its identity from other such lineages over time and space.” That is what I’m talking about when I use the word “species.” I think that is identical or close to what most other biologists mean as well, although they may choose to emphasize the methods they use to recognize species (species delimitation) or mechanisms that produce species when they are asked to define what they mean by “species.” They also might choose different words to describe the same basic idea of lineages isolated through time and space.
My preferred conceptual definition of species (or anyone else’s CONCEPTUAL definition of species) doesn’t say anything about why species remain distinct from one another through time, and it doesn’t say anything about the operations one might use to detect lineages that remain distinct through time and space. There are many different methods that people use to identify and delimit species. There is a lot of evidence (from morphology, behavior, genetics, etc.) that can be used to show that two lineages are (or are not) remaining distinct through time and space. Often, these operations require close examination of contact zones between potential species or geographic variants of species, to determine if they are isolated from one other or not. This diversity of delimitation methods is a strength, not a weakness, of the study of species.
Conceptual definitions of species also don’t tell us WHY species exist. Mechanisms of speciation constitute another important area of study, but this is distinct from species concepts and from species delimitation. In different groups, there are many different behavioral, ecological, and genetic reasons that species remain distinct from one another. The existence of distinct species in life is an observation, but we are still studying all the explanations for this observation.
When people say that there is lots of controversy among biologists about “species concepts,” what they actually mean is that there are lots of arguments about the methods we use to delimit species (species delimitation), or arguments about the relative importance of different speciation mechanisms, or arguments about different conclusions of species delimitation from different analyses. I think there is far less conceptual disagreement among biologists about what we mean by the word “species.”