Do you find something wonderful about the sheer diversity of living things on our planet? If so, you are emotionally involved, at least a bit, in the phenomenon of “biodiversity”. Anyone who is soon encounters the word “species”. The word and its meaning are crucial to thinking about biodiversity. With this word, however, two questions arise immediately. First, what exactly do biologists mean by the word “species”? Second, how many species of living things are there?
These may look like very separate matters. The first looks like a simple matter of definition, probably a simple one. The second question looks like a matter for research, a question of enumeration and tabulation of all the different kinds. That certainly takes a great deal of work to answer but it sounds relatively straightforward.
Yet, that impression of simplicity and straightforwardness is misleading. The two questions are deeply entwined. If one has a definition of the word “species”, you must look at living organisms to see which ones fit the definition and roughly, what percentage, do not. If the latter percentage is large, something must be wrong with the definition. And, for the enumeration task, you need one or more strict criteria for a potential species’ inclusion or exclusion, given by the definition. All this may seem a rather dry academic discussion about terminology but I will try to show why it is not and why it matters.
Let us start with the definitional matter. What could be the problem? “Species” seems a simple word – only seven letters – and it has been in use in English for centuries. (Of course, we have seen an even simpler word, “gene”, which is only four letters, that embraces all sorts of ambiguity and complexity; see this newsletter, no. 5). Generally, “species” denotes a very particular type of something within a broader category. Earlier, it was often used metaphorically. For instance, one might say something like “Bob was of that species of man who saw nothing wrong with cheating people out of their money.” This does not mean that Bob was not a member of our species, a human being, just that he was a particular nasty sub-type of human of a particular kind.
Today, however, “species” is primarily used in its biological sense, to denote a particular type of organism of a very narrow kind, as opposed to a larger category such as insects, frogs, clams, etc. The beginning of scientific discussion and accounting of species in this sense was the work of Carl Linnaeus (1707-1778) to classify and name as many distinct types of living organisms as he could. He recognized that animals and plants often came in clusters where a relatively small number resembled each other but not so closely as to be considered the identical type. In effect, he posited that species could be placed in these larger groups of resemblance, which he termed “genera”. He gave each distinct type the designation of species and a double name, the first being the genus name, the second the species designation. Hence, our species – which he recognized as an animal species – was Homo sapiens.
For Linnaeus, it was not difficult to distinguish and name the different species, as based purely on physical resemblance. If two individual organisms, for instance two different grey squirrels, looked essentially identical, they were both deemed to be members of the same species. There might be slight individual differences in some of their traits but their essential sameness was what was emphasized.
In doing this work, Linnaeus viewed species as essentially fixed in their properties, from presumably, the Creation of the world as described in the Bible. Alteration of organisms over time did not come into the picture. Of course, that changed with Darwin’s evolutionary opus in 1859, The Origin of Species or, to give it its full name, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. If one accepted Darwin’s basic idea, new species could come into existence by a natural process and, indeed, every species of plant or animal on Earth had done so.
Also, the evidence from fossils, which was far more abundant than in Linnaeus’ time, made it a near certainty that species could die as well. In this view, a species could be seen as having a life cycle, from birth to death, with lots of change in-between. The wonderful stability of the Linnaean view of life had vanished if one accepted the Darwinian view of evolution. How then was one to think a species’ characteristics if it could change with time?
Despite the apparent focus on species implied by the title of Darwin’s book, Darwin himself did not spend much time in it discussing the nature of species. Furthermore, since much of his idea about evolution went into relative eclipse in the late 19th century and the first part of the 20th century (an interesting story to which we will return) the problem of not having a clear definition of the word species also took a back seat in biological discussion in this period. But as Darwin’s ideas made a comeback in the 1930s and 1940s, discussion in biology of what we may call the “species problem” also resumed and with vigor.
Nor has that intellectual ferment really stopped. At least ten different ideas and definitions have been proposed in the last eighty or so years. They are not radically distinct but sufficiently different via different emphases of what is deemed most important. The key unifying feature is that all try to explain the quasi-stability of species, at least over centuries and millennia. To do that, one needs the idea of “populations,” namely interbreeding groups of the “same” kind of animal or plant, whose properties are maintained over generations of reproduction. Such groups must have a large measure of genetic similarity, with such individuals mating primarily or exclusively amongst themselves; they do not mate, or “hybridize”, with members of other species. Mating only with members of their species, genetic similarity will be strongly maintained over generations. This allows for slow genetic change over time while maintaining the near homogeneity of the populations of that species.
This is the basic idea of the “biological species concept”, first formalized and enunciated by Ernst Mayr (1904-2005), one of the major figures of 20th century evolutionary biology, in 1942. Despite some differences with some of the other concepts, this has been the closest to a consensus view of what species are for the past 80 or so years. There are, however, two major problems with it, neither of which has been hidden but simply somewhat downplayed.
The first is that it only applies to eukaryotic organisms, namely those whose cells have a true nucleus and which perpetuate themselves by sexual reproduction. This includes all the animals, all the plants and all the fungi, and the protozoa. It excludes all the organisms known as prokaryotes, those lacking a true nucleus and sexual reproduction. The latter, however, are often given species names according to the formula of Linnaean taxonomy. Prokaryotes, or at least many, have a form of reproduction that involves receiving genetic material from other cells but it is relatively small pieces. This is called “horizontal transfer” and it is very different from true sexual reproduction, which involves combining two full sets of genetic material, one from the maternal parent, one from the paternal parent. Such exchange has barriers but there is nothing like the general restrictions between eukaryotic species, and which have been posited to be essential to maintaining species identity.
The result of this much looser connection in prokaryotes between a species identity and its genetic content is embodied in the term “pangenome”. The pangenome is the total set of genes that can be found in a prokaryotic species. For example, take Escherichia coli, which is a common resident in the human gut as part of its microbiome (see no. 6 in this series) and which for several decades was a major experimental workhorse of genetics. The normal circular chromosome of E. coli contains roughly 4000 genes and a bacterial cell that contains those genes is recognized as a cell of E. coli in good standing. But such cells often contain genes of other bacterial cells of other “species”, with the estimated number of these being on the order of 14,000 or so. Those “guest genes” plus the 4000 normal genes of E. coli add up to a total of about 18,000 genes and that is the pangenome of Escherichia coli. For any eukaryotic species, its genome content, as found in all its cells, and compared between individuals, is far more uniform, in its number of genes and their identity. Thus, the “biological species concept” simply does not apply to prokaryotes. There may well be many millions of kinds of prokaryotes but they are not species in the sense that eukaryotes are.
The second problem with the biological species concept is its central premise that true species do not breed with each other. Now, in fact, it has been long been recognized that certain species can mate with each other and produce viable hybrid offspring. Yet, often the offspring, in particular the male offspring, are sterile. That means they cannot themselves reproduce. Such sterile hybrids are an evolutionary dead-end. The mule, a common working animal on farms for centuries, is the perfect example.
Yet, growing evidence over at least the past ten years has greatly expanded our knowledge of species that can mate with each other and produce hybrids that are not only viable but fertile as well. This is especially true for plants where it is now estimated that probably more than 50% of all contemporary plant species actually arose from hybridization events between members of what are or would have been considered separate species. This is less true for animal species but more and more evidence is coming forward that new animal species can arise by hybridization between members of different species. Reproductive isolation between different species is a reality but it is now believed to be much less stringent than had been posited earlier. When a central element of a definition falls, one has a clear warning that the definition is ready if not for the scrap heap than at least for serious modification.
We now come to the second question posed at the beginning: how many species (genetically distinct forms of life that persist over long time-spans) are there on planet Earth? There are, of course, estimates of previously identified, species of both animals and plants. There are roughly 1.2 million identified animal species, most of them invertebrates (vertebrates comprise only about 50,000 known species) and about 320,000 million known plant species (angiosperms and gymnosperms). There are also an estimated 6,000,000 species of fungi. The numbers could be vastly more, however. One estimate is that there are over 8 million animal species, mostly insects and other invertebrates, in jungles and other areas remote from civilization.
As for prokaryotes, the number is simply unknown because of a) the severe problem of definition described above and b) simply lack of information. In general, there is the most information about species numbers in North America and Europe because those are the continents that have the most scientists doing such work and it is easier to do this work on one’s home continent than far away (South America, Africa, Asia). In addition, there are large areas of our planet that simply have not been surveyed adequately because of inaccessibility, such as large parts of the ocean, which covers about 60% of the planet, and Antarctica, which was long regarded as a comparative desert in terms of life forms but has proved to be far more species-rich than previously suspected.)
It is clear that the question of total species number is currently unanswerable. Is this important? Probably not in itself but the process of obtaining the numbers would be invaluable: collecting that information would simultaneously provide information on which species are found in which areas. And that, in turn, would provide insight into the actual ecosystems in each area and how they function. Further monitoring would then provide valuable data on which species are increasing, which are declining, and what new species (“exotics” from other areas) are coming in to a particular region. With the world’s ecosystems under multiple threats, from climate change and various forms of environmental degradation, we desperately need that information to preserve as much of the natural world as possible, for our sake and for future generations and (in my opinion) simply because it is the right thing to do.
And with this thought, we come to the major importance of biodiversity studies, the cataloguing of species richness on our planet. If we want to preserve our civilization, we need to preserve our natural world, or as much of it as we can, and that requires an accurate census of its biological members. We will return to this issue soon.
Supplementary Reading:
Mayr, E. (1942). Systematics and the Origin of Species. Cambridge: Harvard University Press This is the early 20th century classic statement on what animal species are and how they form.
Mayr, E. (1963). Animal Species and Evolution. Cambridge: Belknap Press, Harvard University Press. This is an updated, informative and rich treatment of the species problem, from Ernst Mayr.
Coyne, J.A., Orr, H.A. (2004). Speciation. Sunderland, MA: Sinauer Associates. A detailed, 21st century look at the problem by two eminent evolutionary biologists.
There are, of course, numerous accounts of species numbers in different groups in the literature and one is encouraged to look. Remember, however, that whatever numbers are given, they are almost certainly underestimates.