Biology is beset with deep conceptual puzzles, and biologists do need to take them seriously, even — or especially — when they admit to no clearly best solution.
Philosophy, Who Needs It?, Ford Doolittle
In his essay musing on what, if anything, a zebra might be, the paleontologist Steve Gould asserted that there is no such thing as a fish. A lungfish is more closely related to a bird, or a human, than it is to a trout. If taxonomic grouping is to be cladistic — if it is to derive in a regular and objective way from a branching evolutionary history — then fish do not exist.
About 20,000 species of vertebrates have scales and fins and live in water, but they do not form a coherent cladistic group. Some — the lungfishes and the coelacanth in particular — are genealogically close to the creatures that crawled out on land to become amphibians, reptiles, birds, and mammals. In a cladistic ordering of trout, lungfish, and any bird or mammal, the lungfish must form a sister group with the sparrow or elephant, leaving the trout in its stream. The characters that form our vernacular concept of "fish" are all shared primitive and do not therefore specify cladistic groups.
What, If Anything, Is a Zebra?, Stephen Gould
In their essay musing more generally on what, if anything, a ‘natural kind’ might be, the philosophers A. Bird and Emma Tobin assert that there is no such thing as a reptile. A crocodile is more closely related to a bird than it is to a lizard. Even if natural divisions in types of organisms exist, the division into reptiles is not one.
[B]iologists now believe that reptiles do not form a natural kind. Naturalism is the view that there are in fact natural divisions among things, so that when we attempt to make a natural classification, there is a fact of the matter as to whether that classification is indeed genuinely natural.
Natural Kinds, Alexander Bird and Emma Tobin
In his essay asserting what a species is — “if anything” musings are not his style — the ornithologist Ernst Mayr says that ‘species’ is not an arbitrary man-made concept, but refers to a concrete phenomenon of nature. He happens, though, to use ‘planet’ as an example of a phenomenon so concrete that it could not be arbitrarily redefined.
Some recent authors have dealt with the concept of species as if it were merely an arbitrary, man-made concept, like the concepts of reduction, demarcation, cause, derivation, prediction, progress, each of which may have almost as many definitions as there are authors who have written about them. However, the concept biological species is not like such concepts. The term 'species' refers to a concrete phenomenon of nature and this fact severely constrains the number and kinds of possible definitions. The word 'species' is, like the words 'planet' or 'moon,' a technical term for a concrete phenomenon.
What is a Species, and What is Not?, Ernst Mayr
The realist versus nominalist debate about species is often regarded as different than the realist versus nominalist debate at higher taxonomic levels. The positions that ‘species’ is less real, or more real, than other taxonomic levels, are both taken. Mayr, a species realist, also asserts that some higher-level taxa, at least, are exceedingly real. He cites hummingbirds and penguins as examples. The line between a hummingbird, or a penguin, and any other bird, is clear and well defined.
Such taxa as the hummingbirds, anthropoid apes, or penguins are exceedingly "natural" or "real" (that is, well delimited), and yet the categorical rank given them is subjective, at least for taxa above the species level.
The Growth of Biological Thought: Diversity, Evolution, and Inheritance, Ernst Mayr
The physicist Richard Feynman, at least by customary attribution, once said that the philosophy of science is as useful to scientists as ornithology is to birds. Many varieties of biologist, including ornithologists, however, seem quite fond of debating the nature of existence.
The notion of existence, or the meaning of the term "exist," is a central question of philosophy (Williams, 1981). It is therefore not surprising that it forms a point of argument for proponents of different philosophies of taxonomy, although here the idea of "existence" has been replaced by that of what does, and what does not, constitute a "real" biological (taxonomic) group[.]
On Groups, Existence and the Ordering of Nature, Ian R. Ball
The story of our recently-increased understanding of the origin of birds is entangled in a messy and often heated philosophical debate. We have come, in recent years, to think of birds as being dinosaurs. This change in our way of thinking comes at least as much from a change in philosophical preferences regarding taxonomy, as it does from an increase in scientific knowledge.
Gould took his title questioning the existence of zebras from a 1957 essay by Albert Wood, questioning the existence of rabbits. The rabbit is the mother of all “what, if anything” musings. We have to figure out the difference, if any, between rabbits and hares, in two different ways.
Say that we admit the existence of rabbits and hares. Albert Wood said he was forced to accept their existence, reluctantly, basically because everyone else did. This is a pragmatic rather than a realist position on rabbits, and perhaps a social constructionist one.
I have met no one who questions the existence of hares and rabbits, and I have been reluctantly forced to accept them.
What, If Anything, is a Rabbit?, Albert E. Wood
By “pragmatic” I mean this: in his (in)famous essay about what if anything is a rabbit, threatening intellectual riot that there really isn’t, Wood settles down after only two paragraphs to carefully marshal evidence about what they are.
But for the objections to their existence, first, say that we admit the existence of rabbits and hares, and think that hares are not rabbits. If a rabbit and a hare should mate and produce offspring, what exactly is that?
And second, if we can think of a division into the rabbits and the hares, but also consider a hare as a subtype of rabbit, then hares are unambiguous, but what, if anything, is a rabbit? Are they the larger grouping, or the small? If they might be either, depending on context, are they real?
Generalized, these are a sibling problem, a question of gradations; and a parentage problem, a question of hierarchical nestings. There is also a question of gradations over time.
In the circa-1900 conception of what a reptile might be, above, crocodiles are presented as a central, prototypical example of a reptile. Frogs and turtles are reptiles as well. Birds, however, are not.
Looking at the tree to the right, though, showing the relation of crocodiles and birds, a problem might might been seen in this. If the crocodiles are the prototypical reptile, then birds are closer to the reptilian prototype, than any other living creature.
The problem is more philosophical than scientific. They knew about the relation of birds and crocodiles in 1900. The concept of an Archosaur, the group joining the two, comes from Edward Drinker Cope, in 1869. He nonetheless uses the class names Reptilia and Aves, from Linnaeus, putting the two at equal levels.
[T]he Dinosauria present a graduated series of approximations to the birds, and possess some peculiarities in common with that class, standing between it and the Crocodilia.
Synopsis of the extinct Batrachia, Reptilia and Aves of North America, Edward Drinker Cope
Ten years after the publication of the Origin of Species, taxonomy was not yet taking much account of the theory.
In the Origin of Species, Charles Darwin had foreseen that classification would come to be based on genealogy. A section on his views on classification starts with the observation that defining a number of characteristics common to all birds is easy, but that doing so for crustaceans had been found impossible. What a crustacean is, has a sibling gradation problem. What a bird is, does not.
Our classifications are often plainly influenced by chains of affinities. Nothing can be easier than to define a number of characters common to all birds; but in the case of crustaceans, such definition has hitherto been found impossible. There are crustaceans at the opposite ends of the series, which have hardly a character in common; yet the species at both ends, from being plainly allied to others, and these to others, and so onwards, can be recognised as unequivocally belonging to this, and to no other class of the Articulata.
On the Origin of Species, Charles Darwin
In 1843, Darwin had written to the naturalist George Waterhouse. In classification, what do you mean by “relationship”, he asked. Waterhouse said, it was vague.
Darwin, for example, had to ask G. R. Waterhouse in 1843 what he meant in his letters by “relationship” in classifying organisms. Waterhouse replied “by relationship I mean merely resemblance—I use it in a most vague way—” and he differentiated a relationship of affinity (similarity shared by members of a “natural” group) from one of analogy (convergent similarity). For Waterhouse, relationship was purely a question of degree of “resemblance”, a meaning that 20th century systematists would not accept because it does not strictly presume a genetic basis.
Charles Darwin’s Views of Classification in Theory and Practice, Kevin Padian
In the Origin, Darwin says that by basing classification on genealogy, such vague metaphorical language as “relationship”, would be made real.
The terms used by naturalists of affinity, relationship, community of type, paternity, morphology, adaptive characters, rudimentary and aborted organs, &c., will cease to be metaphorical, and will have a plain signification.
We get to a modern meaning of the word “systematics” as related to classification here. The natural system is founded on descent with modification, and pre-Darwinian classification had been unconsciously seeking to expose this hidden underlying system.
All the foregoing rules and aids and difficulties in classification are explained, if I do not greatly deceive myself, on the view that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike.
But the pre-Darwinian taxonomic style would still be in use not just ten years, but a full century later. When John Ostrom published the idea of a theropod origin of birds, in 1975, for example, he was still talking of Aves and Reptilia being equal-level classes, the same as Edward Drinker Cope had done back in 1869. The practice of basing classification on genealogy was very slow in taking hold.
Although Ostrom suggested that birds were the direct descendants of dromaeosaurs, he used traditional classification to separate these two groups; he placed the birds in their own class, Aves, and dinosaurs in Class Reptilia, contrary to Huxley's grouping. However, his former student, Robert T. Bakker (1975), concluded that birds are actually theropod dinosaurs, not just descended from them. Like Huxley, Bakker argued that birds should not have their own separate class because they fly. Bats fly, too, but they are still considered mammals. Birds are as much dinosaurs as bats are mammals.
The Rise of Birds, Sankar Chatterjee
An argument over stricter versus more flexible alignment of taxonomic branching to evolutionary lineages has been going on at least since Ernst Mayr took on Willi Hennig, and then Willi Hennig took on Ernst Mayr, at about the time of Ostrom’s paper.
Various names are used for the two schools of thought. Evolutionary systematics, associated with Mayr, versus phylogenic systematics, associated with Hennig, has the advantage of parallelism. They can be thought of as population thinking versus tree thinking. The phylogenic school will also often be called cladistics.
Monophyly, what it is and how strongly to desire it in classification, is a part of the debate.
Most definitions of the term monophyly are ambiguous because they do not limit the common ancestor sufficiently. Simpson's (1961) definition does not define monophyly; rather, it attempts to define a nameable taxon. Hennig's (1966a) definition, which requires a group to contain all the descendants of the common ancestor, is precise and useful, but it is narrower than the classic concept of monophyly and is here named holophyly. One way to limit the common ancestor is to require that it be a member of the group of which it is the stem, but this would seem to require a macroevolutionary appearance of groups.
Monophyly and Associated Terms, Peter D. Ashlock
This debate has been quite heated. The 1971 paper I’ve just quoted — blandly, I thought — provoked a furious response, with the argument over it still going on 40 years later. The philosopher David Hull has written a book about the debate.
Much of the debate concerns portions of the phylogenic tree of the reptiles, as shown below. When philosophies of taxonomy are contested and fought about, birds, crocodiles, dinosaurs, and reptiles are popular grounds for the fighting.
In December 2014, the Avian Phylogenics Project published 45 whole genomes and 28 scientific papers. A relevant finding for purposes here, is that the genome of modern crocodiles shows much slower change than the genome of modern birds.
They found the crocodile had the slowest rate of molecular change of any known vertebrate genome.
“The DNA in the modern-day crocodile has changed a lot less, versus the archosaur, than it has in birds,” said [David] Haussler.
By comparison, bird DNA took flight. Their pace of molecular evolution accelerated, and within about 15 million years of evolution, birds radiated out into the bulk of the existing 36 modern avian orders.
When It Comes to DNA, Crocodiles and Birds Flock Together, Geoffrey Mohan
In the school of classification associated with Ernst Mayr, a large change through time without branching, called anagenesis, and with birds as a prime example, is taken account of. Reptiles, or dinosaurs, are not a clade, but a grade, and birds are not part of them.
That we have become quite comfortable calling birds dinosaurs, in recent decades, is an example of this style of thought in decline. It is not only from an advance in scientific knowledge, that we have come to do so, but from the move towards cladistics in taxonomic philosophy. The older style has not entirely disappeared, though. It is still very much in use, perhaps especially, for many taxonomic groupings involving humans.
If our desire for monophyly is strong, if we wish to avoid arbitrarily-created paraphyletic groupings, then what has been done with humans, all up and down the taxonomic ranks, is a problem. Humans are repeatedly made exceptional.
We generally acknowledge ourselves as being apes, but we do not classify ourselves as monkeys. A paraphyly has been invented, that carves out for us a little dignity. Monkeys have tails, we do not.
Being simians, we do not classify ourselves as prosimians. Another paraphyly has been invented, that carves out for us a little dignity, again.
The reptiles have one carve out, the birds, up in the branches, and depending on where reptiles are grounded, maybe also the mammals, down at the trunk, making ourselves exceptional yet again.
Being tetrapods, we do not consider ourselves to be bony fish. We do not consider ourselves to be lobe-finned fish. We do not even consider ourselves to be lungfish-like. We do not generally think of ourselves as being fish of any sort.
We will not generally admit to being amphibians. We do not think that whales are any sort of fish, or amphibian, either.
Being vertebrates, we do not consider ourselves to be invertebrates. We think of the spineless as being a whole different kind from us, rather than us being a subtype of them. We carve out a rather considerable dignity for ourselves, by not sticking to strict phylogenic principles there.
Any good biologist would want to have a clear conception of any group they study. It would be both intellectually distressing, and an intellectual scandal, if they did not. A strong desire for monophyly, though, would muddy any possible clear conception or satisfactory definition of what many groups would be.
Any good biologist finds it intellectually distressing to devote his life to the study of a group that cannot be readily and satisfactorily defined in biological terms; and the abiding intellectual scandal of bacteriology has been the absence of a clear concept of a bacterium.
The Concept of a Bacterium, R.Y. Stanier and C.B. van Niel
And so taking this line of thought to its absurd conclusion: if we have a very strong fondness for monophyly, then we should think of ourselves as prokaryotes. All life is.
A bird, if anything, is a colony of bacteria.
We might call in and apply prototype theory here, if we are looking for an out.
Even those who insist that statements such as "A robin is a bird" and "A penguin is a bird" are equally true, have to admit different hedges applicable to statements of category membership. Thus it is correct to say that a penguin is technically a bird but not that a robin is technically a bird, because a robin is more than just technically a bird; it is a real bird, a bird par excellence.
Principles of Categorization, Eleanor Rosch
Yes, sure, humans are prokaryotes. Technically.
We could admit that different hedges can be applicable to statements of category membership: We are real prokaryotes, in a strict cladistic sense; We are not, however, prokaryotes par excellence. We are prokaryotes in disguise.
Linnaean systematics — collecting, preserving, describing, naming, and ordering the diversity of organisms in a systematic project — was attempting to see the order in creation. Charles Darwin thought that descent with modification was essentially the systematic explanation for the appearance of order that we see, and that classification would come to be based on genealogy. Evolutionary systematics is one way of going about this. It systematically plugs classification into the population-genetic style of thought of the Modern Synthesis. Phylogenic systematics is another way of going about it. It is systematically based on the branching tree data structure, and relations of nodes, branches, and subtrees. Where Steve Gould talked of a cladistic ordering of trout, lungfish, and any bird or mammal, he was addressing the dilemma of reptiles in a tree-thinking way. Take some set of organisms, and find the minimal phylogenic tree that contains them all. Binary trees are a fundamental data structure in computing. Computer science takes to phylogenics, like a duck takes to water.
One way to say what a bird is, is to take all the existing species that we think are birds, trace them back to their last common ancestor, and then gather all of that common ancestor's descendants. We can relate living and extinct organisms, by working in this way, as in the chart above.
Using the method, we can talk of crown group birds. This removes some ambiguity, and makes clear that we are not including, say, the late Cretaceous toothed seabird Ichthyornis, marked to the right with an “O”, or Archaeopteryx, marked with a star.
Another operation we can perform on the tree, is to specify everything closer to any current bird, than it is to a crocodile. This is a stem-based definition, and the result would be called a total group.
In his essay musing on what, if anything, a reptile might be, the herpetologist James Harding considers some results of a particular use of this method, and throws up his hands. Birds do not seem to be dinosaurs. Dinosaurs, if they exist, would be birds. His lifelong academic discipline has been thrown into confusion, and much reduced.
…
Collins and Taggart (2002) claim that their classification (or “phylogenetic hypothesis”) retains “maximum explanatory power.” This statement seems neither intuitive nor readily apparent, especially when comparing their system to the proposal of Modesto and Anderson (2004). What they are largely trying to “explain” is the relationship of birds to the traditional “reptiles.” By dividing either a phylogenetic or a traditional Reptilia into its constituent parts, do we gain or lose information? A “Class Chelonia” is just turtles, and the “Class Crocodylia” is just crocodiles and alligators — its interesting relationship to “Class Aves” (birds) is no longer obvious (without the phylogenetic context offered on the website) and nothing is implied about broader evolutionary relationships or functional morphology. Most unfortunately, the default “Reptilia” of Collins and Taggart is just “lizards and snakes” (when applied in North America), leaving the term stripped of its historical meanings and phylogenetic significance, and setting the stage for taxonomic and linguistic confusion.
What, If Anything, Is a Reptile?, James Harding
We get, then, two nonarbitrary definitions that we could label as a bird, one much broader than the other, or we could choose some arbitrary point in between.
To return to the case of birds, it should be noted that there are only two nonarbitrary definitions of the group Aves — a narrow definition (crown-group Aves) and a broad definition (total group Aves). The crown-group Aves will comprise forms possessing all the characters used to define extant birds (with the proviso that some characters may have been lost secondarily in some forms); and the total group Aves will comprise forms possessing at least one of the characters used to define extant birds. Any definition of the Aves as a group intermediate in extent between crown-group and total group will be arbitrary (i.e. typological or 'essentialist'): any such group would comprise forms possessing some of the characters used to define extant birds . . . but exactly which characters are to be selected as 'some characters' will be a matter of personal choice. Conventional definitions of the class Aves are, in fact, of this arbitrary type; they define the crown-group (Neornithes, or modern birds) plus part of the stem-group (e .g. 'Odontornithes', or toothed birds of the Mesozoic, together with Archaeopteryx). Here an obvious question will arise: which of these definitions, if any, should be employed in practice?
The Avian Relationships of Archaeopteryx, and the Origin of Birds, R. A. Thulborn
Two years after publication of the Origin, an article by Johann Andreas Wagner, translated from the German, was published in the Annals and Magazine of Natural History. Wikipedia says the article is “a vitriolic sneer.” The sneering is at Charles Darwin and his adherents. That they would likely be seeing support for their new theory, in a newly discovered fossil creature, but about which they would be wrong.
At first glance of the Griphosaurus [Archaeopteryx] we might certainly form a notion that we have before us an intermediate creature, engaged in the transition from the saurian to the bird. Darwin and his adherents will probably employ the new discovery as an exceedingly welcome occurrence for the justification of their strange views upon the transformation of animal but in this they will be wrong.
On A New Fossil Reptile Supposed to Be Furnished With Feathers, A. Wagner
Wagner knows what a bird is, though. A bird has feathers. An animal that has feathers is a bird.
”A bird is known by its feathers,” says the old proverb. The universal and exclusive applicability of this is recognized not only popularly, but also in zoology: an animal with feathers is a bird. This distinctive character, hitherto regarded as immoveably fixed, has all at once been brought in question by one of the most unexpected discoveries.
Or, at least, it used to be that simple. Wagner decided that Archaeopteryx was a reptile with some bird-like traits.
The influential non-Darwinian evolutionist Richard Owen, by contrast, thought that Archaeopteryx was a bird with some reptile-like traits.
And Thomas Huxley had argued, that birds are reptiles.
Darwin's friend and champion, Thomas Huxley (1867, 1868a, 1868b, 1870), presented a radical proposal that birds are merely glorified reptiles. He classified reptiles and birds in the same group, Sauropsida. He reasoned that, if birds had not been so outstandingly successful in their aerial adaptation and speciation and if they had remained a relatively small group like pterosaurs, they would now be regarded as an order of reptiles, not a separate class of vertebrates.
The Rise of Birds, Sankar Chatterjee
Ernst Mayr said, birds are different because of their rapid change. Thomas Huxley said, it’s probably the size of their radiation, their success. Something about birds and how to classify them seems surely different, though. If anyone tried to stir up an argument about whether a particular bat ancestor was a bat-like mammal, or a mammal-like bat, an absurdity would be apparent.
Birds, compared to crocodiles, have rapidly changed recently. But crocodiles had drastically changed, longer ago.
The terrestrial lifestyle of early crocodylomorphs was followed by the adoption of a fully amphibious habitat from the Jurassic on, perhaps to avoid confrontation with dinosaurs, with the evolution of a thick, massive, akinetic skull, heavy body mass, a semierect quadrupedal gait, and dense bones, a body plan far from the featherweight and pneumatic design of the avian skeleton. With this dramatic ecological shift, crocodilians moved far away from the main line of bird evolution.
The Rise of Birds, Sankar Chatterjee
And everything in between them is now gone. The modern-day crocodiles, and the modern-day birds, end up about as different, about as widely separated, as two existing vertebrate sibling-clades could be. Birds, today, are clearly identifiable and distinct. Birds are thus an especially fair example to question taxonomic realism with.
On the graduated series of approximations to the bird that Edward Drinker Cope had talked about in 1869, many taxonomic groupings are given a name that is some variation of “approximation to the bird”.
The “having tails like birds”, which includes the toothed Ichthyornis. The “having wings like birds”, which includes Archaeopteryx. The “having bird thoraxes”. The “opposite to the birds,” with an impressive one recently found in amber. The “near birds”. The naming convention makes the various groups of bird approximations look like they have a direction in which they are gradually heading, and that they know what it is.
With the discovery in recent decades of a large variety of feathered or otherwise near-avian dinosaur fossils, particularly in China, the decision on where to draw the line about birds has become more arbitrary than ever. What a bird might be, when fossil specimens are included, is a conceptual puzzle with no one best solution.
With the discovery of very bird-like, though phylogenetically nonavian, theropods, the systematic line blurred, necessitating an arbitrary definition of bird.
Confusing Dinosaurs with Mammals: Tetrapod Phylogenetics and Anatomical Terminology in the World of Homology, Jerald Harris
"Birds became more and more 'birdy' gradually," [Stephen] Brusatte says. "There was no big jump from non-bird to bird among dinosaurs, just a seamless transition."
Birds Evolved From Dinosaurs Slowly—Then Took Off, Dan Vergano
The high definition of a modern bird, its lack of closeness to anything else, is an illusion from the whole graduated series of approximations to them not presently running, flying, or swimming around.
There is a rocks versus clocks debate about the origin of modern flying birds.
Water fowl and land fowl go back to the Cretaceous. But the origin of the general group of neoavian birds, in relation to the Cretaceous-Paleocene extinction event, is contested.
By fossil evidence, some very few lines of birds survived the extinction event, and then explosively radiated after it. By molecular genetic evidence, numerous lines of birds survived the extinction, and have ghost lineages going back fairly deep into the Cretaceous.
The pattern, timing and extent of the evolutionary radiation of anatomically modern birds (Neornithes) remains contentious: dramatically different timescales for this major event in vertebrate evolution have been recovered by the ‘clock-like’ modelling of molecular sequence data and from evidence extracted from the known fossil record. Because current synthesis would lead us to believe that fossil and nonfossil evidence conflict with regard to the neornithine time scale, especially at its base, it is high time that available data are reconciled to determine more exactly the evolutionary radiation of modern birds.
The Evolutionary Radiation of Modern Birds (Neornithes): Reconciling Molecules, Morphology and the Fossil Record, Gareth Dyke and Marcel van Tuinen
The neoavians have, though, a super-radiation, at around the Cretaceous-Paleocene boundary. This makes classifying them hard.
A recent molecular genetic study found a super-radiation of neoavian birds near the Cretaceous-Paleocene boundary, with highly discordant phylogenetic signals from near-simultaneous speciation events. Other studies discuss the explosive radiation in similar terms.
The Dynamics of Incomplete Lineage Sorting across the Ancient Adaptive Radiation of Neoavian Birds, Alexander Suh, Linnéa Smeds, and Hans Ellegren
The repeated finding of poor resolution raises the possibility that the major lineages of Neoaves differentiated so rapidly that the radiation might be considered essentially simultaneous.
Birds in a Bush: Five Genes Indicate Explosive Evolution of the Avian Orders, Steven Poe and Alison Chubb
Fast radiations, with hybridization and incomplete lineage sorting, are messy processes that cannot adequately be represented by simple branching trees.
That classification should be hierarchical, is by assumption. That assumption goes back to Linnaeus, and to the ancient Greeks before that. The farther we get from vertebrates, perhaps, the easier it becomes to object to this.
Debates over the status of the tree of life (TOL) often proceed without agreement as to what it is supposed to be: a hierarchical classification scheme, a tracing of genomic and organismal history or a hypothesis about evolutionary processes and the patterns they can generate. I will argue that for Darwin it was a hypothesis, which lateral gene transfer in prokaryotes now shows to be false.
The Practice of Classification and the Theory of Evolution, and What the Demise of Charles Darwin's Tree of Life Hypothesis Means for Both of Them, Ford Doolittle
In the networked depiction of bird phylogeny, above, complex hybrids of complex hybrids are the only path to any existing species. No simply branching path can be followed to arrive at any bird.
A particular taxon of birds might be thought of as like a probability cloud, in how it got here. It is a superposition of very many paths.
We know that the planetary model of the electron is not real. We often find it useful, though.
Rabbits, carefully considered, might not be real. But Albert Wood, reluctant to admit they exist, and not much believing it himself, could still write a monograph on them.
In one of the two nonarbitrary conceptions of birds, Ichthyornis, shown at the top here, does not count. In the other nonarbitrary conception, Pterodactlyus does. All of the traditionally-conceived dinosaurs, their status as birds quite debatable, sit in between.
More arbitrarily, birds might be defined specifically to include, say, Archaeopteryx, trying to make the line be about flight, or perhaps the line would be about feathers. Where it is so hard to agree on one definition of them, it is also then hard to claim them as very real.
Charles Darwin was not a species realist, or at least doubted that species refers to a real category in nature.
In short, we shall have to treat species in the same manner as those naturalists treat genera, who admit that genera are merely artificial combinations made for convenience. This may not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species.
On the Origin of Species, Charles Darwin
He was able to write a book on their origin anyway.