Evolution is an obstacle course not a freeway; the correct analogue for long-term success is a distant punt receiver evading legions of would-be tacklers in an oddly zigzagged path toward a goal, not a horse thundering down the flat.
— Stephen Jay Gould
In the fall of 1833, Charles Darwin made a trip upriver from Buenos Aires. Near the town of Santa Fe Bajada, he found fossil remains of ancient monsters — giant armadillos, giant ground sloths, mastodons. He also found the fossil tooth of a horse.
The horse tooth was a mystery. Shouldn’t a horse, in South America, be from historical not geological time? Wouldn’t the horse have arrived in South America with the Spanish, rather than existed there before them?
Buenos Aires was in revolution when Darwin got back. This led to Darwin meeting the Beagle, against plan, across the bay, in Montevideo. In Montevideo, Darwin received a mail delivery of the second volume of Lyell’s Principles of Geology. The second volume proposes a relation of geological uniformitarianism, to the uniform introduction and extinction of species. Lyell denied single origin theories of life, and in print, at least, any transmutation of species. By denying these things, Lyell was denying the revolutionary idea from Lamarck, often credited to Darwin instead, about the origin of humans.
It was precisely the proposed beastly link with orangutans that made Lyell reject Lamarck's evolutionary theory and devote more than two chapters of Principles of Geology (1830–3) to a summary and refutation of it. Lyell mocked Lamarck for proposing a “progressive scheme, whereby the orang-outang, having been already evolved out of a monad, is made slowly to attain the attributes and dignity of man.”
Going the Whole Orang: Darwin, Wallace and the Natural History of Orangutans, John van Wyhe and Peter C. Kjaergard
The horse tooth eventually led to a passage, in the Origin of Species, on their extinction. The origin, improvement, and extinction of species are intimately connected together, Darwin said. Catastrophism was dismissed, and the idea of rapid bursts of new species creation was dismissed as well. Where he says below, that no one can have marveled more at the extinction of species, Darwin effectively dismisses Georges Cuvier, the French paleontologist and catastrophist, who had given the name Megatherium to the giant sloths, and who had certainly marveled quite a bit on disappearance.
The whole subject of the extinction of species has been involved in the most gratuitous mystery. Some authors have even supposed that as the individual has a definite length of life, so have species a definite duration. No one I think can have marvelled more at the extinction of species, than I have done. When I found in La Plata the tooth of a horse embedded with the remains of Mastodon, Megatherium, Toxodon, and other extinct monsters, which all co-existed with still living shells at a very late geological period, I was filled with astonishment; for seeing that the horse, since its introduction by the Spaniards into South America, has run wild over the whole country and has increased in numbers at an unparalleled rate, I asked myself what could so recently have exterminated the former horse under conditions of life apparently so favourable. But how utterly groundless was my astonishment! Professor Owen soon perceived that the tooth, though so like that of the existing horse, belonged to an extinct species.
On the Origin of Species, Charles Darwin
I will be giving later here, a simple linear story of our understanding of the evolution of the horse.
But there is a way in which ideas about evolution develop, from Georges Cuvier, to Jean-Baptiste Lamarck, to Charles Lyell, to Charles Darwin, to the present day, that we will be heading back to pick up abandoned bits of Cuvier and Lyell and Lamarck again. Our ideas have taken an oddly zigzagging circuitous and backtracking route.
Darwin’s solution to the astonishing fact of the extermination of the horse in the Americas, was the uniformitarianism of Lyell. Species steadily arise, species steadily go extinct. To this, Darwin had added back in from Lamarck what Lyell had abandoned, that they also gradually change.
Today, we are catastrophists again, and believers in rapid bursts of new species creation. Theories about asteroids hitting Yucatan being responsible for the dying of the dinosaurs, with a radiation of mammals afterwards leading to us, for example, are a part of common understanding.
In his book Blind Watchmaker, the atheist Richard Dawkins intentionally goes back and picks up the argument from design of the natural theologian William Paley. Our ideas take odd routes. Evolution by natural selection can be used to explain the appearance of design in the organic world.
The former horse in the Americas had been exterminated by the sudden arrival of humans, a great climate change, or the combination of both. As catastrophists today, we can come pretty close to reviving old storytelling about humanity wiped out in a rising of the waters, as a consequence of its sin.
The ground sloth Megatherium, from Georges Cuvier’s Recherches sur les ossemens fossiles.
In 1788, fossil bones of a large creature had been discovered west of Buenos Aires. In 1796, Georges Cuvier identified the creature from published accounts as a giant ground sloth. In the same year, he gave a talk on the fossil bones of ancient elephants — the mammoth and the mastodon.
Cuvier supposed that no modern-day giant ground sloth would be discovered in South America, and that no modern-day elephants would be discovered there either. Whether they might still be found, in the more remote areas, was an intellectual debate of the day. The Lewis and Clark expedition, sent out across North America by the paleontologist Thomas Jefferson, was to keep an eye out for them, in case they were still roaming around here.
In Cuvier’s view, ancient giant beasts, now disappeared, seemed “to prove the existence of a world anterior to ours, destroyed by some kind of catastrophe.” This talk, of anterior worlds destroyed by catastrophe, of sudden change, was given in revolutionary Paris.
After the publication of Origin of Species, in 1859, the idea of an origin and an evolution of a species, traced back to a common ancestor, was quickly accepted in the scientific world. But the primacy or sufficiency of a gradualist natural selection as the driving cause of this evolution was not.
A number of good objections existed. Charles Darwin’s notion of blended inheritance made no sense — any selective advantage would be quickly blended out. Natural populations did not seem to contain enough variation for natural selection to create new species. Before the discovery of radioactivity, reasonable estimates of the age of the earth did not seem to allow enough time for Darwinian gradualism to work. And despite what we might say today, that the fossil record shows no such thing.
[L]ike Cuvier, Lyell believed in the stability of species. As evidence continued to accumulate in favour of some kind of organic progression in the stratigraphic record, Lyell's resistance eventually crumbled, but he continued to believe in the extreme imperfection of the fossil record. Like Darwin, he could find no direct evidence from it for evolution.
Lyell's Views on Organic Progression, Evolution and Extinction, A. Hallam
It would take until 1944, and George Gaylord Simpson’s Tempo and Mode in Evolution, before a paleontologist would be likely to put a high stress on the power of natural selection, or to say that the fossil record provided much evidence of it.
Henry Fairfield Osborn, Simpson’s predecessor as curator of vertebrate paleontology at the American Museum of Natural History, saw two profoundly different modes of evolution.
One mode is a change in proportion or degree, as in the lengthening of an elephant’s tusk, or a limb. Natural selection works on this level, Osborn said.
And the higher-level mode is creative change in kind not degree. The origin of absolutely new characters, such as a grinding molar, or a tusk from an incisor. These together make for a form of orthogenesis. An innate tendency in a group to evolve in a direction, to show a frequent recurrence of a trait, due to some internal mechanism or force.
The evolution of the Proboscidea as now observed constitutes a veritable revolution not only in zoology but in biology. It affords the first actual and positive evidence as to the origin of species and reveals that evolution is a dual rather than a single process including the hitherto unrecognized creative principal which we call aristogenesis, as well as the older modifying principle which has been recognized as evolution for the last 2,500 years.
The Thirty-Nine Distinct Lines of Proboscidean Descent, and Their Migration into All Parts of the World except Australia, Henry Fairfield Osborn
The fossil record, conveniently arranged, can provide a convincing illusion of orthogenesis. The proto-Mammoth, marching from Africa to the Americas, sure seems to have a directive evolution to longer tusk and larger size.
In 1902, Osborn published “The Law of Adaptive Radiation”. There is a grandeur to this view of life. He was working at a level of the spreading of mammals into a continent, seen multiple times. Biological radiation is a scientific law like gravity, Osborn said. Knowing that law, and the one about the dual modes of evolution, you can make sense of the fossil record.
In “Law of Adaptive Radiation”, Osborn gives a useful key for understanding fossil mammals and their radiations. The two things to look at are the limbs, and the teeth. Many of the reasonable combinations — choose a limb type, choose a tooth type — will be found.
[P]ractically all the adaptations known among mammals have arisen by combinations of divergence independently pursued in the limbs and teeth; for example, an herbivorous tooth type may combine with a terrestrial, arboreal, or volant limb type, according as the search for plant food is on the earth, in the trees, or in the air. Although every imaginable combination (e.g., aquatic limbs, myrmecophagous dentition) cannot be realized, yet these combinations have been multiplied almost ad infinitum and constitute the fatal defect of Cuvier's law as he conceived it.
As tested by a single case, the Eocene monkeys of the family Notharctidae acquired teeth exactly homoplastic with those of Eocene horses, but the former were provided with arboreal, the latter with terrestrial, limb types. Second, correlation of limb and tooth structure in a given group is further conditioned by the particular combination of specialization of limbs and teeth which the radiation originates with.
The Law of Adaptive Radiation, Henry Fairfield Osborn
Like mammals can be understood by limb and tooth, evolutionary biologists can be understood by a number of fundamental traits. Whether they are strong adaptationists or not, a debate that has been especially persistent.
Whether they are gradualists or saltationists; whether they see mutation as a creative force; whether they see micro- and macro-evolutionary processes as the same or different things; whether they stress genetics or morphology; and such.
The Modern Evolutionary Synthesis brought together Mendelian genetics and Darwinian natural selection, two schools of thought that had previously clashed.
The current Extended Evolutionary Synthesis movement stresses adaptationism but non-genetic and epigenic modes of inheritance. It is as good an example as any, that most all reasonable niches will be filled.
In an essay, “Eternal Metaphors of Palaeontology”, on the idea that what is old is new again, and of available slots being filled, Steve Gould classifies evolutionists on three traits: whether they are steady-statists or directionists; whether they stress environment or internal factors; and whether they are punctuationists or gradualists. At a meta level — theories of the evolution of theories of evolution — this is a non-directionist, environmentalist, and punctuationist stance, which happens to be how Gould would be classified as an evolutionist as well.
Ideas of a directional progression in scientific knowledge get confounded by the mix and match nature of positions on the various debates. Ideas will be abandoned, and then picked up again later in revised form.
Figure demonstrating progressive evolution of the horse, from Thomas Henry Huxley’s American tour.
In 1876, Thomas Henry Huxley traveled to the United States, to give a lecture tour on evolution and natural history. He first visited the natural history museum at Yale.
Whole herds of North American fossil horse species had recently been discovered. Huxley was impressed by the completeness of the record O.C. Marsh could show him at Yale.
For the lecture tour, Huxley used an illustration of horse evolution drawn by Marsh, shown above. The illustration was presented as demonstrative evidence of evolution, and to rebut objection to the theory. But it manages to powerfully convey the idea of long-term gradual directional change, a view both Huxley and Marsh were proponents of.
Joseph Leidy, at this time, was already seeing horse evolution as a bush, not a ladder. Steve Gould points out that the directional view had facts going for it, though. He also indirectly calls attention to the fact that Thomas Huxley, “Darwin’s bulldog”, was in some ways not much of a Darwinist, and also to the political alignments of the debate.
Directionists of the last century did not assert their claims with the blindness of a priori social prejudice alone. They had quite a few facts going for them, especially the appearance of "progress" in vertebrate history, the body of data that eventually converted Lyell to progressionism. Yet Darwin's theory speaks, just as clearly, of adaptation to local environments alone. Since local environments fluctuate stochastically, with no directional trend through time, the basic Darwinian mechanism would seem to offer no rationale for a belief in progress. This apparent dilemma led most nineteenth century evolutionary palaeontologists to believe their own data and reject Darwin for a DI [directional internalist] theory of evolution.
Eternal Metaphors of Palaeontology, Stephen Jay Gould
The illusory view of a gradual direction in evolution, with its notions of progress, has been persistent. Here it is, being taught to schoolchildren today.
Here, by contrast, is a branching phylogeny of horses from Bruce McFadden.
I have said that I will be providing a simple linear story of our understanding of the evolution of the horse. It is the two images directly above.
The horse had been used as the textbook example of gradual linear evolution, from it having a relatively complete fossil record. The horse had later been used as a textbook example of branching evolution and adaptive radiation, again from it having a relatively complete fossil record. The second version is often taught by pointing out the inadequacies and the persistence of the first.
The 55-million-year fossil record of horses (Family Equidae) has been frequently cited as a prime example of long-term macroevolution. In the second half of the nineteenth century, natural history museum exhibits characteristically depicted fossil horses to be a single, straight-line (orthogenetic) progression from ancestor to descendent. By the beginning of the twentieth century, however, paleontologists realized that, rather than representing orthogenesis, the evolutionary pattern of fossil horses was more correctly characterized by a complexly branching phylogenetic tree. We conducted a systematic survey of 20 fossil horse exhibits from natural history museums in the United States. Our resulting data indicate that more than half (55%) of natural history museums today still depict horse evolution as orthogenetic, despite the fact that paleontologists have known for a century that the actual evolutionary pattern of the Family Equidae is branching.
Fossil Horses, Orthogenesis, and Communicating Evolution in Museums, Bruce McFadden et al.
Steve Gould, in the paragraph after the quote about evolution not being a horse thundering down the flat, raises a related higher-level and political concern, which is evolution and the origin of human consciousness. Notions of a direction to evolution are self serving. Our unusually large brains almost seem to have purpose, or to have had an inevitability. They have the appearance of design.
The revolutionary consequence of Darwin’s theory, or Lamarck’s, is avoided just enough to keep humans in a special place.
The population geneticist Michael Lynch points out a theoretical import of the typically small effective population size of multi-cell eukaryotes compared to prokaryotes. In a large effective population, natural selection will be at its most effective. In a small effective population, random genetic drift will have increased power. (Effective population size is an abstract concept; you cannot just go out and count it.)
By small effective population size, where genetic drift will run relatively freely, Lynch does not necessarily mean small, as in tens of thousands of large mammals. He means small, as in millions of insects. He is considering the wild variety of complex embellishments in the multicellular eukaryotic genome, and why selection does not rigorously weed them out. Lynch is mostly concerned, then, at the level of genomic architecture, but the observation applies at an externally observable phenotypic level as well.
Although it is commonly assumed that virtually all aspects of biodiversity, including those at the genomic and subcellular levels, reflect long-term adaptive tuning to persistent ecological challenges, there is substantial empirical evidence that the three major nonadaptive forces of evolution—mutation, recombination, and random genetic drift—vary by orders of magnitude among phylogenetic lineages.
The Repatterning of Eukaryotic Genomes by Random Genetic Drift, Michael Lynch et al.
[T]he origins of many aspects of biological diversity, from gene-structural embellishments to novelties at the phenotypic level, have roots in nonadaptive processes, with the population-genetic environment imposing strong directionality on the paths that are open to evolutionary exploitation.
The Frailty of Adaptive Hypotheses for the Origins of Organismal Complexity, Michael Lynch
When we think about evolution, we strongly tend to think about selection. But we ourselves are down very far to the small end of things, where selection would not be so important. As are horses.
Effective population size of horses (×104), from DNA evidence. a. Last 150 thousand years. b. Last 2 million years.
Below is Sewall Wright’s illustration of the adaptive landscape, from his shifting balance paper. For a population to move at all, meaning for it to evolve, we must have some relaxation of the Hardy-Weinberg assumptions. Wright starts by adding in natural selection — the hilly landscape — and then considers the effects of other relaxations as well.
Even in the presence of selective pressure, evolution is often not a horse thundering towards an optimized adaptation, or even walking slowly towards it. Evolution can be a horse zigzagging around.
George Gaylord Simpson, Horses (1951).
George Gaylord Simpson’s model of adaptive radiation is tied to Sewall Wright’s shifting balance model of evolution, cited above. That is, as Simpson had brought paleontology within the Modern Synthesis, he had brought adaptive radiation within it as well.
To get to Simpson’s version of adaptive radiation, we have to backtrack and zigzag around some.
Charles Darwin’s theory of gradual evolution by natural selection had no room in it for sudden bursts of speciation within a group. There would just not be enough time. If we think we see sudden bursts of speciation in the fossil record, it is an illusion from the fossil record’s imperfection.
On the sudden appearance of whole groups of Allied Species.—The abrupt manner in which whole groups of species suddenly appear in certain formations, has been urged by several palæontologists, for instance, by Agassiz, Pictet, and by none more forcibly than by Professor Sedgwick, as a fatal objection to the belief in the transmutation of species. If numerous species, belonging to the same genera or families, have really started into life all at once, the fact would be fatal to the theory of descent with slow modification through natural selection. For the development of a group of forms, all of which have descended from some one progenitor, must have been an extremely slow process; and the progenitors must have lived long ages before their modified descendants. But we continually over-rate the perfection of the geological record, and falsely infer, because certain genera or families have not been found beneath a certain stage, that they did not exist before that stage.
On the Origin of Species, Charles Darwin
Henry Fairfield Osborn is caught twice dismissing the importance of Charles Darwin in a quote above, in favor of his own veritable revolution. His paper on the law of adaptive radiation cites the neo-Lamarkian Edward Drinker Cope, for observations about say the similar expansions of marsupials in Australia and placental mammals elsewhere, as leading to the law. Osborn strongly stressed August Weismann and the discovery of germ plasm, though, which had been the beginning of the end for Lamarkianism.
George Gaylord Simpson takes Osborn’s theory of adaptive radiation, drops Osborn’s conception of an inherited macro-evolutionary mechanism, and adds in Sewall Wright’s shifting balance model of speciation, which combines a random drift step and a natural selection one.
Osborn’s “hitherto unrecognized creative principal” becomes a much more modern idea about a “key adaptation”. A textbook example of a key adaptation and adaptive radiation would be the time, around 20 million years ago, when the horse in North America becomes a grazer not a browser. After some 25 million years of relative stasis, it branched wildly, with the whole herds of new species taking to the expanding grasslands. The morphological change, and the radiation, are tightly coupled here.
Phylogeny of the horses, emphasizing North America, and distinguishing browsers and grazers, from Bruce McFadden.
With some talk about the potential coevolution of the horse and the grasslands, and some reconsideration of the timing of the grassland expansion and morphological change in the horse, that brings us to the present day.
Phylogeny of horses, with continents marked by color, showing species radiations around 18, 11, and 4.5 million years ago.
Paleontologist Juan Cantalapiedra and colleagues have reviewed the research on 138 species of horses. This was in February, in Science.
There is a grandeur to their view of life. They are working at the level of a spreading of horses on a continent, seen multiple times. They see a radiative burst of new species in North America, 18 million years ago. And other radiative bursts of new species when horses entered Eurasia, 11 and 4.5 million years ago.
It is a contrarian paper.
This result is contrary to long-proposed evolutionary theory.
How Did Horses Evolve?, Tom Edathikunnel
Horses were once a textbook example of gradual linear evolution. They are now a textbook example of adaptive radiation. The results in this paper are contrary enough to expectations, that what the textbooks say needs reconsidered again.
Cantalapiedra and colleagues note the irony of their findings: “the radiation of equids […] has been cited as a textbook example of adaptive radiation for more than a century, as it is crucial in the development of evolutionary theory linking trait evolution and adaptive success.”
Horse Study Reins in Evolutionary Orthodoxy, Jana Howden
Here is Peter Grant, with a general discussion of adaptive radiation, in context of the textbook example of the finches of the Galápagos. During and after the speciation in a radiation, selection will morphologically differentiate the species, for the exploitation of different niches.
Studies of these radiations help reveal the causes of their evolution. As a result of natural selection during and after speciation, descendant species differ morphologically or physiologically in the way they exploit different environments.
...
The term adaptive radiation was coined in 1902 by a paleontologist, H. F. Osborn, and the phenomenon it refers to was popularized by another, G. G. Simpson, about 50 years later. Simpson viewed the evolutionary radiation of a major group of animals, such as marsupial mammals, as various lines of descent from a common ancestor arising more or less simultaneously and diverging in different morphological and ecological directions, rather like spokes radiating from the hub of a wheel. This image is powerful yet fails to represent the correct evolutionary pathway of bifurcating branches in a treelike structure. Nonetheless, the term has stuck. The adaptive adjective is applied because the products of a radiation are conjectured or known to be adapted to exploiting the environment in different ways.
Adaptive Radiation, Peter Grant
The traditional story of horses sees a coupling of morphological adaptation for grassland grazing, and the branching radiation around 18 million years ago.
The early radiation of American equid tribes(18 to 15 Ma) has traditionally been explained as the direct outcome of morphological adaptation linked to the onset of grass-dominated habitats(17, 20).
Decoupled Ecomorphological Evolution and Diversification in Neogene-Quaternary Horses, J. L. Cantalapiedra et al.
It is a decoupling of branching speciation and morphological diversification, and this seen not once but multiple times, that is contrary to expectations.
“This knocks traditional notions that rapid diversification of new species comes with morphological diversification as well,” says paleontologist Bruce MacFadden of the University of Florida in Gainesville. “This is a very sophisticated and important paper.”
Horses Buck Evolutionary Ideas, Rachel Ehrenberg
Henry Fairfield Osborn had suggested, to understand adaptive radiations, look at the limbs and the teeth. Their mode of getting about, and what they eat. In the figure below, degree of hypsodonty — teeth suitable for an abrasive diet — is on the right. Body size, related to their way of getting about, on the left.
In the top figure, high rates of morphological change are in warm colors, low rates in cool. A general coolness at times of high branching is apparent. In the density diagram in the bottom figure, low speciation and high speciation times are divided out, with rate of morphological change on the vertical axis, as another way of showing the tendency, though note that the text says that the body size difference is not statistically significant.
Here is the abstract for the paper:
Evolutionary theory has long proposed a connection between trait evolution and diversification rates. In this work, we used phylogenetic methods to evaluate the relationship of lineage-specific speciation rates and the mode of evolution of body size and tooth morphology in the Neogene and Quaternary radiation of horses (7 living and 131 extinct species). We show that diversification pulses are a recurrent feature of equid evolution but that these pulses are not correlated with rapid bursts in phenotypic evolution. Instead, rapid cladogenesis seems repeatedly associated with extrinsic factors that relaxed diversity bounds, such as increasing productivity and geographic dispersals into the Old World. This evidence suggests that diversity dynamics in Equinae were controlled mainly by ecological limits under diversity dependence rather than rapid ecomorphological differentiation.
Decoupled Ecomorphological Evolution and Diversification in Neogene-Quaternary Horses, J. L. Cantalapiedra et al.
And here, some of the introduction:
The radiation of equids in the Neogene has been cited as a textbook example of adaptive radiation for more than a century (11), as it is crucial in the development of evolutionary theory linking trait evolution and adaptive success (1,12). The rich equid fossil record provides a suitable data set for testing these ideas within a phylogenetic framework. Much work has focused on the evolution of body size and dental morphology (12), as these two traits condense multiple dimensions (such as population density, range size, diet, and environmental pressures) of a species’ adaptive zone. Early studies based on dental proportions suggest that phenotypic change accelerated during an early Miocene radiation (1,13,14), although recent analyses show that body size disparity did not increase during diversification pulses (15). Yet, previous work has been nonphylogenetic and has not directly investigated the connection between diversification dynamics and phenotypic evolution.
…
We found evidence for repeated speciation bursts across Equinae, but none of these were associated with rapid ecomorphological evolution.