Saltation
Natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps. If it could be demonstrated not by numerous, successive, slight modifications, my theory would absolutely break down. ~ Charles Darwin
A sudden leap in specific evolution is termed saltation. Before Darwin, most evolutionary theorists were saltationists. Lamarck was generally a gradualist but thought that saltation was possible.
Darwin’s speculations on evolution were not immediately favored because of his insistence on gradualism alone.
Mr. Darwin’s position might have been even stronger than it is if he had not embarrassed himself with the aphorism, “Natura non facit saltum,” which turns up so often in his pages. Nature does make jumps now and then, and a recognition of the fact is of no small importance in disposing of many minor objections to the doctrine of transmutation [i.e., Darwin’s theory]. ~ English biologist Thomas Henry Huxley in 1864, in review of Darwin’s On the Origin of Species (1859)
William Bateson, who coined the term genetics, rhetorically riposted in 1894: “Species are discontinuous. May not the variation by which species are produced be discontinuous too?” Like Huxley and others, Bateson bemoaned the “gratuitous difficulties which have been introduced by this assumption” by Darwin of gradualism.
Others thought that Darwin’s emphasis on gradualism had not gone far enough. Scottish engineer Fleeming Jenkin wrote an article in 1867 that convinced Darwin that he had not been insistent enough about incrementalism. In response, Darwin added a section in the 1869 5th edition of Origin that slammed the door on saltation.
In the early 20th century, incipient geneticists rallied around the notion that saltation might be caused by large mutations. English geneticist Reginald Punnett supported a saltational theory in his 1915 book Mimicry in Butterflies.
German-born American geneticist Richard Goldschmidt is considered the first to integrate genetics, development, and evolution. In 1933, he proposed macroevolution via macromutation, an idea that was universally rejected and became ridiculed as the “hopeful monster” hypothesis.
The change from species to species is not a change involving more and more additional atomistic changes, but a complete change of the primary pattern or reaction system into a new one, which afterwards may again produce intraspecific variation by micromutation. ~ Richard Goldschmidt in The Material Basis for Evolution (1940)
Saltation staggered in the wake of the neo-Darwinist school that reconciled Mendelian genetics with natural selection. This became the reigning religion of evolutionary biologists and remains so to this day.
As an a priori bias, phyletic gradualism has precluded any fair assessment of evolutionary tempos and modes. The general preference that so many hold for gradualism is a metaphysical stance embedded in the modern history of Western culture: it is not a high-order empirical observation, induced from the objective study of Nature. ~ American evolutionary biologist Stephen Jay Gould in 1977
Saltation’s slump did not last, as continuing research found hopeful monsters.
The past 20 years have vindicated Goldschmidt to some degree. With the discovery of the importance of regulatory genes, we realize that he was ahead of his time in focusing on the importance of a few genes controlling big changes in organisms, not small-scales changes in the entire genome as neo-Darwinians thought. The hopeful monster problem is not so insurmountable after all. ~ American paleontologist Donald Prothero in 2007
Saltational evolution is targeted and relatively rapid change in specific characters and their coding genes. ~ American entomologist Daniel Rubinoff & South African evolutionary biologist Johannes Le Roux
Saltation works via genic edits, often epigenetic in origin, which modify development and produce a significant change.
Plant hybridization, and its resistance, sometimes suggests saltation. Hybrids via allopolyploidy are saltational. Mixed populations of related plants that naturally resist hybridization indicates saltation.
Common Cordgrass
Common cordgrass originated in southern England around 1870: an allotetraploidal offspring of European small cordgrass and American smooth cordgrass. (Allopolyploidy is polyploidy of chromosomes of distinct species.) Transposable elements played a key role in the creation of this saltational hybrid.
Common cordgrass is a quick-growing sturdy grass that forms large, thick colonies on coastal salt marshes. With its dense root systems binding mud and increasing silt deposits, common cordgrass was seen as a means to control coastal erosion. Hence cordgrass was planted at coastal sites throughout the British Isles, and in Asia, Australia, New Zealand, and North America.
New colonies take time to become established, but once ensconced, their rapid vegetative spread smothers competitors and even prevents wading birds from feeding. The introduction of common cordgrass invariably caused extensive damage to natural saltmarsh ecosystems.
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The vast variety of flowering plants provides ample testimony to the frequency of plant saltation. Descent through saltation is not necessarily obvious, as a parent and saltational offspring may appear selfsame.
Descent in the evening primrose family – such as from the twolobe clarkia to the rare Merced clarkia – is shown to be a product of saltation, because the 2 annual species do not reproductively hybridize.
Floral saltation seems to be more common in annuals than in other plants, but evidence suggests saltation may have played a significant role in the early evolution of woody plants.
Major developmental events, such as the specification of organ identity, are often under the control of a limited number of developmental control genes. Changes in these loci can bring about profound yet coordinated morphological changes. ~ German evolutionary biologist Günter Theißen
Floral saltation sometimes results in homeosis: the transformation of organs. Peculiar flowers, including tulips and orchids, emerged through saltation.
The western rosinweed, a daisy endemic to northern California, is emblematic of saltation caused by rapid chromosome reorganization, which is a stress response common in plants.
The diversity and success of insects owes in part to homeosis. Insect evolution offers innumerable instances of saltation. The instant expansion of legged segments in centipedes was a saltational effect.
As Reginald Punnett observed, mimicry is another well-known example of saltation.
Predators form categories to decide on prey suitability. If the categorization is based primarily on a single prey trait, a relatively small genetic change in prey may produce a large change in appearance as perceived by predators. Such feature saltation could cause a qualitative shift in categorization from suitable to unsuitable prey, thereby initiating mimicry evolution. ~ Swedish evolutionary biologist Gabriella Gamberale-Stille et al
Turtles
Of the amniotes, turtles have a unique body. Their shell is made of modified ribs, and their shoulder girdle is inside the rib cage.
Turtles (chelonians) abruptly appeared 250 MYA, in the Late Triassic, with no intermediate ancestry. (Pappochelys was a mid-Triassic diapsid with a few traits suggestive of turtles, including expanded ribs and related skeletal structures which allude (in hindsight) to being a precursor of a shell. The extinct, aquatic, turtle relative Odontochelys appeared in the Late Triassic. Like Pappochelys, Odontochelys lacked several features found in turtles, including a carapace (the characteristic shell of turtles), though Odontochelys did have a plastron (the flat, bottom shell found on turtles). Variations on a theme are common in evolutionary history.) Bony antecedent turtle-like shells would have been preserved in the fossil record if they had existed. This strongly suggests a body plan of saltational suddenness, and a quite decent design, as the basics of turtles have little changed since.
Turtles have the unique ability to quickly extend or retract their heads. This was long thought to have evolved solely for defense, but it emerged as a way to spring the head forward to snatch prey. Turtles sometimes act as ambush hunters, using their shell as a blind.
In an instance of convergent evolution, turtles retract their necks 1 of 2 distinct ways. Most turtles are cryptodires, which evolved during the Jurassic. These include most freshwater and sea turtles, snapping turtles, box turtles, soft-shell turtles, and tortoises. Cryptodires retract their necks straight back into their shells via muscles that fold vertically.
The other turtle group – pleurodires – bend muscles horizontally to pull their neck back to the side and tuck it next to their shoulder. Pleurodires are freshwater turtles found in Africa, South America, and Australia. They include snake-necked turtles and the peculiar matamata.
Sphinx Moths
Sphinx moths, also known as hawk moths, are a family of sizable moths with rapid, sustained flying ability. There are ~1,450 species of sphinx moths in ~200 genera.
Prosperpinus is a sphinx moth genus of 7 species. In general, these moths are green with red or orange hindwings. Like many other hawk moths, they hover like hummingbirds in front of the flowers from which they nip nectar. With 1 exception, Prosperpinus larvae feed exclusively on plants in the evening primrose family.
3 Prosperpinus species are unique in their own ways: the yellow-banded day sphinx, which lives in the boreal forests of North America; the Terloo sphinx, which is endemic to a region ranging from southern Arizona to Sonora in Mexico; and the Pacific green sphinx, which lives on the Pacific coast, from British Columbia to Mexico.
The yellow-banded day sphinx abandoned the green and red color scheme of its genus to make itself an excellent mimic of the bumblebees which share its forest habitat.
Terloo sphinx larvae feed only on spiderling plants, which are unrelated to the evening primrose plants that other Prosperpinus larvae dine upon. Spiderlings are favored because they are common during the mid-summer monsoon in the upland desert habitat where the moth resides and are in bloom during the few weeks when adults live.
Certain body parts of the Pacific green sphinx differ markedly from other Prosperpinus moths, so much so that it is was given its own monotypic genus: Arctonotus. That withstanding, it is the life history of the Pacific green sphinx that really sets it apart. Adult moths are active during early evening in mid-winter. Other Prosperpinus require heat and moisture to emerge from pupation. The Pacific green emerges when there are cold winter rains, where temperatures might fall below freezing.
There are virtually no blooming plants during the adulthood of the Pacific green sphinx. As a result, these moths don’t feed. They could not anyway, as their proboscis is nonfunctional. Adults rely wholly on energy acquired when they were larvae, more than 6 months prior.
Not only are Pacific green adult moths able to withstand frigid temperatures, but the eggs and larvae continue to develop during the winter, exposed to the bracing cold. This thermal robustness required unique cold-weather adaptations.
These 3 unique sphinx moth species independently speciated between 5.5 and 4.6 million years ago, in developments that are unrelated.
The rapid derivation of divergent unique traits, each in a separate species, strongly suggest that these adaptations are saltational. ~ Daniel Rubinoff & Johannes Le Roux
