Conceptual History of Evolution Theories
Though not a long-held widespread belief, the idea of evolution has echoed through human history. Intertwined with the concept of evolution was the issue of how life itself came about.
Ancient Indian Vedic texts, dating 3,000 years ago or even earlier, blithely considered life as a fact: particles of life pervade the universe, taking form when conditions allow.
In the 6th century bce, Greek philosopher Thales of Miletus declared that the world started from water, as did life. A century later, Greek philosopher Empedocles asserted several insights that had lasting impact. One was that all things were of 4 primal elements: air, fire, earth, and water.
In a seminal variant of the conservation of energy, Empedocles believed that no new matter was ever created. Only its form changed, as an admixture of the 4 elements. Empedocles also formulated theories of inheritance and evolution to explain adaptation to habitat.
Ancient Greek philosophers Plato and Aristotle had their own abstractions about the nature of things. Plato was an idealist. Everything in Nature derived from nonmaterial eternal ideas (“forms”).
Aristotle one-upped that by bifurcation, embracing dualism. The material world exists, but material living beings are also suffused with a vital principle – entelechy – that conducts the processes of organisms. Entelechy is nonmaterial, and therefore undiscoverable by scientific investigation.
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Though reared in poverty, Al-Jahiz thirsted for knowledge. He moved to Baghdad as a young man and became one of the greatest, and most prolific, Arab scholars of all time. In his encyclopedic Book of Animals (~860), Al-Jahiz wrote: “Animals engage in a struggle for existence: for resources, to avoid being eaten and to breed. Environmental factors influence organisms to develop new characteristics to ensure survival, thus transforming into new species. Animals that survive to breed can pass on their successful characteristics to offspring.”
Persian polymath Nasīr al-Dīn Tūsī wrote Akhlaq-i-Naseri (1232), a treatise on ethics which proposed a hierarchy of life that was accepted by other natural philosophers and remains a falsity that still rings true with many, including many biologists who should know better.
Animals are higher than plants, because they are able to move consciously, go after food, find and eat useful things. There are many differences between the animal and plant species. The animal kingdom is more complicated.
Reason is the most beneficial feature of animals. Owing to reason, they can learn new things and adopt new, non-inherent abilities. The first steps of human perfection begin from here.
The human has features that distinguish him from other creatures, but he has other features that unite him with the animal world, vegetable kingdom or even with the inanimate bodies. Before humans, all differences between organisms were of the natural origin. The next step will be associated with spiritual perfection, will, observation, and knowledge.
All these facts prove that the human being is placed on the middle step of the evolutionary stairway. According to his inherent nature, the human is related to the lower beings, and only with the help of his will can he reach the higher development level. ~ Nasīr al-Dīn Tūsī
An evolutionary hypothesis underlay this hierarchical scheme.
“The organisms that can gain the new features faster are more variable. As a result, they gain advantages over other creatures. The bodies are changing as a result of internal and external interactions.” ~ Nasīr al-Dīn Tūsī
Spontaneous Generation
Although the Copernican revolution deposed humans from being the cosmic center of all of God’s creation, humans nevertheless remained the pinnacle of God’s works. Natural philosophy positioned science and religion in close harmony, with the remarkable design so clearly manifested in creatures great and small being seen as evidence of God’s hand. ~ Roger Lewin
In 1688 Italian physician Francesco Redi demonstrated that flies laid eggs that developed into maggots which begat flies. This and other such proofs eventually dispelled spontaneous generation as spawning life. But it was a long time coming.
Until the mid-19th century, the widespread belief was that, while large creatures reproduced by sex, small organisms could arise spontaneously from mud or organic matter. Folklore suggested that large dead animals decomposed to smaller ones.
It was not until the end of the 19th century that spontaneous generation was entirely abandoned. The experiments of French microbiologist Louis Pasteur and Irish physicist John Tyndall were compelling. Tyndall disproved spontaneous generation by showing that putrefaction did not occur in dust-free air. Pasteur founded the science of microbiology in showing that “germs” caused disease. He then invented pasteurization and developed vaccines for several diseases, including rabies.
Preformationism
An infinite series of plants and animals were contained within the seed or the egg. They are infinite trees in only one seed. All the bodies of humans and animals were perhaps produced as soon as the creation of the world. ~ Nicolas Malebranche
As the 18th century dawned, while spontaneous generation was slipping, another theory of life generation gained public acceptance. Preformationism embodied the idea of new life coming into being as miniatures, growing only in size.
Preformationism had much earlier roots. The concept itself was preformed and merely growing in approval. Christians long believed that all life was created in the same week, and on the 7th day the Creator rested (apparently it having been a tough work week).
In stark contrast to preformationism, epigenesis posits that organisms develop through cellular differentiation, by which distinct organs and tissues form. Epigenesis was a radical idea long resisted.
Spermism
Has the semen soul, or not? ~ Aristotle
The name of 6th-century-bce Greek philosopher Pythagoras would live on for his triangle theorem, among other mathematical feats. Pythagoras also originated spermism: the notion that fathers provided the essential characteristics of their offspring, while mothers merely contributed material substrate. This testosterone-fueled foolishness reverberated through history.
A century and a half after Pythagoras, Aristotle further ejaculated on spermism. In his treatise On the Generation of Animals, Aristotle fixated on semen, wildly speculated on inheritance, and tossed about preformationism and epigenesis, whereupon he came down solidly in favor of preformationism. In the inevitable cerebral coin tosses of ignorance, one must ultimately pick a side.
The microscope was instrumental in resolving developmental biology. But a tool is only as good as its user.
In the 1660s, Dutch lens-grinder Antonie van Leeuwenhoek considerably improved the resolving power of the microscope, and was one of the first to see spermatozoa, going on to chronicle the sperm of about 30 species. His description confirmed spermism, upon seeing what he took to be:
… all manner of great and small vessels, so various and so numerous that I do not doubt that they be nerves, arteries and veins. And when I saw them, I felt convinced that, in no full-grown body, are there any vessels which may not be found likewise in semen.
Jan Swammerdam
In 1668, Dutch biologist and microscopist Jan Swammerdam was first to observe and describe red blood cells. Swammerdam was a pioneer in using the microscope in dissections. His techniques were employed for hundreds of years.
17th-century knowledge of insects was largely inherited from Aristotle, who considered the critters so insignificant that they were largely ignored as unworthy of scientific study, thus ensuring that entomology would be neglected for over a millennium. Swammerdam studied insects under his microscope. Swammerdam’s principal interest was in disproving metamorphosis: that insect life stages could radically differ in form. In this he was dead wrong. Nevertheless, Swammerdam observed that late-stage larval flies had undeveloped adult features, such as wings. This led him to cast his lot with preformationism.
There is never generation in Nature, only an increase in parts. ~ Jan Swammerdam
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The biblical myth of preformationism was strongly favored as the dominant Christian theology. Epigenesis instead suggested that unorganized organic matter could somehow generate life without the need for God’s intervention. How dare it.
The microscope brought forth a religious quandary, centered on sperm (shades of Aristotle). Observing sperm as a life form, how could it be that so many little animals were wasted by even a single ejaculation?
In the early 18th century, German philosopher Gottfried Leibniz gave spermism a lift with the notion of panspermism: that wasted sperm could be scattered by the wind and generate life upon finding a suitable host. Leibniz also believed that “death is only a transformation enveloped through diminution.” For Leibniz, not only have organisms always existed in their living form, they will always exist, body united with soul, in an endless cycle of death and rebirth.
English physician William Harvey was first in the Western world to write of the circulatory system: of blood being pumped around the body by the heart. Harvey’s 1651 On the Generation of Animals pumped epigenesis, contradicting Aristotle’s conjecture of preformationism (in his identically titled treatise).
Harvey famously asserted ex ovo omnia: all animals come from eggs. This was misunderstood in some biased quarters as suggesting ovist preformationism, egg-based preformationism, but Harvey disclosed epigenesis as the basis for biological development.
As gametes were too small to be seen under the best magnification at the time, Harvey’s account of fertilization was speculative rather than descriptive. Early on, Harvey proposed that a “spiritous substance” fertilized a female, but he later spurned it as superfluous, and so unscientific.
Harvey’s 2nd guess was that fertilization transpired through a mysterious transference, either by contact or contagion – contact indeed, of a most contagious sort.
Preformationism clung in quarters given to Christianity for some time. In the late 17th century, French philosopher Nicolas Malebranche sought to reconcile the teachings of Augustine of Hippo and Descartes, to prove that God was omnipresently active. Malebranche proposed that each embryo held ever smaller embryos ad infinitum: a biological Matryoshka doll.
Preformationism dominated in the 18th century, competing with spontaneous generation and epigenesis, as those theories held that inert matter could create life without God’s intervention. It wasn’t until the late 19th century that cell theory would succeed preformationism as the accepted model of life.
Natural Selection
From so simple a beginning endless forms most beautiful and most wonderful have been and are being evolved. ~ Charles Darwin in The Origin of Species (1859)
Preformationism eventually bowed to cell biology, though to this day evolution is rejected by Christian sects which cannot reconcile their delusional belief system with actuality.
For the more sober-minded, the mechanics of evolution have been the grounds for discovery and argument, revolving around 3 key questions: 1) whether evolution is random or adaptive; 2) whether the impetus behind evolution is competitive or environmentally adaptive; and 3) whether evolution is purely genetic, or influenced by lifestyle prior to conception. The answers to these questions are known beyond doubt yet still argued over. Like physics, economics, and politics, evolutionary biology is as much religion as science.
Jean-Baptiste Lamarck
Behaviour may drive evolutionary diversification. ~ Spanish evolutionary biologist Oriol Lapiedra
French naturalist Jean-Baptiste Lamarck (1744–1829) considered evolution to be adaptive. His book Zoological Philosophy was published in 1809, the year Darwin was born.
Lamarck proposed 2 axioms of evolution. The 1st was about adaptation based upon biological need, and its corollary: depreciation by disuse.
In every animal, more frequent and sustained use of an organ strengthens that organ, while the constant disuse of an organ imperceptibly weakens it and ends in its disappearance. ~ Jean-Baptiste Lamarck
Lamarck cited the long neck of the giraffe as an example of usage – stretching to reach leaves – driving adaptation. 2 centuries later, this proved true.
The most primitive giraffe already started off with a slightly elongated neck. The lengthening started before the giraffe family was even created, 16 million years ago. ~ American zoologist Melinda Danowitz in 2015
Lamarck’s 2nd axiom was that the lifestyles of parents affected their offspring.
Everything that Nature has caused individuals to acquire or lose by the influence of the circumstances, it preserves by heredity and passes on to the new individuals descended from it. ~ Jean-Baptiste Lamarck
In the 1950s, after geneticists discovered DNA, they thought that they had unlocked the secret of inheritance. The recipes of life were encoded in DNA; simple as that.
With that in mind, Larmarck’s hypotheses became moribund. It was not until the end of the 20th century, with the discovery of epigenetics, that Lamarck’s prescient 2nd axiom, on heritable variation, was vindicated.
The possibility that acquired (epigenetic) ‘marks’ can be passed from parents to children has a deliciously Lamarckian flavour that has proved difficult to resist as a potential antidote to genetic determinism. ~ English geneticist Adrian Bird
Roundworms develop resistance to a virus and can pass that immunity onto to their progeny: an acquired trait conferred for many consecutive generations. No genetic mutation is involved.
The immunity is transferred in the form of small viral-silencing agents called viRNAs, working independently of the organism’s genome. ~ Israeli neurobiologist Oded Rechavi
RNA interference (RNAi) is a process that cells use to silence specific genetic expressions. It is commonly employed to fight off viruses and other genomic parasites. RNAi waylays messenger RNA (mRNA), a courier for gene expression. Without mRNA in the act, a gene is left inactive.
What works for worms works all the way up the evolutionary ladder. Epigenetic mechanisms let animals and their offspring evolve while maintaining the same genetic pedigree. This flexibility, coupled to gene conservation, is a great advantage in adaptability, both in uptake and in rapid reversibility.
Around the world, several fish species evolved in caves, without sunlight, all sharing common adaptations, including eye loss, enhanced senses on their skin to assist in navigation, and loss of skin pigment. Disuse has an evolutionary effect owing to a natural tendency to conserve resources.
Animals’ biorhythmic clocks typically rely upon sunlight for calibration. Cave fish lack the Sun’s cue. The circadian clock is a highly conserved physiological timing mechanism, as many bodily functions are synchronized to the internal clock. A cave fish’s clock is set by its feeding schedule.
Taste sensitivity depends upon diet. As meat mostly stimulates the umami taste receptors, carnivorous mammals are overly sensitive to that particular taste, but lose their ability to savor sweetness, as it confers no benefit. In contrast, the giant panda, which feeds primarily on bamboo, lacks a functional umami taste receptor.
Raccoons and bears eat a lot of meat, but their diets are fairly broad. They retain the genes to taste sweetness.
Sea lions and dolphins descended from land mammals, returning to aquatic environs 35 and 50 million years ago, respectively. Adaptive to their diets, these animals experienced atrophies in their taste systems.
Conversely, where specific senses dominate, they become especially acute. Sometimes such adaptations are a matter of both sensory sensitivity and behavior. Rats rely heavily upon their sense of smell: so much so that they can target their sniffing to specific objects of interest.
When a bird changes from terrestrial foraging to finding food in the trees adaptations accelerate in certain physical characteristics. A bird adopting an arboreal lifestyle grows shorter hindlimb bones and longer tails, providing a perching improvement.
10 MYA a more arid climate caused East Africa to change from woodland to grassland. The elephants there switched from browsing to grazing by 8 MYA, when the terrain was still a mosaic of vegetation. Within 3 million years, elephant teeth had evolved to better accommodate the shift in dietary preference. Elephant behavior shaped morphological adaptation.
A female dolphin in western Australia started protecting her sensitive rostrum from damage when foraging on the seabed by covering it with a protective sponge. Others in the pod learned it, and it was taught to offspring. This cultural knowledge, passed on for generations, took genetic expression.
A socially transmitted behaviour like tool use can lead to different genetic characteristics within an animal population. ~ Swiss evolutionary geneticist Anna Kopps
As de novo protein synthesis is required for memory formation, memories are epigenetically encoded. These gene expression programs are readily transmitted to offspring. This is the molecular facet of precocious knowledge evolving.
(Material substrates, such as nuclei acids and associated chemical compounds, are ever only coincidental, not causal, to the energetics by which Nature is emergently composed.)
Epigenetic processes play a role in memory consolidation and help to transmit acquired memories even across generations in a Lamarckian manner. ~ German molecular biologist André Fischer
Giovanni Brocchi
Italian Giovanni Brocchi (1772–1826) was a geologist and mineralogist, interested in natural history. Unlike Lamarck, Brocchi saw species as discrete and fundamentally stable unless driven to extinction by external forces. Brocchi thought that new species replaced extinct ones.
Brocchi’s book on evolution was read by Darwin when he was a student at Cambridge. Influenced by Brocchi, Darwin’s 1st hypothesis of species origin was explicitly saltational, invoking geographic isolation as provoking abrupt appearance of a new species. Darwin later rejected saltation.
Charles Darwin
I am almost convinced that species are not immutable. ~ Charles Darwin in 1844
English naturalist Charles Darwin (1809–1882) took an unpaid position on the survey ship HMS Beagle when he was 22 (1831). Near the end of it 5-year voyage, Darwin observed life on the Galápagos Islands, isolated off the coast of Ecuador. Each island had its own variety of tortoise, mockingbird, and finch.
Inspired by Charles Lyell’s theory of a gradually changing Earth, and Thomas Malthus’ theory of food supply limiting population growth, Darwin derivatively developed a similar hypothesis of biological evolution: gradual change in speciation based upon competition for survival.
(Darwin hardly ever used the word evolution, preferring “descent with modification.” Darwin did not coin “survival of the fittest.”)
In the struggle for survival, the fittest win out at the expense of their rivals because they succeed in adapting themselves best to their environment. ~ Charles Darwin
This was the thrust of Darwin’s evolution hypotheses on variation and selection, described in his 1859 book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. The photo shown was taken shortly after the book’s publication, when Darwin was 51.
Darwin’s “natural selection” was concocted from the idea of there being a struggle for survival. Darwin pondered in 1858:
Can it be doubted, from the struggle each individual has to obtain subsistence, that any minute variation in structure, habits, or instincts, adapting that individual better to the new conditions, would tell upon its vigour and health? In the struggle it would have a better chance of surviving; and those of its offspring which inherited the variation, be it ever so slight, would also have a better chance. Yearly more are bred than can survive; the smallest grain in the balance, in the long run, must tell on which death shall fall, and which shall survive. Let this work of selection on the one hand, and death on the other, go on for a thousand generations, who will pretend to affirm that it would produce no effect?
Darwin’s initial hypothesis was that random mutations are passed to the next generation, resulting in incremental evolutionary change. In Darwin’s imagination, these variations are winnowed by “natural selection.” Those variations that do not confer an advantage to the breeding success of a species are not passed on: survival of the fittest as a competitive exercise.
English parson Thomas Malthus published an Essay on the Principle of Population (1797), where he argued that public policies designed to help the poor were doomed because they would simply cause unchecked breeding that could lead to famine and misery for all.
From reading Malthus, Darwin conceived of natural selection as constant environmental competitive pressure forcing change to survive. From this, Darwin believed that “natural selection almost inevitably causes much extinction of the less improved forms of life.” Darwin had no evidence upon which to base this ludicrous claim.
Natural selection acts solely by accumulating slight successive variations. It can produce no great or sudden modification. It can act only by very short steps. ~ Charles Darwin
Darwin’s choice of the term natural selection derived from selective breeding of domestic animals. In Origin he opined:
Good domestic breeds are valued by the negroes of the interior of Africa who have not associated with Europeans. Some of these facts do not show actual selection, but they show that the breeding of domestic animals was carefully attended to in ancient times, and is now attended to by the lowest savages. It would, indeed, have been a strange fact, had attention not been paid to breeding, for the inheritance of good and bad qualities is so obvious.
At the present time, eminent breeders try by methodical selection, with a distinct object in view, to make a new strain or sub-breed superior to anything existing in the country.
In contrast to “actual selection” by breeders, Darwin considered “natural selection” to be Nature competitively selecting the species that survive.
Darwin axiomatically assumed a geologic timeframe for natural selection: a slow, incremental progression. Instead, evolution continuously happens, producing visible results in spurts, from both environmental demands and opportunities.
Darwin wrongly surmised that every aspect of an organism was subject to selection. He had not a glimmer of core conservation: that cellular fundamentals were conserved.
Lamarck haunted Darwin. Darwin initially proposed mutation as occurring first, followed by winnowing, but considered this hypothesis incomplete, as it lacked a mechanism for selection.
Darwin increasingly retreated to Lamarck’s more mysterious but purposeful selection: that living can demand change, that the environment can facilitate or induce adaptation, and that incremental changes from life’s experiences are somehow passed on to offspring.
In 1868, Darwin conjectured pangenesis: a complex mishmash of heredity enveloping sexual reproduction, life-experience, and developmental phenomena, such as cellular regeneration. Darwin’s proposed mechanism for pangenesis were tiny organic particles – gemmules – which transmitted heritable information from all parts of the body to the gonads.
Pangenesis was criticized for its Lamarckian roots: that parents could pass on traits acquired during their lives. Nonetheless, Darwin persisted in tinkering with pangenesis. His last word on the topic emphasized that traits could be inherited which were not manifest in a parent organism, that a developing organism could exhibit trait transition, and that certain traits depended upon an organism’s sex.
While some of his individual conjectures panned out (make enough guesses and you might get some right), Darwin’s speculations fell far short of a well-founded hypothesis outlining a biomechanism for evolution, which is what Darwin sought.
Gregor Mendel
The value and utility of any experiment are determined by the fitness of the material to the purpose for which it is used. ~ Gregor Mendel
Gregor Mendel (1822–1884) was an Austrian monk who made the most of his life through patience and diligence. His chosen occupation and precise nature would delay appreciation of his work until long after he died.
Mendel came from a family of peasant farmers. His parish priest and teacher, impressed with Mendel’s intelligence in elementary school, helped him get secondary schooling; unusual for a peasant.
Mendel struggled for funds. One of his professors suggested joining the Augustinians, whose main work was teaching, and who would pay for Mendel’s schooling. Mendel became a monk at 21.
The abbey was a remarkable place: access to scientific instruments, an extensive library, and an excellent botanical collection. The abbot shared with Mendel a love of plants.
Thanks to his mentorship, Mendel was able to study at the University of Vienna from 1851 to 1853. He was sent to Vienna partly because Mendel had failed his teaching examination.
Mendel took courses in plant physiology and experimental physics. His professors emphasized studying Nature via experiments, underpinned by mathematical models. While Mendel had enthusiasm for his studies, upon his return from Vienna, he again failed his teaching exam.
This limited Mendel to substitute teaching. On the bright side, it also let him spend more time doing what he loved: gardening.
Between 1856 and 1863, in the monastery’s garden, Mendel cultivated and examined 29,000 pea plants, tediously chronicling his observations about their flowers (color and position), seeds (color and shape), pods (color and shape), and height. The variety of their easily identifiable traits, quick generation, and copious offspring made peas a fortuitous choice.
Mendel detailed laws of genetic heredity in Experiments on Plant Hybridization, published in a regional natural science journal in 1866. Given its title, Mendel’s paper was understood as being about hybridization, not heredity, and had negligible impact for over 3 decades, until it was rediscovered.
Mendel hypothesized hereditary units, how inheritance manifested, about hybridization, and expression of dominant or recessive characteristics. Mendel’s heredity laws were: 1) the law of segregation, and 2) the law of independent assortment.
While genes are paired in normal cells, they are segregated in sex cells (eggs or sperm), which unite to form a gene pair. The pair express either a dominant or recessive trait.
A dominant gene trumps a recessive gene, so it takes 2 recessive genes for a recessive trait to be expressed; whence Mendel’s law of segregation.
Mendel’s law of segregation is nominal and subject to violation. Plants are known to sometimes employ ancestral alleles, not parental genes. This paramutation may occur by inheritance via double-stranded RNA, not DNA.
Mendel’s 2nd law, independent assortment, was that the expression of any 1 genetic trait is not influenced by another. It may have seemed that way for pea plants at first blush, but inheritance is more intricate than that. Most genes are linked. For example, biorhythms determined by an organism’s circadian clock are the product of a gene complex.
Many processes are under biorhythmic sway, from organs to tissues to cells. Even the production of ATP in mitochondria oscillates by a molecular clock.
Done with peas, Mendel turned to honeybees, creating a hybrid strain so vicious they were destroyed. His work was inconclusive as to heredity in mating bees.
Perhaps Mendel would have had better luck if he had chosen fruit flies, the genetics of which were first studied by American evolutionary biologist Thomas Morgan at the turn of the 20th century. Morgan followed in Mendel’s footsteps, though he was initially skeptical of Mendel’s laws of heredity.
Fruit flies (particularly Drosophila melanogaster) would go on to be geneticists’ favorite victim, even though they fail to exhibit some important genetic mechanisms found in other animals. While helpful for learning the fundamentals, the rather exclusive focus on fruit flies by researchers retarded for decades a more thorough understanding of genetics, especially epigenetics.
Mendel was promoted to abbot in 1868. His increased administrative responsibilities, especially a fight over taxes with the local government, spelled an end to his scientific work.
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Though failing to explain how evolution occurred, Darwinism held tremendous sway over generations of biologists; another example of the staying power of sophistic explanation by the well-respected, and a tendency to favor simplistic stories, thus crowding out more accurate but involved scenarios.
The term natural selection remains ubiquitous and is widely considered in much the same way that Darwin conceived: evolution via competition.
Lamarckism fell from favor in the 1880s, after German evolutionary biologist August Weismann facilely concluded that changes from use, such as lifting weights to increase muscle mass, and disuse, such as lazy lifestyle, were not heritable. Weismann also found Darwin’s theory “inadequate.”
In their stead, Weismann proposed the germ plasm theory: that the only carriers of inheritance are germ cells (eggs and sperm). The supposition lurking behind germ plasm theory is that heredity is entirely hard-wired, which snugly fits with DNA as the sole messenger of inheritance. What was unknown for decades is that DNA provides only a partial code, subject to interpretation and limited expression.
