The way in which each man's contribution came to pass, and in turn came to signify a new way of looking at all organic evolution since the world began, will be discussed here.
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Since there are so many cultures and so many histories and so many cosmologies among humanity's various pasts, this paper is necessarily going to be focussing on the western traditions which led to the work of Darwin and Mendel, essentially ignoring most of the works of non-western cultures. But while the medieval western mindset which reluctantly gave birth to the age of discovery may have left a legacy of religious dogma cluttering the intellectual edifice of modern scientific thought, there were times in mankind's past when some people grasped the significance of the fossil record and the apparent vastness of geological time to formulate a world view entirely consistent with a modern scientific one.
The ancient Hindu and Mayan world views seem to have held out at least the possibility of a great cycle of evolutionary time, where nothing is fixed once for all. But not so the western medieval scholastic world view.
Our western thinkers were so fixed on their sacred texts, that they constructed a fortress of ideas to protect the precious few fixed ideas which held, among the rights of kings and the wrongs of witches and the certainty of a bad judgement to come to most, that there was a single week, just a few thousand years before Moses walked, during which the entire universe was rushed to order, and brought out of the shop for all to use. Before modern evolutionary theory, with its understanding of genetic structures and geological time, and before its conceptual framework of environmental influences on the survivability of natural variations within species, there were, in Europe, some very fixed ideas about the nature of living things.
Not all men followed the lead of the Church, especially since Luther, but it was difficult to change the world view, based as it was on experience and common sense. It was self evident that the sun and stars revolved around the earth, that species were fixed at creation, and that humanity had souls and the other animals were for eating.
So slowly did Euclid's physics, Al-Khwarismi's algebra, Al-Rhazi's medicine, Al-Farghani's astonomy, Ibn Hayyar's chemistry, Copernicus' revolutions, Galileo's evidence, and the ever present fossil record, all chip away at the explanation of creation so well constructed by Thomas Aquinas, that even today it is easy to find people of religious bent who prefer the creation story told to them by their catechists to the logic of evidence which we have since inherited.
Metaphysical systems in the century preceding Darwin and Mendel were a dime a
dozen. The explanations current for heredity resolved themselves around innate attributes and mystical fluids and particles. There was some idea that all creatures were special creations, eggs or ovums made unique on the spot, with only some mystical contributions from the parent's bodily essences. What was missing was an experimental approach to heredity. The first proper heredity charts were made in the eighteenth century by two men studying independantly some common human mutations which are known to run in families. R. de Reaumur studied a family with extra digits popping up on hands and toes, charting the occurances through four generations, but drew no conclusion. Pierre Louis Moreau de Maupertuis, however, studied a family of the digitally enhanced, and did many breeding experiments with dogs, and as a result of his observations very nearly formulated a complete theory of evolution. He wrote in 1751, his paper Systeme de la Nature, "Could one not explain by that means [mutation] how from two individuals alone the multiplication of the most dissimilar species could have followed? ...in which the elementary particles failed to retain the order they possessed in the father and mother animals; each degree of error could have produced a new species...but to which perhaps the passage of centuries will bring only impercepticle increases" (Glass 1968: 77).
In Charles Darwin's time, the resistance to evolutionary theory was so strong that Darwin was reluctant to engage the public wrath by making his views generally known. Few other men in the history of science have waited twenty two years to present their discoveries in public forum. So, when he did publish his theory of natural selection, which flew in the face of the officially sanctioned doctrine of Divine selection, he opened his vast argument with a listing of all the great thinkers who had come before him with essentially the same frame of reference.(1) Having thus established that he is not a voice crying alone in the wilderness, but is firmly established among a vast company of learned men, Darwin begins to outline his great theory of the origin of species.
In addition to the mountain of evidence backing his contention that species have evolved, Darwin presents his variation on Malthus' theory of survival of the fittest among human populations, which Malthus called natural selection. Darwin extends this theory to the whole of animated creation, and presents his case that natural selection is the mechanism by which species are apt to change over time, in response to changes in their environment. It
is a grand theory, logical in its modest suppositions, immodest in its utter rejection of special creation by a Divine power. But Charles Darwin had no choice but to call it the way he saw it.
Darwin supposed that "the early male forefathers of man were probably furnished with great canine teeth; but as they gradually aquired the habit of using stones, clubs or other weapons for fighting their enemies or rivals, they would use their jaws less and less. In this case the jaws, together with the teeth, would become reduced in size" (Fagan 1996: 45) The great difference between the apes and the hominids shows up strongly in the skull, jaw and teeth of the two classes. If there were ever an evolutionary change from a small early apelike anthropod and human ancestors, it would surely show up in the fossil record as an apelike anthropod without the remarkable canine fangs typical of apes. When professor Raymond Dart had just such a fossil delivered to him in 1924, in South Africa, complete with fossilized infant sized brain, larger than any ape's, in an apelike skull sporting no canine fangs, but only incisors and grinding teeth, Dart recognized that it was the fossil of a link in the chain of human evolution, confirming Darwin's theory. Said Dart later, "I stood in the shade holding the brain as greedily as any miser hugs his gold, my mind racing ahead. Here, I was certain, was one of the most significant finds ever made in the history of anthropology." (Fagan 1996: 37-44) Indeed, he had just discovered Australopithecus.
The actual mechanism by which species are apt to change over time, as we now know, is through successful reproduction of a genetic variation of alleles, selected for survival naturally. (Even today, however, the specific mechanism for the successful addition of whole chromosomes in a zygote nucleus, such as is required of cross species evolution, is unknown and therefore remains theoretical.) In Darwin's time, the concepts of embryonic development and heredity were not yet separated.
There was, however, a very bright boy named Johann born in 1822 to a german farmer named Anton Mendel in what is now the Czech republic. Since the family was poor, and his schooling so costly, Johann surrendered to the obvious imperative and joined the religious order of the Augustinians at Brno in order to continue his studies without losing the struggle for survival. When he entered holy orders he took the name Gregor, and that is how he is remembered. He studied in Vienna under such luminaries as the physicist Christian Doppler and the biologist Franz Unger, who wrote "the endeavour to trace the diversities of species to the effects of outward influences, such as the nature of the soils, assuredly misses the true cause" (George 1975: 30)
Mendel was fascinated by natural history, and by plant hybridization in particular, so that it became his special study, while he worked as a school teacher, to experiment with plant breeding, and to report his findings to the local Natural History Society. Based upon his meticulously ordered experiments, his paper, Versuche uber Pflanzen-Hybriden, published in 1866 was a tour de force of theoretical clarity, founded entirely upon experimental data. But he mentions evolution on only three occasions, citing a viewpoint [Gartner's] that "a species has fixed limits beyond which it cannot change", Mendel expresses the opinion that Gartner's statement cannot be accepted unconditionally.(Hartl 1992: 247) Gregor Mendel had read Darwin. The 1863 german translation of the
Origin of Species found in the monastery at Brno is riddled with Mendel's own notations. But Mendel made no connection between his own study, of inherited traits in plants, and the theory of evolution by natural selection, perhaps sharing Unger's view that the environment was not a factor in inheritance.
So, what did Mendel deduce as a result of his experiments? From Mendel's Versuche: The difference of forms among the progeny of hybrids, as well as the ratios in which they are observed, find an adequate explanation in the principle [of segregation] just deduced. The simplest case is given by the series for one pair of differing traits. It is shown that this series is described by the expression: A + Aa + a, in which A and a signify the forms with constant differing traits, and Aa the form hybrid for both. Participating in fertilization are thus:
Pollen cells: A + A + a + a
Germinal cells: A + A + a + a
The result of fertilization can be visualized by writing the designations for associated germinal and pollen cells in the form of fractions, pollen cells above the line, germinal cells below. In the case under discussion one obtains
A/A + A/a + a/A + a/a = A + 2Aa + a
In the first and fourth terms germinal and pollen cells are alike; therefore the products of their association must be constant, namely A and a; in the second and third, however, a union of the two differing parental traits takes place again, therefore the forms arising from such fertilizations are absolutely identical with the hybrid from which they derive. Thus, repeated hybridization takes place. Mendel said this formula was the "law of combination of differing traits according to which hybrid development proceeds." He explained, "...[it] remains more than probable that
a factor that so far has received little attention is involved in the variability of cultivated plants....[Our] cultivated plants, with few exceptions, are members of different hybrid series whose development along regular lines is altered and retarded by frequent intraspecific crosses " (Hartl 1992: 252) The studious monk had discovered that hereditary traits are determined by hereditary cellular particles (now called genes), that evidently exist in pairs, undergo segregation and independent assortment, and persist unchanged through successive generations of hereditary transmission. We call this process meiosis, and it occurs in the
reproductive sex cells only. It is the mechanism by which variation within a species is reproduced.
His paper was forgotten almost as soon as it was published, since it answered questions about heredity that nobody had been asking. From the inaccessibility of the journal in which it had been published, to the fact that development and heredity were not understood to be separate concepts in his time, Gregor Mendel went unhailed until his work was uncovered
thirty four years later by scholarly researchers who had repeated his work and made the same discovery at the beginning of the twentieth century. They alone understood what he had been saying.
The celebrated variation in the population of the peppered moth of England can be explained by the work of Mendel, just as it can be explained by the theory of natural selection. In fact, while the theory of natural selection explains why the white moth is selected for destruction by the predatory bird population when the trees have been soot stained, and the darker moths camoflaged from sight, only Mendel's principle of genetically inherited variation explains how the moths can reproduce both varieties within one
population under varying environmental conditions. Mendel explains how both the variations are hereditary, and Darwin explains how one of the variations is naturally selected for destruction.
The most compelling instance of the evolution of a species in our time is the alarming evolution of antibiotic resistant strains of infectious bacterium in hospitals around the world. By the Mendelian processes of segregation of alleles and independant assortment of the resistant strains and the susceptible strains, the resistant alleles, even if recessive, will
dominate the population if the susceptible strains are constantly destroyed by the presence of antibiotics in their environment. Eventually, only resistant alleles will be able to be reproduced, the susceptible ones having been removed from the population.
Thus, the infectious bacterium population, relentlessly exposed to antibiotics, over time becomes immune to the medicine's effect, and the medicine is no longer antibiotic......
We learned from the combination of both Darwin and Mendel how evolutionary processes work. From Darwin and his fellow evolutionists we learned that the process of species variation was slow and took place over long periods of time. Darwin himself pointed out the particular role natural selection played in culling some varieties and encouraging others, and the role specific environments have played in steering the selection process.
Mendel has shown us how the stable, particulate (genetic) nature of inheritance allows for both hereditary traits and a variety of traits at the same time: genetically inherited variation. The sythesis of their two concepts provides us with the modern theory of evolutionary processes among living organisms.
Endnotes:
(1)
(2)
The difference of forms among the progeny of hybrids, as well as the ratios in which they are observed, find an adequate explanation in the principle [of segregation] just deduced. The simplest case is given by the series for one pair of differing traits. It is shown that this series is described by the expression: A + Aa + a, in which A and a signify the forms with constant differing traits, and Aa the form hybrid for both. The series contains four individuals in three different terms. In their production, pollen and germinal cells of form A and a participate, on the average, equally in fertilization; therefore each form manifests itself twice, since four individuals are produced. Participating in fertilization are thus:
Pollen cells: A + A + a + a
Germinal cells: A + A + a + a
It is entirely a matter of chance which of the two kinds of pollen combines with each single germinal cell. However, according to the laws of probability, in an average of many cases it will always happen that every pollen form A and a will unite equally often with every germinal-cell form A and a; therefore, in fertilization, one of the two pollen cells A will meet a germinal cell A, the other a germinal cell a, and equally, one pollen cell a will become associated with a germinal cell A, and the other a.
The result of fertilization can be visualized by writing the designations for associated germinal and pollen cells in the form of fractions, pollen cells above the line, germinal cells below. In the case under discussion one obtains
A/A + A/a + a/A + a/a = A + 2Aa + a
In the first and fourth terms germinal and pollen cells are alike; therefore the products of their association must be constant, namely A and a; in the second and third, however, a union of the two differing parental traits takes place again, therefore the forms arising from such fertilizations are absolutely identical with the hybrid from which they derive. Thus, repeated hybridization takes place. The striking phenomenon, that hybrids are able to produce, in addition to the two parental types, progeny that resemble themselves is thus explained. This represents the average course of self-fertilization of hybrids when two differing traits are associated in them. In individuals flowers and individual plants, however, the ratio in which the members of the series are formed may be subject to not insignificant deviations. (Hartl 1992: 249)
Evolutionary Theory
manuscript by
Hugh Bibbs
The synthesis of evolutionary thinking which we call the Modern Theory of Evolution comprises two central contributions from the physical sciences which were originally articulated in the mid-nineteenth century (Turnbaugh 1996: 95). They are, firstly, the theory of natural selection articulated by Charles Darwin in his 1859 publication of On the Origin of Species by Means of Natural Selection, and, secondly, the principle of genetically inherited variation as first detailed by Gregor Mendel in his famously overlooked paper Versuche uber Pflanzen-Hybriden printed in 1866 in Brno's obscure science journal, Verhandlungen des naturforschenden Vereines in Brunn.
Darwin credits Buffon for having a scientific outlook, and Lamarck for having brought attention to the evolutionary process at work in living creatures. He recalls Geoffroy St-Hilaire, a Dr. Wells, a Mr. Rowley of the United States, no less, Mr. Brace, the Dean of Manchester and Rev. W. Herbert. He praises Professor Grant with having had the hindsight to observe change within species, and he reaches for an incidental appendix to a Mr. Patrick Matthew's treatise on naval architecture. While confessing that "I am not sure that I understand some passages..." he nevertheless declares that the said Mr. Matthews "...clearly saw, however, the full force of the principle of natural selection [at work in the tree population] "(Darwin 1996: 57). Darwin cites Von Buch's geography, Rafinesque's flora, and a professor Haldeman, as all backing his view. He praises an anonymous work, Vestiges of Creation, and a paper by the geologist M. J. d'Omalius d'Halloy. All of these expert witnesses support his view, he claims, that new species have been produced by descent with modification instead of separate creation. He goes on to quote professor Owen's interpretation that archetypal skeletal structures found in various bird species worldwide are evidence of common ancestry. He continues to cite other comrades in evolutionary theory such as Wallace, Dr. Freke, Herbert Spenser, the french botanists M. Naudin and M. Lecoq, the geologist Count Keyserling, Dr. Schaaffhausen, the Reverend Baden Powell, the zoologist Von Baer, professor Huxley and finally a Dr. Hooker, "who" says Darwin "admits the truth of the descent and modification of species, and supports this doctrine by many original observations" (Darwin 1996: 63).
From Mendel's Versuche, a more complete quotation: