* The realization that the classical theory of evolution by natural selection and Mendel's work on heredity complemented each other led to intensive work to dovetail the two concepts. The result of this effort was what would become known as the "Modern Synthesis" of evolutionary science. In the meantime, biochemists worked towards understanding the molecular basis of heredity, leading to the discovery of the double-helix structure of the DNA molecule.
* Although early genetics research seemed to be at odds with classical evolutionary theory, from the mid-1920s the two concepts began to draw together, largely through the work of the Scots biochemist John Burdon Sanderson Haldane (1892:1964), the English geneticist Ronald A. Fisher (1890:1962), and the American geneticist Sewall Wright (1889:1988). Together they formed the quarrelsome triumvirate of a revitalized evolutionary science.
J.B.S. Haldane was by all accounts overbearing and contrarian, inclined to assail religion and embrace Leftist causes. He was also brilliant, and in a series of ten research papers published from 1924 into 1934 showed that Mendelism -- genetics -- and evolution were not merely compatible, they fit together like hand in glove. The idea that the two worked together was not new, having been around for about two decades previously, but Haldane was able to demonstrate its plausibility mathematically. He started with the example of the peppered moth.
The pale-colored peppered moth was long known to British insect collectors, but in 1848 one was found with a dark coloration. In 1896, the British naturalist J.W. Tutt (1858:1911) discovered that while the pale peppered moth was still predominant in the English countryside, the dark peppered moth was overwhelmingly predominant in sooty industrial areas. The conclusion was that the dark peppered moth was better camouflaged for life in industrial areas, allowing it to avoid predators -- birds in particular -- and giving it a selection advantage over the pale peppered moth.
Haldane applied statistical methods to the story of the peppered moth, pointing out that if dark camouflage gave it a fairly modest survival advantage in an industrial environment, the dark form would outbreed the pale form and predominate within a fairly small space of generations. He also showed that simple mutations couldn't do the job, since it would demand that every fifth moth born to pale parents be a dark mutant, which was absurd; natural selection had to be involved.
* Haldane's mathematics, not so incidentally, did not constitute a proof of evolution by natural selection -- a point worth emphasizing before going further, since people are sometimes impressed by elaborate mathematics without realizing exactly what it can and cannot do. Scientific modeling, or its modern extension computer simulation, is merely a description of how the real world is believed to work, not a proof of how it works.
The sciences obtain data from observations of the real Universe, or experiments when possible, and then attempt to account for the relationships between observations, constructing a model, a theory, that ties them together. There is an inclination, even among some scientists, to read more into the model than is there. Ultimately, there is no more substance to the model than the observations that establish its foundation; any element in the model that isn't directly linked to observations is a necessary accounting rule at best, useless excess baggage at worst.
Isaac Newton's law of universal gravitation -- that gravity is proportional to the product of two masses and inversely proportional to the square of the distance between them -- is a simple mathematical model derived through Newton's intuition from observations of the motions of heavenly bodies. It has proven a useful tool for centuries and is still used today to plan the flight of interplanetary spacecraft. However, Newton was careful to say he did not know why gravity behaved the way it does -- "I frame no hypothesis." -- he simply said all observations available to him showed that gravity worked the way he believed it did.
Newton's law of gravity is extremely useful, but the equation itself doesn't amount to a proof of anything. It is no more valid than observations show it to be, and in fact, in modern times it is known that it isn't valid under extreme circumstances that Newton had no knowledge of, such as the "warped space" around the collapsed stars known as black holes.
If a model can't be validated by observation even in principle, it's useless. If a model gives results that don't match observations, then something's wrong with the model. Sometimes double-checking shows the original observations were bogus, and that the model is correct; but if double-checking confirms the observations, then the model's broken.
It should be noted that when the results of a model don't match reality, the model may be tweaked until it does. There is nothing inherently wrong with that, the notion not being much different from that of adjusting a gunsight of a target match rifle to ensure hitting the bull's eye. For example, the actual values provided by Newton's law of gravity are subject to a constant factor "G" that effectively gives the strength of the gravitational force, and has to be derived from observations. However, having obtained that value of G, the law of gravity is then generally applicable. If it wasn't, if the value of G had to be tweaked for every different case, the model would be worthless.
* As a footnote to this section, although the peppered moth is a classic example of evolutionary natural selection, it is also one of the most heavily criticized. One of the later investigators, Bernard Kettlewell (1907:1979), performed experiments in the 1950s that suffered from a number of potential flaws, for example placing moths on tree trunks despite the fact that they don't generally perch there. This error was aggravated by texts that showed obviously staged photographs of dark and light peppered moths on tree trunks as illustrations.
The matter was played up by creationists as the scientific scandal of the century. In reality, the rise and fall of the peppered moth relative to industrial pollution was already well established, and the differential predation of light versus dark moths remains the obvious driving force. Indeed, one of the criticisms of Kettlewell's work -- that he didn't realize that birds see into the ultraviolet, and so a moth camouflaged to a human might not be camouflaged to a bird -- evaporated when it was determined that the camouflage of the moths worked just as well in the ultraviolet as it did in the visible light range. Accusations that Kettlewell fudged his data haven't been validated by careful reexaminations of his work.
In any case, Kettlewell's experiments are old news. Clean air acts were passed in the UK at about the same time that Kettlewell was performing his experiments. Since that time the British landscape, both rural and urban, has become vastly cleaner. The result is that light-colored peppered moths once again predominate, with the dark form all but becoming extinct.
BACK_TO_TOP* Haldane's pioneering mathematical studies were paralleled by those of R.A. Fisher. Fisher was a single-minded man, a die-hard eugenicist whose studies were almost entirely focused to that end. The culmination of his work was his GENETICAL THEORY OF NATURAL SELECTION, published in 1930, in which he mathematically demonstrated how a small mutational change would propagate through a population, the rate of propagation being proportional to the advantage of a change, and also showed how shifts in the environment would lead to shifts in the makeup of the population.
Confronted with the work of Haldane and Fisher, geneticists increasingly accepted that the gross changes observed by Mendel with his pea plants were much more the exception than the rule, and that traits influenced by multiple genes could produce what seemed to be a gradual, continuous range of differences. For example, a Swedish biologist named Hermann Nilsson-Ehle showed that if a trait were controlled by ten different genes, that trait would have about 60,000 variations, a range that in practice might well appear to be seamlessly continuous. By the 1930s, genetics had made its peace with evolutionary theory, and in fact the two had formed up a close collaboration.
However, field biologists were unconvinced by the musings of theoreticians -- Haldane, incidentally, was an entirely inept practical biologist -- and had a strong tendency to cling to Lamarckism. To them, natural selection just didn't seem able to account for the remarkable features of organisms that were observed. Sewall Wright, in contrast to Haldane and Fisher, was actually a practical biologist, with a background in animal breeding, who also had a mathematical bent. As a result, Wright had a close focus on the real world: instead of relying on broad generalities, he was able to zero in on real-world scenarios, and as a result field biologists found him much more persuasive.
He was also more persuasive since, instead of hosing down his audience with calculations, he gave them an easily visualizable model: the "adaptive landscape" or "fitness landscape". In a 1932 paper, he suggested that evolution might be compared to a landscape with peaks and valleys, corresponding to adaptations providing good and poor fitness respectively. Mutations in species would cause organisms to migrate around the landscape in a blind fashion, one step per generation, guided by the "terrain" to drift up peaks and away from valleys as fitness improved.
In a modern computer simulation, the organism could be seen as a sort of simple-minded "evobot" that moved over the landscape through genetic changes at random, generation by generation, with the only sort of guidance being a bias towards moving uphill and against moving downhill. Wright saw the landscape as smoothly curved, since the genetic variations from generation to generation were seen to be generally small and gradual.
It should be emphasized that the fitness landscape is a sheer abstraction. Trying to actually describe a general fitness landscape would be difficult since fitness is a broad term, with many possible "dimensions" -- it can be a function of the ability of a life-form to deal with climate, to avoid predators, to find prey, to digest foods, to find a mate, to deal with pathogens, and so on in a long list -- making it hard to represent except for specialized cases. Since there are "trade-offs" in the adaptations of an organism, with an adaptation that gives an advantage sometimes inescapably also providing a disadvantage, what amounts to a "peak" in one dimension of the fitness landscape may be a "valley" in another. For example, the size of a full-grown bull elephant makes it almost invulnerable to predation by lions or the like -- but makes it more vulnerable to starvation in times when forage is scarce.
The overall "fitness" of an organism is a complicated balance of a great number of factors, reflecting a "fitness peak" that is a composite result of many environmental pressures, or "selection pressures". In addition, the factors are not necessarily fixed, with changes in climate, introduction of competitors and predators, and so on shifting the fitness peaks and valleys around. In fact, if an organism reaches a fitness peak, that organism will become more common, making it a target for predators and parasites, inevitably shifting the peak underneath it. Despite its abstract nature, the fitness landscape still is an elegant visualization tool for various specific scenarios.
There is an implication in this model that if an organism reaches a fitness peak, it is likely to stay there indefinitely, enduring down through the generations with little change. The fact that the landscape can, in fact is almost certain to, shift, with the peaks moving or disappearing, ensures ongoing change. However, Wright pointed out another mechanism to show why organisms didn't remain static indefinitely: "genetic drift". If mutations are occurring in organisms all the time, then they don't really occupy a static position in the fitness landscape anyway, with the locus jittering around at random due to hereditary "noise". In other words, the evobot's motions on the fitness landscape are a bit drunken and unsteady -- becoming steadier when selection pressures are strong, unsteadier when they are weak.
The effects of this "noise" are much more visible in a small population than a big one. In many cases, this "noise" may not have any impact on fitness one way or another, at least not at the outset. Even if it did have a negative effect, if the fitness peaks are not too tall and the (un)fitness valleys not too deep, the noise might well drive the organism towards another fitness peak. If the new fitness peak were "taller" than the old, the new form of the organism might then drive the old form to extinction.
What Wright was suggesting was that small, isolated populations might be the precursors of new species, a notion that appealed to field biologists because it was what they had observed. Wright had practical experience to back up his ideas as well, having conducted research on inbreeding of guinea pigs and shorthorn cattle. In both cases, he had observed how a particular mutation could be emphasized and then "fixed" in a small population through inbreeding, and then introduced to the wider population. His experiments of course involved artificial selection, but believed that the same process applied to natural selection -- in much the same way that Darwin had leveraged off artificial selection to provide an argument for natural selection.
Fisher, a strict selectionist, didn't like Wright's concept of genetic drift, leading to a more or less polite scientific dispute between the two men. Wright wasn't as inclined to quarrels as were Fisher and Haldane, though it with such colleagues, it was hard for him to avoid squabbling. The feud proved very valuable to population genetics, since it produced volumes of work that did much to finally dismiss all lingering traces of Lamarckism in evolutionary thought. By the 1930s, the challenges that Mendelian genetics had seemed to pose to classic evolutionary theory, resulting in a marriage of the two concepts.
BACK_TO_TOP* While geneticists such as Haldane, Fisher, and Wright learned how heredity worked, biochemists worked to nail down its underlying mechanisms. By the 1920s, biochemists had effectively demolished the idea that there was some magical "life force", an "elan vital", undermining the vitalist mindset by showing that basic life processes were chemical in nature and followed chemical laws, successfully demonstrated many life processes in the test tube using biocatalysts known as "enzymes".
However, the biochemists were stumped by heredity, having no idea of how heredity could be implemented by biochemical processes. In 1927, Hermann Muller, then working in Texas, provided a significant clue that heredity was in fact a biochemical process by showing that X-rays could cause mutations in the genes of the Drosophila melanogaster fruitfly. Later research showed that mutations could also be caused by high temperatures and certain chemicals. Obviously, such "mutagenic" influences affected the physical mechanisms controlling heredity -- whatever they might be -- just as they affected other biochemical mechanisms in the cell. Incidentally, Muller's work also allowed geneticists to extend their studies of fruitflies and other organisms through the creation of new traits and genes. They were no longer limited to the genes provided by an organism as it existed.
Despite this clue, the actual mechanisms of heredity remained mysterious. The next clue was found by the English biochemist Frederick Griffith (1879:1941), who in 1928 reported the results of his studies on Pneumoccocus bacteria. Griffith was able to obtain an extract from virulent Pneumoccoci that would turn benign strains of the bacteria virulent, and the descendants of these modified bacteria would continue to be virulent. Griffith discovered that he could kill bacteria with heat treatment and continue to obtain an extract that could instill virulence, long after the bacteria were dead.
In the mid-1930s, three researchers working at the Rockefeller Institute in New York -- Oswald Avery (1877:1955), Colin MacLeod (1909:1972), and Maclyn McCarty (1911:2005) -- began work to find the element in such extracts that instilled virulence in benign strains of bacteria. In 1944, they showed that "deoxyribonucleic acid (DNA)", a class of the "nucleic acids" found in cell nuclei, that had been extracted from virulent strains was entirely adequate to turn benign strains virulent. Selective destruction of DNA rendered the extracts ineffective in transmitting virulence, while selective destruction of proteins -- the long-chain molecules that provide most of the machinery to keep an organism living -- still left the extract capable of passing on virulence.
Their suggestion that DNA was the agent of heredity was controversial. DNA had long been known, and though its precise structure was not understood at that time, it was regarded as a simple long-chain molecule, composed of four nitrogen-based molecular subunits called "nucleotides", that apparently provided structural reinforcement for chromosomes. Complicated proteins seemed to offer many more possibilities as basic structures for the mechanisms of heredity than the simple and "boring" DNA.
However, continued work showed that DNA did in fact encode genetic information. The most conclusive experiments were performed in 1952 by Alfred Hershey (1908:1997) and Martha Chase (1927:2003), who performed studies on "bacteriophages", or viruses that infect bacteria. Viruses were known to consist essentially of DNA and protein. Hershey and Chase showed that the bacteriophages injected their DNA into target bacteria, but left their protein components outside. The challenge remained as to how DNA actually worked, but by that time other biochemists were hot on the trail.
BACK_TO_TOP* The work of Haldane, Fisher, and Wright in establishing new evolutionary thinking was soon followed up by others. One of the first disciples was Theodosius Dobzhansky (1900:1975), a Ukrainian field naturalist and geneticist who had come to the USA in 1927 and signed up with John Hunt Morgan. Dobzhansky didn't much care for the untidy Columbia fly room, but when Morgan transferred to the California Institute of Technology in 1932, Dobzhansky followed him and found the land pleasant.
Dobzhansky was interested in the subject of genetic diversity. The general assumption of the time was that one member of a species was genetically very similar to another, a notion which was supported by Morgan's lab experiment with the Drosophila melanogaster fruitfly. After all, hadn't Morgan found it difficult to obtain mutant variants? Dobzhansky wanted to test this idea, scouring the continent for samples of the related wild Drosophila pseudoobscura fruitfly and then examining their chromosomes under a microscope for distinctive markers.
Using this approach, Dobzhansky determined that the variability of the wild fruitflies he caught was much greater than anyone had anticipated, and that the variations were generally distinct to each population of flies. Dobzhansky was observing at the chromosomal level the branching tree of life postulated by Darwin. Later examinations of the genetics of other species showed that Dobzhansky's wild fruitflies were much more the norm than the exception for organisms in terms of variability.
While Dobzhansky was an excellent field biologist, he was also interested in theory, and found theoretical work by his contemporaries inspirational in helping piece together the implications of the genetic variability he had found in his wild fruitflies. He was absolutely taken with Wright's notion of an adaptive landscape -- Dobzhansky claimed he fell "in love" with it -- and in 1937, he published the landmark book GENETICS & THE ORIGIN OF SPECIES, the title of which explicitly married Mendel and Darwin.
Dobzhansky was in complete agreement with Wright's ideas about species that consisted of multiple isolated colonies of various sizes -- in fact, "it is very common in nature" -- and observed that these isolated subdivisions gave rise over generations to different races or varieties, and ultimately into different species. Although Dobzhansky was also initially convinced of the importance of genetic drift, in the light of further research by Fisher and others, he gradually became a much stricter selectionist.
In his work, Dobzhansky discussed in detail the impact of geographic or other factors that led to isolated populations, and also emphasized the "hidden variability" implicit in recessive genes. In particular, he demonstrated how heterozygous genes -- in which there were two different forms, or alleles, of a gene -- provided much more diversity and drive to evolutionary processes than homozygous genes -- in which the particular gene was invariant. It was such insights, as well as a popular writing style, that gave Dobzhansky stature. While crediting Haldane, Fisher, Wright, and others for key insights, Dobzhansky acknowledged that he was spreading the word: "What that book of mine ... did was, in a sense, to popularize this theory. Wright is very hard to read."
GENETICS & THE ORIGIN OF SPECIES didn't become a best-seller that took the public by storm, but it did make many key converts in the scientific community. In the USA, admirers included the German-born field zoologist Ernst Mayr (1904:2005), the paleontologist George Gaylord Simpson (1902:1984), and the plant geneticist G. Ledyard Stebbins JR (1906:2000).
In 1942 Mayr published SYSTEMATICS & THE ORIGIN OF SPECIES, which used Dobzhansky's work as a starting point. In his book, Mayr described species as simply inbreeding populations that were reproductively isolated from other groups, with issues of physical distinctions, such as morphology, regarded as irrelevant in that context. Mayr described the fragmentation of a population into several reproductively isolated groups, what he called "emergent species" with the term "adaptive radiation". He suggested what he called the "founder principle", which had a clear debt to the thinking of Sewall Wright: small isolated groups could undergo rapid evolution, since genes providing adaptive improvements didn't have to propagate through a large population, and if geographic or other barriers between the small population and a larger, less well adapted population broke down, the improved variant might come into competition with the parent stock and quickly replace it.
Simpson, as a paleontologist, was familiar with the fossil record, which clearly identified a succession of forms. However, this progression of forms had proven misleading, since it gave the impression of straight-line progress to increasingly refined forms, suggesting pre-Darwin evolutionary mechanisms such as Lamarckism. Simpson was able to show that the fossil record was full of side branches and dead ends, just as Darwin had predicted it should be. The seeming straight-line progress was simply due to looking backwards over the line of evolution from its end and disregarding the branches.
Simpson also noted that the fossil record was marked by confusing discontinuities in its history. In his 1944 book MODE & TEMPO IN EVOLUTION, Simpson interpreted this seemingly contradictory evidence by suggesting that evolutionary change was not a completely smooth process, instead being marked by rapid starts, long intervals of stabilities, and dead ends that vanished from the Earth in extinctions. The idea didn't catch on at the time, but it wouldn't be forgotten, either.
For his part, Stebbins extended the concepts of the new evolutionary thinking to the domain of plants, whose heredity tends to differ from that of animals -- for example in their greater inclination to form polyploid hybrids, with duplicated sets of chromosomes. Despite the differences, Stebbins was able to show in his 1950 book VARIATION & EVOLUTION IN PLANTS that the same principles relevant to animal evolution applied to plant evolution.
* While the new generation of biologists worked at the leading edge of evolutionary theory, field studies were reaching back to its beginnings for insights. One of Darwin's inspirations during his survey of the Galapagos had been the remarkable finches he had found there, varying according to size, beak, diet, and behavior, representing a branching of species from a common ancestral finch that somehow made its way from the South American mainland.
Darwin hadn't actually made too much of the finches, mentioning them in VOYAGE OF THE BEAGLE, but saying little or nothing about them in THE ORIGIN OF SPECIES. Field biologists of the 1930s found them puzzling because of their extreme diversity, the belief being that natural selection would have trimmed off the proliferation of branches, involving populations of birds overlapping in form and range, into a few stable species. In 1935, the centennial of Darwin's visit to the Galapagos, the suggestion was floated that "Darwin's finches", as they were then named in his honor, deserved a closer examination in the field, where observations and breeding experiments could be performed.
Julian Huxley (1887:1975) -- Thomas Huxley's grandson, secretary of the Zoological Society of London, and a prominent British advocate of new evolutionary concepts -- decided to back a shoestring expedition to the islands, obtaining modest funding for the effort, then finding a young schoolteacher and amateur birdwatcher named David Lack (1910:1973) who was interested in the job. Lack left England in 1938 and went to the Galapagos on a commercial steamer. He spent four months on his field study, not finding the arid, desolate, and impoverished islands any tropical paradise, which at least helped him to focus on his work. Lack obtained detailed observations of the finches, determining that they constituted thirteen distinct species. He went to the US and continued his studies using preserved specimens, working first in San Francisco and then in New York City.
In New York City, Lack roomed with Ernst Mayr, then the curator of the American Museum of Natural History. Not surprisingly, when Lack published his first paper on Darwin's finches in 1940, he was strongly influenced by Mayr, leaning heavily on Mayr's notions of genetic drift and adaptive radiation to explain the diversity of the finches. However, the idea didn't sit comfortably with Lack, and when he published the book DARWIN'S FINCHES in 1947, he retained notions of adaptive radiation but made use of the notion of the "ecological niche", a notion that had been circulating among the biological community for about two decades.
An ecological niche can be thought of in a broad way as a "professional category" or "trade" applied to animals: ant-eater, grasslands herbivore, mole, large predator, and so on. When the first South American finches arrived on the Galapagos, as Lack had it, they found a domain full of empty ecological niches, allowing the descendants of the pioneering finches to expand into roles that would have been closed to them otherwise -- if an ecological niche had been occupied by another species, selection pressure would have worked against the finches, since they would not have been able to compete against organisms that were already well adapted to the "trade". The fact that the finches occupied several relatively isolated islands in the Galapagos chain helped boost their diversification, but there were also overlapping species on each island. Competitive pressures gradually led to specialization among the various species.
Darwin's finches became such an icon of evolutionary theory in the 1950s that a false impression arose that Darwin himself had used them as one of the main supports of his argument. They have since been investigated in extreme detail, in particular by the husband-and-wife team of Peter and Rosemary Grant, who began a meticulous field study of the finches beginning in 1973 and still ongoing. The Grants were able to monitor the shift of the characteristics of finch populations in response to a multiyear drought, and their shift back to previous characteristics after the drought ended. The rate of shift as observed by the Grants suggested that if there were long-term changes in conditions on the islands, new species would emerge in only a few centuries.
* In any case, by the end of the 1950s, the work of Dobzhansky, Mayr, Simpson, Stebbins, and Lack had established a thoroughly renewed evolutionary theory, the "Modern Synthesis", grounded in a level of rigor and detail that would have astounded even the meticulous Charles Darwin. Disputes over the fundamental elements of evolutionary theory disappeared, with few contesting Dobzhansky's simple claim that "nothing in biology makes sense except in the light of evolution."
Without evolutionary theory, the species of the Earth would be no more than entries in a catalog that could provide no real insights into their origins and relationships, making their characteristics no more than arbitrary. The science was not standing still, either; new insights had emerged that would take evolutionary science several more steps forward.
BACK_TO_TOP* While biologists worked on the Modern Synthesis, the puzzle of how DNA, or whatever molecule that encoded heredity, stored a "genetic code", the instructions for building an organism and keeping it alive, had proven sufficiently intriguing to attract some of the best minds in physics. The Danish physicist Niels Bohr (1885:1962) and his student Max Delbrueck (1906:1981) examined the physical constraints on the mechanisms of heredity, and their thoughts and those of others were presented to the public in 1944 by the Austrian physicist Erwin Schroedinger (1887:1961) in a popular landmark book titled WHAT IS LIFE?
Schroedinger speculated that the genetic code was embodied in an "aperiodic crystal" that consisted of a string of a few different elements. The order of the elements provided the genetic code, just as a simple dot and dash could encode messages in Morse code. Schroedinger was a theoretical physicist and had, as he admitted, an imperfect grasp of practical biochemistry, but his basic ideas were astute and proved very influential. They inspired a young American biochemist named James Watson (born 1928), who was investigating DNA in Cambridge, England, working in collaboration with a British physicist named Francis Crick (1916:2004).
The Austrian biochemist Erwin Chargaff (1905:2002) had discovered that the four nucleotides that make up the DNA chain -- adenine, thymine, cytosine, and guanine (A, T, C, and G) -- had a clear regularity: the amount of A in DNA was the same as the amount of T, and the amount of C was the same as the amount of G. This was a significant clue to the structure of DNA, and another major clue was provided in 1952 when Rosalind Franklin (1920:1958), a collaborator with Crick and Watson, performed X-ray crystallography studies of DNA, obtaining images of patterns produced by shining X-rays through crystalline samples of DNA.
In 1953, Crick and Watson used the clues to develop their famous "double helix" model of DNA, with DNA composed not of a single polymeric chain but a pair of linked chains, conceptually resembling a ladder twisted into a spiral. The bases were linked together by alternating sugar and phosphate groups, with the steps of the ladder consisting of the "base pairs" A and T, or C and G, linked together by a relatively weak bond. The DNA molecule could replicate by splitting down the middle of the helix along the weak central bonds, with the two half-chains then reconstituting themselves into two full chains by linking new nucleotides. Mutations arose through errors in the DNA replication process.
* The sequence of the nucleotide "bases" in the DNA molecule clearly defined the genetic code, but Crick and Watson did not know how to interpret the sequences. By 1964, through the work of Marshall Nirenberg (1927:2010), H. Gobind Khorana (1922:1911), and others, the details of how DNA specified the genetic code had been broadly worked out.
Proteins are primary components of the cell, providing structural elements, useful tools, and, in the form of enzymes, protein-based catalytic systems to support cell reactions. Proteins are chains of "amino acid" subunit molecules, with 20 different amino acids used in human proteins:
_____________________ alanine (ala) arginine (arg) asparagine (asn) aspartic acid (asp) cysteine (cys) glutamic acid (glu) glutamine (gln) glycine (gly) histidine (his) isoleucine (ile) leucine (leu) lysine (lys) methionine (met) phenylalanine (phe) proline (pro) serine (ser) threonine (thr) tryptophan (trp) tyrosine (tyr) valine (val) _____________________
It was the fact that proteins had 20 different possible subunits that sent researchers down the wrong track of thinking that proteins were the agents of heredity, since the four different nucleotide subunits of DNA seemed to be too limited in number to do the job. This was overlooking the reality, obvious in hindsight, that groups of several of the four nucleotides could provide as many variations as needed.
The primary function of DNA is to "code" the production of proteins. The DNA chain is organized in "triplets" or "codons" of bases, coding a particular amino acid in a protein chain as follows:
_______________________________ ala: GCU GCC GCA GCG arg: CGU CGC CGA CGG AGA AAG asn: AAU AAC asp: GAU GAC cys: UGU UGC glu: GAA GAG gln: CAA CAG gly: GGU GGC GGA GGG his: CAU CAC ile: AAU AUC AUA leu: UUA UUG CUU CUC CUA CUG lys: AAA AAG met: AUG phe: UUU UUC pro: CCU CCC CCA CCG ser: UCU UCC UCA UCG AGU AGC thr: ACU ACC ACA ACG trp: UGG tyr: UAU UAC val: GUU GUC GUA GUG START AUG STOP UAG UGA UAA _______________________________
A single DNA strand may encode a number of proteins, with a section encoding a single protein called a "gene" and marked out by the START and STOP sequences listed above. The general principle is: "one gene, one protein" -- though this is a bit of an oversimplification. The entire complement of genetic information in an organism is known as the "genome".
Protein synthesis involves DNA, enzymes, and a cellular organelle called the "ribosome". Under the control of enzymes, a DNA strand is split in half down the middle, and then replicated with a half-strand of a closely related molecule known as "messenger RNA (mRNA)" -- which is almost identical to DNA, except that it uses a slightly different sugar molecule, and the nucleotide uracil (U) instead of thymine. This process is known as "transcription", and when it is complete, the DNA half-strand releases the mRNA half-strand.
The mRNA then leaves the cell nucleus and links to a ribosome. The ribosome moves along the mRNA strand, reading a triplet at a time. With each triplet, the ribosome acquires an amino acid that is connected to a short RNA sequence named "transfer RNA (tRNA)", which allows the ribosome to match a specific amino acid to a specific triplet code.
The ribosome strips the tRNA off the amino acid, links the amino acid into the emerging protein chain, and then moves on to the next mRNA triplet. This scheme of protein construction is known as "translation". Later work revealed increasing levels of elaboration to this process, with "regulatory sequences" encoded in DNA working with a system of enzymes to control cell construction and operation, and indeed led to the discovery of entire suites of genes, discussed later, that synthesized nothing directly but provided high-level control over those that did.
* The discovery of the genetic code was one of the giant scientific achievements of the 20th century; manned landings on the Moon were trivial in comparison. The understanding of the molecular basis of heredity finally killed off the last lingering traces of vitalism, and also would greatly reinforce evolutionary science.
BACK_TO_TOP* Although officials of the Soviet Union liked to proclaim the "scientific" basis of their society and Soviet scientists figured prominently in the list of Nobel Prize winners, during the era in which modern genetics and evolutionary theory were being created in the West, the USSR was going down a bizarre blind alley that would prove a memorial to the triumph of ideology over sensibility.
In the late 1920s, Trofim Denisovitch Lysenko (1896:1976) was a technician at an agricultural research station in Azerbaijan who came up with the idea of planting a winter crop of peas. It worked, but only because the winter was unusually mild. Lysenko then decided that he might be able to encourage crop plants to grow under winter conditions by simply "conditioning" them to a harsh climate. In essence, he was resurrecting Lamarckism. As was later observed, he believed that he could get normal crop plants to grow in winter by "giving them a stern talking-to."
The idea was unworkable on the face of it, and further efforts along such lines were largely marked by failure. However, Lysenko's initial "success" led to the publication of a flattering article in the state newspaper PRAVDA -- the name was Russian for "Truth", though the paper was not noted for objective reporting. The article called him the "barefoot professor" and praised his work. The idea that a peasant lad could make major scientific discoveries had a strong appeal to Soviet ideology, and Lysenko quickly realized that he was on to something.
Although he was an incompetent agricultural scientist, Lysenko had an instinct for political propaganda, littering his work with citations from Marx and Lenin, while smearing his enemies as counter-revolutionaries. Soviet ideology also had a built-in bias towards Lamarckism, since it offered the hope of improvement of the people by their own efforts -- while Western genetics implied "genetic determinism", as biological determinism had been recast, which Lysenko denounced as a reactionary and bourgeois concept that implied a static class system. In addition, Lysenko was promising quick results, much quicker than could be obtained by mainstream geneticists through selective breeding. Why waste time in silly experiments with fruitflies?
Crackpots tend to believe that the world is against them, but unlike most crackpots Lysenko could do something about his enemies, since he had the ear of Soviet dictator Josef Stalin. In 1935, Lysenko declared in a speech at the Kremlin: "Both within the scientific world and outside it, a class enemy is always an enemy, even if a scientist." Stalin, who had little feeling for science but was very keen on the suppression of class enemies, stood up and applauded: "Bravo, Comrade Lysenko, bravo!"
Later Lysenko would announce: "I do not consider formal Mendelian-Morganist genetics a science" -- and one of his cronies added: "The teaching of genetics must be eliminated from secondary schools." Mainstream geneticists were arrested and sent to the prison camps for "treason" -- Stalin's USSR had a frighteningly broad definition of the term -- and some did not return. Soviet agricultural science went from failure to failure under Lysenko's direction.
Josef Stalin died in 1953; there were many who hoped that Lysenko's star would then fall, but he was too entrenched. Under the new regime led by Nikita Khrushchev, Lysenko would remain in a position of authority, even though Premier Khrushshev's son Sergei, a rocket engineer, told his father that Lysenko was a charlatan. Indeed, with Watson and Crick's discovery of the structure and function of DNA, Lysenko was moving on to new heights of crackpottery, announcing that it was "impossible to ascribe an attribute of life, IE heredity, to a nonviable substance, deoxyribonucleic acid, for example." As described by Watson & Crick, the structure and function of DNA left no apparent room for Lysenkoism, and so Lysenko and his kind had to reject it.
Khrushchev was nowhere near as repressive as Stalin, however, and by the time of Khrushchev's ouster in 1964, criticisms of Lysenko were increasing in volume among the Soviet scientific community. Physicist Andrei Sakharov, father of the Soviet hydrogen bomb and later a world-famous political dissident, denounced Lysenko to the Soviet Academy of Sciences:
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He is responsible for the shameful backwardness of Soviet biology and genetics in particular, for the dissemination of pseudo-scientific views, for adventurism, for the degradation of learning, and for the defamation, firing, arrest, even death of many genuine scientists.
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Khrushchev's sacking was, ironically, partly due to the fact that Soviet agriculture had suffered a series of crop failures under his leadership. The crop failures appear to have been due to mismanagement by the clumsy Soviet agricultural system in general, not Lysenko's crackpot ideas in specific -- but he couldn't have helped matters any. A commission of the Academy of Sciences reviewed Lysenko's work and delivered a highly critical report. Lysenko was stripped of his rank and privileges, to live in bitter obscurity until his death.
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