Thursday, February 12, 2009

Part Two

The Laws of Variation.

When Darwin proposed his evolution theory of natural selection there were scientists working on the very problem that Darwin had with the means of variation and its mechanism. None of these people made the connections. On the one hand we have Gregor Mendel working on the laws of inheritance while there were others wanting to find out the chemistry behind it all. All completely independently without taking any notice of each other It was almost a hundred years later that it all came together.
Our ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part differs, more or less, from the same part in the parents. But whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus.[Chapter Five. laws of Variation]
Yes this is what gave Darwin his greatest problem and he cast around for an answer. Darwin did not know that the mechanism of natural selection had been worked out by Gregor Johan Mendel with his work on peas, but because it was published in an obscure journal it never found its way into Darwin's hands. Gregor Mendel was born in Heinzendorf, Austria on July 22, 1822. He died in Brno, Austria January 6, 1884. Mendel's first presentation was on his eight years of experimentation with artificial plant hybridization. During his studies he became a member of the Zoologist-botanisher Vernin in Vienna. His first two communications were published in 1853 to 1854. Both articles contained information about damage to plants by insects. Between 1856 to 1863 Mendel cultivated and tested almost 28,000 plants. His explanation was that hybrid germinal and pollen cells that are in their composition correspond in equal number to all constant forms resulting from the combinations of traits united through fertilization. He bred two lines of peas, amongst others, through inbreeding. Short and Tall. Once he was satisfied that they bred true he crossed them obtaining only tall plants. These first crosses were planted when tall and short plants resulted. These he counted. The experiments were repeated again and again using different traits.The results were always the same. A 3:1 ratio. His experiments were simple,elegant.

Unfortunately Mendel's work never reached Darwin though Mendel had Darwin's The Origin in his possession. Mendel had sent all the results of his experiments to a Swiss German botanist Karl Wilhelm von Nageli, son of a physician, who studied botany in Vienna and eventually became Professor of botany at Munchen. Nageli seems not to have read it properly or not understood its importance. All Mendel's publications collected dust on shelves around Europe having never been read. He was involved and was the first to see chromosomes and report on them. Even more strange Nageli corresponded with Darwin.While this was going on there were arguments on the chemistry of inheritance and the molecules involved. They were centered on the nucleus which contained two substances, protein and what was then called nuclien. This is a whole story of its own. To add too the intrigue Friedich Meisenger was the man working on this and he was the one who discovered DNA. He too corresponded with Nageli who now held all the cards but the light was too dim for him to see.Just think,Nageli had been given Mendel's work to peruse,it was clear work,elegant and almost perfect in explaining the mechanism of inheritance. He himself had been the first to observe chromosomes which of course are made of thousands of genes. These genes are made of DNA discovered by Meisenger who had made it known to Negeli. In turn Negeli knew of Darwin and his work. Every thing was there in his hands.It would have been at that time impossible for any mortal to understand. We had to wait till 1953.These three disciplines eventually came together to form one coherent theory of evolutionary Biology. Mendels works were rediscovered in 1900 independently by three workers and brought a close to an era of speculation on heredity and opened up a new pathway of study on heredity to reveal a new mechanism operating in the sense of evolution[.Parts submitted by Peter Koerner]

The writer first learned about Mendel in 1946 in his Matriculation year. His class was being taught by a pretty young student teacher for a short period and he remembers being taught about mitosis and meiosis.The class gave her a bad time as she was not that much older than they were. In 1955 he did a course in Genetics and learned all the basics of Mendelism with the 3:1 ratios. He understood it well at the time, though he never really tied it in with Evolution even though all of Botany and Zoology had used evolution as a foundation quietly in the background, though not taught as such or made much of. Was it a sensitive issue? Bill Hamilton , a tall, white-haired man of sixty-three, a Royal Society professor at Oxford widely considered the most influential evolutionary thinker since Darwin, had been deeply dissatisfied with the lectures on evolution he received as an undergraduate at Cambridge University, where it seemed to him that his professors did not give Darwin's mechanism of natural selection its proper due.[ Volume 9, NO. 8 - November 1999 OH MY DARWIN! Who's the Fittest Evolutionary Thinker of Them All by James Schwartz ]

Thomas Hunt Morgan

The work of Mendel was soon to be confirmed by Thomas Hunt Morgan. To a geneticist Morgan had a very good pedigree. Morgan's father, Charlton Hunt Morgan, was a U.S. consul, and his uncle, John Hunt Morgan, had been a Confederate army general and leader of a guerrilla group known as 'Morgan's Raiders'. A great great grandfather John Wesley Hunt amassed a fortune and founded a railroad. and Francis Scott Key another great great grandfather wrote the Star Spangled Banner. A good pedigree indeed, genes do count.Early in life Morgan showed an interest in natural history. In 1886 he received the B.S. degree from the State College of Kentucky (later the University of Kentucky) in zoology and then entered Johns Hopkins University for graduate work in biology. At Hopkins, Morgan studied under the morphologist and embryologist William Keith Brooks.the subject of his PhD thesis was the classification of spiders where his research was in the naturalist tradition.

In 1904, he accepted an invitation to assume the professorship of experimental zoology at Columbia University, where, during the next 24 years, he conducted most of his important research in heredity. Like most embryologists and many biologists at the turn of the century, Morgan found the Darwinian theory of evolution lacking in plausibility. It was difficult to conceive of the development of complex adaptations simply by an accumulation of slight chance variations. Moreover, Darwin had provided no mechanism of heredity to account for the origin or transmission of variations, except his early and hypothetical theory of pangenesis. Although Morgan believed that evolution itself was a fact, the mechanism of natural selection proposed by Darwin seemed incomplete because it could not be put to an experimental test. Morgan had quite different objections to the Mendelian and chromosome theory. Both theories attempted to explain biological phenomena by postulating units or material entities in the cell that somehow control developmental events. To Morgan this was too reminiscent of the preformation theory--the idea that the fully formed adult is present in the egg or sperm--that had dominated embryology in the 18th and early 19th centuries.

Although Morgan admitted that the chromosomes might have something to do with heredity, he argued in 1909 and 1910 that no single chromosome could carry specific hereditary traits. He also claimed that Mendelian theory was purely hypothetical: although it could account for and even predict breeding results, it could not describe the true processes of heredity. That each pair of chromosomes separate with the individual chromosomes then going into different sperm or egg cells in exactly the same manner as Mendelian factors, did not seem to be sufficient proof to Morgan for claiming that the two processes had anything to do with each other.Morgan could have been famous because of his ancestry and for opposing Darwin,evolution chromosome theory and Mendel, but who becomes famous for being so very wrong. Luckily for him he started work on Drosophoila which led to him to the truth and confirmed Mendel's work and enabled him to support evolution and Darwin.

The work on Drosophila.

Morgan apparently began breeding Drosophila in 1908. In 1909 he observed a small but discrete variation known as white-eye in a single male fly in one of his culture bottles. Aroused by curiosity, he bred the fly with normal (red-eyed) females. All of the offspring (F1) were red-eyed. Brother-sister matings among the F1 generation produced a second generation (F2) with some782 white-eyed flies, all of which were males. 3470 were red eyed of which 1011 were males. This was supposed to be near enough to the 3:1 ratio. There seems to be less than 50 /50 ration of males to females. To explain this curious phenomenon of having no white eyed females, Morgan developed the hypothesis of sex-limited--today called sex-linked--characters, which he postulated were part of the X-chromosome of females. Other genetic variations arose in Morgan's stock, many of which were also found to be sex-linked. Because all the sex-linked characters were usually inherited together, Morgan became convinced that the X-chromosome carried a number of discrete hereditary units, or factors. He adopted the term gene, which was introduced by the Danish botanist Wilhelm Johannsen in 1909, and concluded that genes were possibly arranged in a linear fashion on chromosomes.So much to his credit, Morgan rejected his skepticism about both Mendelian and chromosome theories when he saw from two independent lines of evidence--breeding experiments and cytology--that one could be treated in terms of the other. In collaboration with A.H. Sturtevant, C.B. Bridges, and H.J. Muller, who were graduates at Columbia, Morgan quickly developed the Drosophila work into a large-scale theory of heredity.

Particularly important in this work was the demonstration that each Mendelian gene could be assigned a specific position along a linear chromosome "map." Further cytological work showed that these map position could be identified with precise chromosome regions, thus providing definitive proof that Mendel's factors had a physical basis in chromosome structure.. To varying degrees Morgan also accepted the Darwinian theory by 1916. .In 1924 Morgan received the Darwin Medal; in 1933 he was awarded the Nobel Prize for Medicine or Physiology for his discovery of "hereditary transmission mechanisms in Drosophila"; and in 1939 he was awarded the Copley Medal by the Royal Society of London, of which he was a foreign member. In1927-31 he served as president of the National Academy of Sciences; in 1930 of the American Association for the Advancement of Science; and in 1932 of the Sixth International Congress of Genetics. The work was carried on by Theodosius Dobzhansky, who was a geneticist and evolutionist whose work had a major influence on 20th-century thought and research on genetics and evolutionary theory. In 1927 Dobzhansky went to Columbia University in New York City as a Rockefeller Fellow to work with the geneticist Thomas Hunt Morgan.

Between 1920 and 1935, mathematicians and experimentalists began laying the groundwork for a theory combining Darwinian evolution and Mendelian genetics. Starting his career about this time, Dobzhansky was involved in the project almost from its inception. His book Genetics and the Origin of Species (1937) was the first substantial synthesis of the subjects and established evolutionary genetics as an independent discipline. Until the 1930s, the commonly held view was that natural selection produced something close to the best of all possible worlds and that changes would be rare and slow and not apparent over one life span, in agreement with the observed constancy of species over historical time. Dobzhansky's work with Drosophlia pseudoobscura was important in that he was able to produce flies through heat treatment that were virtually different species in that they were unable to breed with each other.

The Chemistry of Inheritance

This part of the story began at about the same time as Gregor Mendle did his work on peas and published his Origin of Species. and has continued up till the present time. Criek and Watson brought the work all together, by adding to the work of Mendel with their discovery of the very secret to the way DNA passed information from one cell to another.In 1953 Francis Crick and James Watson built a model of the double helix of the DNA molecule. Afterwards Francis Crick said to a friend in a pub 'I think we have discovered the secret of life'. Their discovery [and I should include Rosalind Franklin] remained the best kept secret for much of the rest of the century, so little was known of it by the general public and still remains so. It was really earth shattering but only in the cloistered halls of biological institutions did some scientists appreciated its implications. Now it is recognised by many scientists as the most important biological discovery of the twentieth century and perhaps only second to Darwin's a century earlier.

The solution was both simple and elegant,but it just crept up on us .I spent six years at university studying at various times the parts of what has is now known as Molecular Biology, the coming together of Biochemistry, microbiology and genetics All that has come after has its foundation in the double helix of the phosphate and pentose sugar, with its rungs of the bases, Adenine, Cytosine, Guanine and Thiamine, spelling three letter words which make up the amino acids, as for example ACGACGAGTAGTAGT. The Three letter triplets of bases each form an amino acid and chains of amino acids form Proteins.

Central Dogma of Molicular Biology.

This states that once “information” has passed into protein it cannot get out again. In more detail, the transfer of information from nucleic acid to nucleic acid, or from nucleic acid to protein may be possible, but transfer from protein to protein, or from protein to nucleic acid is impossible. Information means here the precise determination of sequence, either of bases in the nucleic acid or of amino acid residues in the protein.

In the beginning was the word and the word was made flesh, and never once remember hearing about DNA. This work has been expanded in the latter part of the twentieth century with the sequencing of the human genome. Many genes have been identified along with so many mutations that have caused diseases. The whole understanding of genetics has exploded to such an extent that it is filling whole areas of Darwin's jigsaw picture,no longer a a puzzle..Matt Ridley in 1999 has enthuastically written about the Human Genome Project that we will' be able to know more about our origins, our evolution, our nature and our minds than all the efforts of science to date. It will revolutionise anthropology, psychology, medicine, palaeontology and virtually every other science'.... 'In a few short years we will have moved from knowing almost nothing about our genes to knowing everything'. [Matt Ridley. Genome]and continues 'we are living in the greatest intellectual moment in history. Perhaps Matt Ridley is a bit optimistic but this all fits perfectly into Darwin's jigsaw picture. What is impressive is that the science of genetics is that it is a young science and yet it has made such great advances. In a Penguine book on Genetics written by H Kalmus in 1945 there is little to find fault with and little that is incorrect. Many of the gaps were recognised and ideas advanced on their solutions which have proved almost correct. The same can be said about a book 'Genetics and Man' written by C D Darlington in 1976. There have been no big mistakes, no back tracking, no new direction, just a relentless advance in filling in Darwin s picture from the 1945 book to the present. This ongoing research is of the utmost importance and will tell us so much about ourselves, our past , present and future.


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