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Evolution: The Origin and Subsequent Elaboration of the 'Chromosomal' Hypothesis ofHybrid Sterility Evolution SELECTED PAPERS AND COMMENTARY To really understand evolution we must first understand the historical development of ideas on evolution. But to really understand its history, we must first understand evolution. OVERVIEW  Introduction: Species Barriers Origin of Species (Darwin 1859), Revisited Two Levels of Information in DNA Haldane's Rule Non-Genic ("Chromosomal") Speciation Allen, Romanes and Gould Heredity as Information Transfer (Butler) SELECTED PAPERS on the Four Black Boxes: 1. Variation 2. Heredity 3. Phenotypic ("Natural") Selection/Isolation Hybrid Sterility (Darwin 1862) Variation (Hooker 1862) Pangenesis (Darwin 1868) Inutility of Characters (Gulick 1872) Natural and Artificial Selection (Belt 1874) Inutility of Characters. Paradox of . Random Drift (Delboeuf 1877) 4. Reproductive ("Physiological") Selection/Isolation An Unnoticed Factor in Evolution (Catchpool 1884) Physiological Selection (Romanes 1886) Physiological Selection (Romanes 1887) 4. "Physiological" Selection/Isolation, the "Chromosomal" basis Hybridism and the Germ-Cell (Guyer 1900, 1902) Cytological Basis for the Mendelian Laws (Cannon 1902)  Chromosomes of the Germ Cells (Montgomery 1901) Chromosomes in Heredity (Sutton 1903) Two Levels of Genetic Information (Bateson & Saunders 1902) Heredity and Variation in Modern Lights (Bateson 1909) A Phenomenon of Arrangement (Bateson 1914) Chromosomes, Polyploidy and Why Evolved (Winge 1917) Speciation in Retroviruses (1995) Origin of Species (1996) Thinking about Stem-Loops (1998) Two Levels of Information in DNA (1999) Haldane's Rule (2000) Non-Genic (Chromosomal) Speciation (2003) Allen, Romanes and Gould (2004) Chromosomal Speciation: A Reply (2004) Heredity as Information Transfer (2006)Positive Selection of Synonymous Mutations Initiates Species Divergence (2007) Acknowledgements Other Web Sites New Book on Evolution Introduction:Species Barriers The idea of barriers against members of other species was evident in the nineteenth century in the context of infectious disease (Click here). These barriers are both external (e.g. hygiene), and internal (e.g. the immune response). Internally, our bodies ("self") can detect and destroy members of other species ("not-self"; i.e. viruses, bacteria, protozoa). Another, no less subtle form of self/not-self discrimination, involves our detection of a mate ("near-self") who will be our "physiological complement" such that the union will produce healthy offspring ("hybrids"). An incestuous relationship with a close relative ("too near-self") will probably result in less healthy offspring. On the other hand, extreme out-breeding, such as with an ape (not-self), is prohibited by species barriers. There is more to this than just the inability to copulate (the gamete transfer barrier). There are both external components (e.g. mate choice), and internal components. Even if the male sperm could meet and fuse with the female ovum, the resulting cell ("zygote") might be unable to grow into an organism (developmental barrier resulting in "hybrid inviability"). Even if these transfer and developmental barriers were overcome, in the gonad (testes, ovary) the two sets of parental chromosomes might be unable to pair for gamete-production (gonadal barrier resulting in "hybrid sterility"). Our modern understanding of these barriers began with Charles Darwin's The Origin of Species (1859). The debate on the primary mechanism of "speciation" continues to this day. The issues involved are complex. What are the relative importances of the three barriers (transfer, developmental, gonadal) in keeping species separate ("reproductively isolated")? Do the barriers appear sequentially, and if so, which appears first? Are there fewer barriers between closely allied species, than between distantly related species, or does one barrier just replace another barrier? Are the barriers we find between closely related species indicative of the first barriers to appear? Does a barrier arise suddenly in an all-or-none fashion, or does it arise slowly, so that reproductive isolation is initially only partial? If one barrier is replaced, does it disappear completely? Is there a group of species, members of which, for some reason related to their biology, have not progressed beyond the first barrier? What is the molecular basis of each barrier? Four years after the death of Darwin in 1882 a major advance was made by Darwin's close research associate, George John Romanes. Whereas the work of Gregor Mendel (1865) may have been unrecognized until 1900 because of the relative obscureness of its originator and his location, George Romanes was at centre stage.   George Romanes and, his wife and biographer, Ethel Romanes.Photographs of these portraits were kindly provided by their grandson Giles Romanes (also the grandson of Almroth Wright). The portrait on the left is signed by T.H. Huxley's son-in-law, John Collier (1850-1934), who, at Romanes' suggestion, was commissioned by the Linnean Society to provide the famous 1881 portrait of Darwin. Yet Romanes' contributions to evolutionary theory are only just gaining recognition. Perhaps for the reason given at the top of this page, Biohistorians have not found it easy to tell the story. John Lesch observed in 1975 that: "The development of evolutionary theory in the two decades from Darwin's death to the turn of the century remains very largely terra incognita for the historian." William Provine concluded in 1986 that: "Evolutionary biology in the period 1859-1925 is extraordinary complex". These web pages makes available some of the primary documentation in the area, so that you can try to sort out the issues for yourself. It is obviously biased towards my own particular evolutionary views, but I provide links to other sites, giving alternative perspectives. D. R. Forsdyke The Origin of Species, Revisited: A Victorian who Anticipated Modern Developments in Darwin's Theory By Donald R. Forsdyke Queen's Quarterly (1999) 106, 112-133 (With copyright permission from the Editor, Boris Castel.) In his later years Charles Darwin's closest professional relationship was with George John Romanes, to whom he entrusted the burden of his life's work. Four years after Darwin's death, Romanes published a theory of the origin of species by means of "physiological selection". This resolved the major problems in Darwin's theory, but replaced them with a "peculiarity" of the reproductive system which would allow selective fertility between "physiological complements". To most of his contemporaries, and those who came afterwards, this did not convey much. However, bioinformatic analysis of DNA sequence data emerging from genome projects now allows an interpretation. Surprisingly, the words of a "pore flahr gel" can help us understand work which Alfred Wallace, with the unknowing aid of a Kingston lady, had condemned to a century of obscurity. 1848 was a good year for the Reverend George Romanes, professor of classics at Queen's College, Kingston, Ontario. He inherited a "considerable fortune" and his third, and most illustrious son, George John Romanes, was born. The Presbyterian minister had been in Canada for 14 years, and "relieved of any necessity to continue the duties of his chair", he returned to Europe (1850), eventually settling in London. The children, two surviving boys and two girls, grew up as free spirits. The publication of Charles Darwin's The Origin of Species by Means of Natural Selection, when George John was eleven, went unnoticed. Some last-minute private tutoring facilitated George John's entrance to Cambridge University, where he bloomed. His initial interest in theology, gave way to a life-long interest in biology while maintaining his spiritual concerns. The biography relates that he "finally abandoned the idea of a profession" (medicine or the church), and "resolved to devote himself to scientific research."1,2 In 1874 Romanes published some of his views on evolution in the scientific journal, Nature3. Darwin sent him "a friendly little note" and invited him to visit. "From that time began an unbroken friendship, marked on one side by absolute worship, reverence, and affection, on the other by an almost fatherly kindness and a wonderful interest in the younger man's work and in his career. ... Mr. Darwin met him, as he often used to tell, with outstretched hands, a bright smile, and a 'How glad I am that you are so young!'"  Darwin had postulated that organisms with variations conferring some advantage in the "struggle for existence" would more likely survive and produce offspring. This process of "natural selection" also discriminated against organisms with disadvantageous variations. The codiscoverer of evolution by means of natural selection, Alfred Russel Wallace, held rigidly to the theory as originally formulated, while Darwin was more flexible. Wallace was also a leading advocate of the credibility of supernatural phenomena, arguing that natural selection was insufficient to account for the evolution of the human brain (1869)4, and publishing The Scientific Aspect of the Supernatural (1866)5, and Miracles and Modern Spiritualism (1874)6. In 1876 Romanes and his elder brother James were deceived by a "medium" who claimed to be able to communicate with spirits. Romanes wrote two letters to the sceptical Darwin expressing an inclination (short-lived) to believe in the phenomena he had observed. James, who was 14 when the family left Canada, had a friend in Kingston with an interest in spiritualism and he sent her drafts of the letters. Romanes later (1880) expressed doubt publicly concerning "the ascertained facts of clairvoyance and mesmerism" which had been proclaimed in a letter in Nature7. This brought a first contact with Wallace8. There were two meetings at which Romanes made no mention of his earlier credulity. Meanwhile, Romanes had formed a close working relationship with the elderly Darwin. Well aware of inconsistencies in the theory of the origin of species by natural selection, Darwin had made their resolution his life's main focus. At the time of his friendship with Romanes, Darwin was much concerned with "Pangenesis" as a possible explanation for the inconsistencies9. Pangenesis suggested that the testes and ovaries (gonads) were merely collecting centers for hereditary (and perhaps acquired) information dispersed about the body as independent units which he called "generative elements" or "gemmules". In an 1875 letter he wrote to Romanes:  "I hope with all my heart that you are getting on pretty well with your experiments; I have been led to think a good deal on the subject, and I am convinced of its high importance, though it will take years of hammering before physiologists will admit that the ual organs only collect the generative elements."1, 10 A later letter (1876) began: "As you are interested in Pangenesis, and will some day, I hope, convert an 'airy nothing' into a substantial theory..."10. Darwin's correspondence suggests a sharing, not only of the experimental, but also the theoretical burden of his life's work. Romanes's experiments to prove the gemmule hypothesis came to nothing, but served to focus his attention on the gonads. Following Darwin's death (1882), Romanes devoted much time trying to find errors in August Weismann's alternative (but correct) proposal that the germ line (contained in the gonad) was quite distinct from the rest of the body ("soma"). Romanes's book, An Examination of Weismannism eventually appeared in 1893, but by this time Weismann's ideas were becoming widely accepted11. Key features of Weismann's proposal, and the Darwinian alternative, appear in Figure 1. Here we see that the germ line is part of an unending cycle, and so is potentially immortal. The soma (that's you and me) lasts for one generation and is then discarded.  Figure 1. The eternal cycling of the germ line. Following the gonad-specific process known as "meiosis", the male gonad (testis) produces male gametes (spermatozoa), and the female gonad (ovary) produces female gametes (ova). [1 = ovum. 2 = polar body. 3 = zona pellucida]   These unite (fertilization) to produce a unicellular "zygote", which then multiplies and differentiates to produce an organism.    Tissues of the organism are either the "soma" (e. g. liver, brain, kidney), or the "germ-line" (contained in the gonad). Weismann proposed (correctly) that the soma (mortal) merely provides a supporting role for the germ-line (potentially immortal). Darwin proposed (incorrectly) that throughout life the germ-line would collect "gemmules" of information from the soma. The germ line cycle can be interrupted, so that an organism is "reproductively isolated", either because the gametes cannot meet ("transfer barrier"), or because the zygote does not develop to produce a healthy ("viability barrier"), or because meiosis fails in the gonad ("sterility barrier"). Romanes died in 1894 at age 46, and is remembered today mainly as Darwin's protege, and for the annual Romanes Lecture series which he endowed at Oxford. However, Darwin's mantle had been safely entrusted. Four years after Darwin's death Romanes published an extraordinary paper in the Journal of the Linnean Society, entitled "Physiological Selection: An Additional Suggestion on the Origin of Species"12. In a formidable display of deductive reasoning, paralleled only by that of Darwin himself, Romanes resolved the three major problems in Darwin's theory -- "inutility of characters", "blending inheritance", and "hybrid sterility". We are concerned here mainly with the latter. Darwin's theory was based on observations of plant and animal species "under domestication". Darwin thought it appropriate to extrapolate these observations to natural species. Yet he was aware (and the biologist Thomas Huxley constantly reminded him) that, if crosses between members of closely related natural species were possible, the "hybrid" offspring were invariably sterile. If we consider the horse and the ass as members of separate natural species, when a cross is made between them the resulting mule, although otherwise healthy, has maldeveloped testes or ovaries, and cannot reproduce. Conversely, the offspring of crosses between members of "species" created by man (the breeder and horticulturist), are invariably fertile. If one defines a species as a group of organisms which do not breed (produce fertile offspring) with members of other species (e.g. cats do not breed with dogs), then "species" arising under domestication (e.g. poodle and bulldog) are not true species even though they may differ greatly from each other anatomically; they are "varieties", or "races". Hence Darwin confronted a serious problem. Returning again to Figure 1, we see that failure to produce offspring may have three fundamental causes. The germ-line cycle may be interrupted because: (i) The sperm and ovum (gametes) are unable to reach each other or will not fuse to form the zygote. (ii) the zygote cannot develop into an organism, so that the "soma" is not present to support the survival of the germ-line in the gonad. (iii) The formation of gametes in the gonad is impaired. These are sometimes referred to as the transfer barrier, the viability barrier, and the sterility barrier, respectively. Romanes focussed primarily on the sterility barrier, often the only barrier separating members of closely related species (i. e. species likely to have arisen recently from a common originating species). He suggested that, like cells of all other organs and tissues, germ line cells might undergo random variations. Variations, for example, in height or eye colour were familiar to everyone. No one knew what caused variations, but no one doubted their existence. Romanes emphasized one possible class of variation affecting germ line cells, which would make an organism less fertile with other members of the species but not influence somatic characters. Normally the loss of fertility would be highly disadvantageous because the organism would leave no offspring. However, he further argued that if two organisms underwent the same type of variation, they would still be fertile between themselves. They would be "physiological complements". Hence, at any time-point a species would consist of the parental group (comprising the majority of species members), and numerous small variant groups. Members of each variant group would be less fertile with members of the parental group and of the other variant groups. Members of each group, to varying degrees, would be "reproductively isolated" from members of the other groups, but remain quite fertile with each other. Hybrid sterility would be an extreme manifestation of the phenomenon. Romanes demonstrated that irrespective of any selective factors in the environment (no "survival of the fittest" required), members of the reproductively variant groups would tend also to be somatically variant, just as members of the parental group would tend to be somatically variant (e.g. changes in height or muscular strength). In the absence of environmental selection, however, somatic variations of members of the large parental group would not be sustained, because these members were crossing freely with each other, a process which tended to blend and neutralize variations. For example, although humans vary in height, tall people cross with small people and the average height tends to remain constant. On the other hand, variations within a small variant group would not be subjected to this "swamping" effect due to blending with members of the parental group (i.e. there would be less non-variants to dilute the variation). Because of this reproductive isolation, the variation would be sustained. If, by chance, the variation happened to confer some advantage to members of the variant group, relative to members of the parental group, that would be a bonus. The selective advantage conferred would allow the commencement of classical Darwinian natural selection. The major difference between the old Darwinian formulation (natural selection), and the new Romanesian formulation (physiological selection) was that in the first case, natural selection preceded reproductive isolation, where in the second case reproductive isolation preceded natural selection (Figure 2). Reproductive isolation, in any shape or form, would suffice, but the most usual form of isolation would result from physiological selection. Of course in a large population it would be very unlikely that a male and a female whose gonads had undergone the same rare variation would encounter each other. Thus the variation would probably not be perpetuated. Indeed, as Romanes pointed out, successful speciation was rare.  by D. R. ForsdykeWWW page access counter Since 30th March 1999 |
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