Moss - What genes cant do - 2003, страница 7

Only the dual concept seems to fit the facts, as we see at present. Tobe consistent, we should supplement Virchow’s well known tenet of the celltheory: “Omnis cellula e cellula” by its counterpart: “Omnis organisatio exorganisatione.” If the former denies spontaneous generation of living matter, thelatter denies spontaneous generation of organization. In admitting this, wemerely paraphrase what Whitman has called the “continuity of organization.”But within these specified limits the cell, even in development, is still, as Schwannhas said, an individual.In the remainder of this chapter we will see how the question of the continuity of organization, in fact, became further and further removed fromthat which became the mainstream of work on heredity. We will returnto the question of organization in subsequent chapters.Chopping up the AnlagenThe last three decades of the nineteenth century saw a proliferation ofattempts to theorize about the relationships between evolution, heredity,development, and cytology.

Developmental morphology after Darwinand under the influence of Ernst Haeckel, had become tinged withromantic intuitionism and subordinated to the purposes of phylogeneticreconstruction as guided by Haeckel’s biogenetic law. Increasinglyhowever, late nineteenth-century embryologists were turning toward anew experimentally oriented embryology devoted not to classificationbut to further revealing the mechanisms of development.With the advance of cytological microscopy, the continuity of thenucleus and the existence of chromosomes had become empirically well20Chapter 1established. Recalling the model of a cell-within-a-cell articulated by vonBaer, it is not surprising that a pattern of centrifugal movement, this timefrom the nucleus outward, would be invoked. But how would the Keimeund Anlagen of pre-Darwinian, teleomechanist thought be reconfiguredto fit into a “Darwinian nucleus”? Darwin conceived of a process ofgradual change requiring the presence of a range of heritable variationsexposed to the forces of natural selection.

Where variation in the neoKantian mold was construed as systematic and adaptive, Darwinianvariation had to be based on the generation of many independently variable characteristics whose variants were mostly the product of randomprocesses.At least one way to assimilate the legacy of the teleomechanist Keimeund Anlagen into a Darwinian framework would be to chop it into manylittle pieces such that natural selection could favor or disfavor small differences. Of course, in allowing for gradualist change in this manner, onemust abandon the systematic and generic quality of the older model. Thegeneric preformationism of the neo-Kantians was holistic.

The adaptivepotential of the Keime und Anlagen was necessarily understood to be afunction of the purposeful organization of the whole—it could therebynot be reduced to merely a sum of its parts. When the Keime und Anlagenbecomes chopped up into particles, the generic potential—i.e., the potential to produce a wide range of forms which is present in the germ asa whole—is forsaken. One returns to a kind of pre-Kantian preformationism, now in the form of a patchwork homunculus. Darwin himselfconceived of heredity based upon an aggregation of particulate gemmulesor pangenes which are distributed throughout the body and capable ofmigrating into germ cells to provide the full complement of particles necessary to produce a new organism. Darwin’s model did not exclude thepossibility of the inheritance of acquired characteristics in as much assuch particles, produced by the active mature body, could well be influenced by life experience.That separation between heritable factors which could or could notbe influenced by life experience was put forward, as a veritable cordonsanitaire, by August Weismann.

Weismann, an embryologist, Darwinist,anti-Larmarckian, and leading expositor of late nineteenth-century speculative biology, offered a vision aimed at a unified account of evolutionGenesis of the Gene21and development. Weismann postulated the ironclad separation of tissuedestined to be germ cells from that destined to give rise to the remainder of the body, or soma. Remaining isolated from the soma, theso-called germ plasm can’t be influenced by the life-experience of individuals, thereby ruling out the possibility of passing on adaptive changes.Weismann conceived of the chromosomes as a complex hierarchy ofnested determinants of form, smaller particles packed into progressivelylarger particles (the primary units, or “biophores,” aggregated to form“determinants”; the determinants grouped into “ids”; and the idsgrouped into “idants”; identified with observable chromosomes (Wilson1893).

Weismann, in effect, not only chopped the Keime und Anlageninto pieces but also chopped the pieces into pieces, and those pieces intopieces as well. The process of development, he believed, could beaccounted for by the unequal distribution of piecemeal determinants ineach successive division of the fertilized egg and its descendants. Somaticcell types, then, would be determined by the subset of heritable particulate material with which they were endowed.Although purely speculative, Weismann’s model had the virtue ofoffering a unified theory of development and evolution, taking its cuesfrom the requirements of a Darwinian gradualism and a commitment toan unmitigated anti-Larmarkianism.

Perhaps the popularity of these latercommitments among many twentieth-century evolutionary thinkers hastended to immunize Weismann’s still often celebrated reputation againstwhat could be the more corrosive effects of empirical disconfirmation.10Weismann left an influential legacy in many ways, but his work did notlead directly to what became the predominant research program of thetwentieth century.A Bifurcation in EmbryologyBy the 1890s the United States was gaining importance in the worldof biology.

The Marine Biological Laboratory (MBL) at Woods Holewas becoming a focal point for debates among leading practitioners ofthe new experimental embryology. The contrasting views of two of theprimary figures at Woods Hole prefigured the principal axis of opposition that came to define the subsequent course of American biology.22Chapter 1Charles Otis Whitman was the first director of the MBL in 1888 andfounder of the Journal of Morphology. He rejected the autonomous-celltheory of development and considered the organization of the egg torepresent the unity of the organism prior to any cell division. Rather thanfocusing on the primacy of the nucleus his emphasis was on the organization of the egg cytoplasm and its constitutive role in the generation ofdifferentiated cell lineages:The organization of the egg is carried forward to the adult as an unbroken physiological unity, or individuality, through all modifications and transformations.The remarkable inversions of embryonic material in many eggs, all of which areorderly arranged in advance of cleavage, and the interesting pressure experimentsof Driesch by which a new distribution of nuclei is forced upon the egg, withoutany sensible modification of embryo, furnish, I believe, decisive proof of a definite organization of the egg, prior to any cell formation (Whitman 1893).Whitman emphasized the continuity of organization from one generation to the next, with the organization of the egg cytoplasm constitutingthe key link between generations.

This approach to development gaverise to studies which traced the origins and subsequent history of cell lineages, beginning with identifying that part of the egg cytoplasm whichgives rise to particular cell lineages.By contrast, Edmund Beecher Wilson, who was to become the leadingAmerican cellular biologist during the first decade of the twentieth century, staunchly defended the autonomous-cell-centered model of development.

If all cells are equal in their developmental potential, thenthe cytoplasmic organization of the egg need not be granted a privilegedstatus. Both Whitman and Wilson were keenly aware of theirstandpoints with respect to recasting the status of the preformationismversus epigenesis debate, and both were motivated to avoid the perceivedextremes at either end (Maienshein 1987). This opposition came to beplayed out largely in terms of nuclear versus cytoplasmic interactions,with the former perceived as associated with the preformationistpole and the latter with that of epigenesis. Wilson thus favored a modelwhich emphasized the influence of the nucleus on the developing cytoplasm, whereas Whitman emphasized the role of the organized cytoplasm in regulating the activity of the nucleus.

Wilson’s position andcognizance of its historical standpoint is ably indicated in his MBLlecture of 1893:Genesis of the Gene23It is an interesting illustration of how even scientific history repeats itself thatthe leading issue of to-day has many points of similarity to that raised twohundred years ago between the prae-formationists and the epigenesists.

Manyleading biological thinkers now find themselves compelled to accept a view thathas somewhat in common with the theory of prae-formation, though differingradically from its early form as held by Bonnet and other evolutionists11 of the18th century. No one would now maintain the archaic view that the embryoprae-exists as such in the ovum. Every one of its hereditary characters is, however,believed to be represented by definite structural units in the idioplasm of thegerm-cell, which is therefore conceived as a kind of microcosm, not similar to,but a perfect symbol of, the macrocosm to which it gives rise (Wilson 1893).It is strikingly clear from this quotation, seven years prior to the rediscovery of Mendel, that what Gregor Mendel’s famous paper providedwas not a novel concept about the nature of inheritance, but in Kuhnianterms, an examplar for use in turning this new particulatepreformationism into a research program.Mendel’s ExemplarWhat Thomas Kuhn (1962) came to highlight in his 1969 postscript asthe central insight of his celebrated work was the recognition of thecentrality of exemplars in the history of science.