Theoretical
Evolution: the Future of Selection
By H.F. Hoenigsberg
Departamento de Genética & Evolución Universidad Manuela Beltrán
Bogotá D.C. Colombia
Neo-Darwinism is a wholly organismic, reductionistic, gradualistic, extrapolationistic theory that pretends to encompass the whole of evolution from the first single-celled organism to the most complex of animals. Thus, life has been orchestrated by natural selection acting on heritable changes occurring largely at random in every generation. As viewed by classical Neo-Darwinian selection is a one way street that leads towards a unidirectional transformation of populations, as in orthoselection, based on adaptation behind each allelic substitution and offering gradual accumulation of selected micromutations for the evolution of higher taxa. It is presented as a unified theory applying the statistical and elegant mathematical law of the Hardy-Weinberg equilibrium in large populations.
Classic Neo Darwinism, by claiming a gradualistic step by step single gene survival through selection of alternative alleles, rejects recent theory of lateral quantum genetic transfer where animal eukaryotic genomes show plasticity, dynamism and chimerism (Doolittle, 1999 a, b).
In other analysis we find that the theory of reciprocity through game theory (Axelrod and Hamilton, 1983) utilizing the Prisoner’s Dilemma by Rapoport and Chammah (1965) and the reciprocal altruism through repeated opportunities for interaction of individual’s recognition and learning by Trivers, do not find a satisfactory answer in step by step single gene survival in which only one selective level functions among individuals. As discussed later in this paper, both reciprocal altruism and the repeated cooperation between a pair of opponents exist in slime molds and myxobacteria (among many other interspecific symbiotic microorganisms) that do not have even brains. Therefore, we maintain that New-Darwinism does not have a satisfactory explanation for the evolution of behavior. How can Darwinism account for the giving of alarm calls as reported by many workers in the field?
Both Kin selection and mutualism need an explanation that go back to the very roots of individuality. There is evolution in the transition of individuality, in a continuum all the way from unicellularity to multicellularity. This is a new way of analyzing present day selection and adaptation.
Prebiotic Models in Evolution
It is tempting to view, in prebiotic models, some important biological processes, such as spontaneous aggregations, in their first moments, as dissipative structures “ŕ la” Prigogine, in polypeptide synthesis by condensation over catalytic surfaces (Keller and Segel, 1970; Prigogine, 1967; Prigogine and Stengers, 1984; Nicolis and Prigogine, 1977). In Eigen’s evolution of molecular populations and the formation of the genetic code through a succession of instabilities a fundamental problem emerges. Natural Selection cannot act where no individuality exists. Originally, only molecular competition and conflict between RNAs occur. Although different proto-genes, serving different functions, should be able to co-exist without excluding each other. And in protocells and in molecular hypercycles there may or may not be competition because functional interactions between different RNAs are necessary at times, especially because these polymers have some enzyme activity (replicases). In the hypercycles no sequence can exclude another in the cascades of replications. A molecule in some cases will altruistically aid the next serially until the closure of the cycle. Here we have replicators but without natural selection. With mutation present, even parasites (a mutant) appear, but also altruistic mutants can exist.
Problems beyond Darwin
There are other recent results that do not find in Darwinism or in Neo-Darwinism a satisfactory account of speciation. Contreras, Torres-Mura and Spotorno (1990) having found the largest chromosome number in a South American desert rodent had already “sensed” an autonomous genome size evolution although they never explicitly advocated whole genome sudden adaptability.
Gallardo (1997) had proposed a saltational karyotypic evolution in the Octodontoidea (Mammalia, Rodentia) to explain relatively sudden niche jumps that assume ontogenetic changes within related Taxons (Order, Genus) even before he discovered the unexpected and healthy tetraploid rodent Tympanoctomys barrerae (Gallardo et al., 1999), a finding that questioned Muller’s Neo-Darwinian gradualistic theory in which rarity of polyploidy among animals stresses sexual and developmental barriers in triploids derived from diploids and tetraploids. Nevertheless, there are structural difficulties with polyploidy in mammals because genome duplications have the theoretical possibility of breaking up dosage compensation derived from the inactivation of one X chromosome in females. In fact, following Mullerian theory in the XY mammalian sex determination system the progeny of tetraploids would have no chance of surviving because of the dosage-sensitive regulatory mechanism. However, how does structural compensation “senses” dosage ?
The discovery of tetraploidy in a mammal has shaken the foundation of a darwinian evolution by single gene changes and proclaims the dynamic nature of whole genome readjustment. Tympanoctomys barrerae (2n=100+XY) is a specialized desert rodent that represents the South American radiation of octodontids. Genome size estimates with flow-cytometric analysis shows T. barrerae with 16.8 pg of DNA, which is approximately double the DNA content of its close relatives and other members of the superfamily Octodontoidea and Cavioidea. In mammals, the largest genome size prior to T. barrerae was found in the rodent Proechimys trinitatis with 12.6 pg of DNA. Variation in genome size cannot be attributed to the amount of the whole-arm heterochromatin; C-band patterns do not support centric fusions and more extra heterochromatic additions to explain the origin of the large genome size of T. barrerae (For the recent work done on T. barrerae, see the review article by Gallardo, 2001).
If DNA sequence data of whole genomic analysis over the past ten years in Eukarya cannot permit interpretations other than chimerism and plasticity (Doolittle 1999), then even single genes cannot be traced back to their historical ancestor through Mendelian (vertical) transmission and only McClintock’s lateral transmission can be of use. How can we continue to utilize the traditional definition of individual fitness due to single gene survival through selection of alternative alleles in which Mendelian transmission is indispensable? Here we are confronted with a challenge to traditional Darwinism.
I venture to predict that, in the future, the new theory of evolution will include many non–adaptive features and not only allometry and pleiotropy, where speciation can occur through the allopatric set-up, but also sympatry because demic structures can produce abrupt isolation and eventual speciation even without drift. Drift as promoter of evolutionary change will not be abandoned, it will be there to push demes out of an adaptive peak with selection forcing them to another (Hoenigsberg, 1988, 1989).
The future of Evolutionary Studies will have to deal with the following as yet unsolved topics:
The Hierarchical Levels of Evolutionary Change
Evolutionary change may occur at a number of biological levels. Changes can appear in gene frequency, in the average phenotype of a population of organisms, and in how biological communities are made up. Maynard Smith (1983) was the first to assert that all of these units of evolution exhibit the properties of heredity, reproduction and variation, and natural selection may be operating at any one of or at all three levels. I maintain that chromosomes, plasmids, parasitic particles and even demes as a synchronized consanguineous and coherent unit (as a colony or beehive) are also self-reproducing entities and therefore can be subjected to the action of natural selection at any of these levels (Hoenigsberg, 2001 in press).
We can start out with the means of communication of a primitive species that usually involve the use of chemical signals. Communication not only integrates the group, but also can provide some adaptive advantage, as in the case of microorganisms in gathering or dispersing food. The Myxobacteria constitutes an excellent example. The individual bacteria feed in a swarm that permit them to conquer large prey that they cannot digest as separate cells. Moreover, their group excretion of extracellular enzymes could only be accomplished by cooperation. Furthermore, they not only show their cooperation during feeding, but also during fruiting body formation. A fruiting body is a tall structure, about a millimeter, that contains many resistant spores or cysts, each of which can initiate a new generation when it germinates in a suitable, nutritious medium.
The bacterial or prokaryotic cell is exceedingly primitive and yet it has evolved several communal characteristics. Besides feeding and fruiting body formation (the latter for dispersal), the feeding cells are phototactic and go toward light when they are in a group (Aschner and Cronin-Kinsh, 1970), and they are chemotactic and orient themselves in a chemical gradient as well. According to Dworkin (1972 ) these bacterial cells are able to merge in larger and larger groups mainly because they are chemotactically attracted to one another. It appears that they have division of labor during the formation of fruiting bodies, i.e. some of the cells specialize as resistant spores, while others disintegrate and form the large supporting stalk (Wireman and Dworkin, 1975). Like social insects, social bacteria have the equivalent of two castes. The spores constitute the reproductive caste, and the stalk are the “workers”, for they do not propagate, but altruistically lift the reproductive cells into a more convenient position for dispersal, after which they disintegrate and dye in the process. This is an example of a higher level selective process which we have been discussing and it is a remarkable series of communal feats, when one considers that these are brainless prokaryotic cells which in the evolution of microbial life, arose long before the higher eukaryotic cells of our bodies.
Slime molds are made up of amoebae (some are flagellated at some stage in their life cycle), that, in some of the species, duplicate all the social behavior we have just described for Myxobacteria. So, the cellular slime molds, too, have fruiting bodies and division of labor where some cells turn into resistant spores, while others form the supporting stalk by getting large and vacuolate, with thick cell walls. Moreover, as in Myxobacteria, these stalk cells dye as they form, again a good example of altruism. In sensu stricto, slime molds aggregate by chemotaxis to form a multicellular mass that communicate by chemical signals at one time in their life cycle and, at a later stage, they become essentially one large multicellular individual. These slime molds are social at early stages and simply multicellular at later stages. These are examples of the fundamental principle of a social existence (Bonner, 1980). What about the colonial diatoms? (W. Smith, 1856, A Synopsis of the British Diatomaceae etc. London). These also serve as perfect examples of a hierarchical biological construct where nature has passed from lower primitive single cellularity to higher level communal life, where we have evidence of a continuum.
From Boltzmann to Darwin
Eigen’s theoretical work demonstrates that where there are systems formed exclusively by proteins (no nucleic acids) the succession of instabilities would continue on and on. On the other hand, interactions between polynucleotides and proteins would permit the system to reach a final state in the genetic code that would correspond to a considerable stability if compared to the “errors” of Kinetics. Thus, the succession of the events would be the theory of Boltzmann’s order, Prigogine’s dissipative structures and the genetic code. These, we believe, are the links in a sequence of events that bring the physical laws in the thermodynamic equilibrium to biological order and Natural Selection.
Thus, for individuality to emerge, there has to be, first of all, selection for conflict modifiers of within-organism change, including modifiers for mutation rate. Moreover, the characteristic that promotes modifiers to reduce within-organism conflict will have the adaptive value that serves individuality and the well-being of the organism. Thus, for the organism to improve as a biological machine where higher order selection can operate, it has to improve its own coherence by frustrating its primitive unicellular tendency of within-organism variation, competition, conflict and selfishness! In the cases described for slime molds and Myxobacteria, the unicellular tendency that had to be improved to become altruistic, was that of within-multicellular variation, competition, conflict and selfishness!
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