Evolution as a Self-Limiting Process

EVOLUTION AS A SELF-LIMITING PROCESS

 

Introduction

As a physiologist I am fascinated by how things work. It is clear to me that Darwinism simply doesn't work and so it isn't interesting to me. The first section will summarize the evidence that Darwinian mechanisms have not produced new life forms and so must be abandoned. Since the devices involved in Darwinism are sexual in nature, it follows that sexual reproduction cannot produce new kinds of living things.

Before I get to the hard evidence, I would like to comment on biological systems generally. Cells and organisms grow and then they stop growing. Cells differentiate during embryonic development and then they stop. In other words, most biological systems are self-limiting, in contrast with geological mechanisms which are not self-limiting. The geologist Charles Lyell introduced the term Uniformation to describe his view that the physical forces shaping the earth today are the same forces that have acted in the past. Darwin embraced this notion and applied it to his evolutionary hypothesis. It is evident in the very last words of his book: ".....endless forms most beautiful and most wondrous have been and are being evolved" (Darwin 1896).

Is it not reasonable that evolution like other biological processes might also be self-limiting? I think that the evidence for that conclusion is enormous and leads to the perspective that a primary effect of sexual reproduction is to bring evolution to a halt and to permit only minor adaptations to a relatively stable environment. In support of that notion isn't it interesting that we see rampant extinction today but no one has seen a single higher form of life undergo a significant evolutionary change? I am not saying that such changes cannot occur. What I am saying is that no one has observed the mechanism by which such changes have occurred. Of course, nearly all attempts to produce new life forms have involved sexual reproduction. Since all these attempts have failed, we must look to another mechanism for the answers.

For purposes of argument, let us accept the conclusion that evolution is largely finished. How then did it occur? I propose that the answer has been before us for a long time and we have simply not recognized it. The answer resides in an understanding of the nature of sexual reproduction and the manner in which eggs and sperm are produced. In all diploid animals and plants the chromosomes occur in pairs. One member of each pair comes from one parent, the other from the other parent. Thus, each egg or sperm must have only one of each kind of chromosome, a condition known as haploidy. The process by which this reduction takes place is known as meiosis or chromosome reduction. The manner in which this reduction takes place provides us with the answer to the question. If one were to imagine the simplest way that chromosome reduction could take place it might be as follows. The chromosomes would align in pairs and a single division would take the chromosome number from diploid to haploid. Not a single living creature undergoes meiosis in this way. Instead, each chromosome is duplicated taking the chromosome number from diploid to tetraploid. Then while they are in alignment (synapsis) a very important step occurs. Breaks occur in some of the chromosomes and exchanges occur between the original pairs of chromosomes a phenomenon known as crossing over. Following this event, the chromosomes undergo the first meiotic division returning the chromosome number from tetraploid to diploid. The way in which this division occurs is critical to an understanding of the evolutionary process. The two originally identical chromosomes (known as sister strands) always remain together. Note that this first meiotic division is a perfectly valid form of diploid reproduction and forms the basis for what I have called the semi-meiotic hypothesis for organic evolution (Davison l984, l993). Also, since meiosis involves two steps, it is mandatory that the first meiotic division must have evolved prior to the second, and accordingly can be considered a primitive and presexual form of diploid reproduction.

Is evolution finished?

Several years ago when it first occurred to me that evolution might be a self-limiting process I discovered that others had already suggested that evolution might be over. In order to give due credit to them I will present their statements in the form of direct quotation so their meaning cannot be colored by interpretation. The first of these is Julian Huxley, the grandson of Thomas Henry Huxley, the great defender of Darwin. In 1940 Huxley published "Evolution: The Modern Synthesis." The book is in effect an agreement among certain geneticists, systematists and paleontologists that evolution is a Darwinian phenomenon. On page 571, seven pages from the end, Huxley offers this revealing commentary.

"Evolution is thus seen as a series of blind alleys. Some are extremely short - those leading to new genera and species that either remain stable or become extinct. Others are longer - the lines of adaptive isolation within a group such as a class or subclass, which run for tens of millions of years before coming up against their terminal blank wall. Others are still longer - the links that in the past led to the development of the major phyla and their highest representatives; their course is to be reckoned not in tens but in hundreds of millions of years. But all in the long run have terminated blindly. That of the echinoderms, for instance, reached its climax before the end of the Mesozoic. For arthropods, represented by their highest group, the insects, the full stop seems to have come in the early Cenozoic. Even the ants and bees have made no advance since the Oligocene. For the birds, the Miocene marked the end; for the mammals, the Pliocene."(Huxley 1963).

Note Huxley's language: blind alley, terminal blank wall, terminated blindly, full stop and marked the end. This passage also contains another interesting observation. Huxley says that new genera or species either remain stable or become extinct Are these conclusions compatible with Darwinian evolution? I do not think so. Huxley was not alone in these opinions. The non-Darwinian paleontologist Robert Broom reached similar conclusions.

"In Eocene times - say between 50,000,000 and 30,000,000 years ago-small primitive mammals rather suddenly gave rise to over a dozen very different Orders - hoofed mammals, odd-toed and even-toed, elephants, carnivores, whales, rodents, bats and monkeys. And after this there were no more Orders of mammals ever evolved. There were great varieties of evolution in the Orders that had appeared, but strangely enough Nature seemed incapable of forming any new Orders. What is equally remarkable, no new types of birds appear to have evolved in the last 30,000,000 years And most remarkable of all, no new family of plants appears to have been evolved since the Eocene. All major evolution has apparently come to an end. No new types of fishes, no groups of molluscs, or worms or starfishes, no new groups even of insects appear to have been evolved in these latter 30,000,000 years." (Broom l951).

"There is, however, no doubt that evolution, so far as new groups are concerned, is at an end. That a line of small generalized animals should have continued on till in Eocene times the Primates originated and then ceased, and that except for specializations in Eocene types there has been no evolution in the last forty million years, and that the evolutionary clock has so completely run down that it is very doubtful if a single new genus has appeared on earth in the last two million years,.."(Broom 1933).

Just as others had suggested that evolution might be finished, I found that others had questioned the capacity of sexual reproduction to produce evolutionary progress. The horticulturist Luther Burbank was not an academician. He described himself as having been educated at the University of Nature. From his autobiography he writes:

"There is a law of which I have not yet spoken that is useful to plant-breeders, as well as being a limitation on them. It is called the 'Law of the Reversion to the Average.' I know from my experience that I can develop a plum half an inch long or one two and a half inches long, with every possible length in between, but I am willing to admit that it is hopeless to try to get a plum the size of a small pea, or one as big as a grapefruit I have daisies on my farms little larger that my finger nail and some that measure six inches across, but I have none as big as a sunflower, and never expect to have ... In short, there are limits to the developments possible, and these limits follow a law" (Burbank 1939).

Before introducing my next skeptic, I will place Darwinism in historical perspective. In the year 1900, Mendel's papers were rediscovered and with that discovery Darwinism was given a great impetus with the identification of Mendel's factors which we know now as genes. Variation was no longer a mystery. A major exponent of the new science of Genetics was William Bateson, properly regarded as the father of modern genetics. His early enthusiasm for Mendelism was indicated when he named his son Gregory when he was born in 1903. However, that enthusiasm waned toward the end of his life. In 1970 Arthur Koestler was completing research on his book "The Case of the Midwife Toad," concerning the career of the Lamarckian zoologist Paul Kammerer. As part of that research, Koestler interviewed Bateson's son Gregory who offered the following recollection to Koestler:

"By 1924, Bateson had come to realize, and told his son in confidence, 'that it was a mistake to have committed his life to Mendelism, that it was a blind alley which would not throw any light on the differentiation of species, nor on evolution in general'" (Koestler 1971).

Mendelism is of course the genetics associated with sexual reproduction and here we have Bateson like Burbank suggesting that sexual reproduction is incapable of producing progressive evolutionary change.

The failure of selection

Long before Darwin man had been experimenting with selection. Dogs, which many believe were originally derived from wolves, offer an excellent example of the failure of selection to produce new species. In addition to the many working breeds, man has also created some bizarre creatures like the Chihuahua and giant animals like the St. Bernard and the Great Dane. In addition to size, great variations in coat quality, color and temperament have been produced. All of these differences are due to the action of Mendelian genes segregating and recombining in sexual reproduction, with the result that dogs are still able to hybridize freely with wolves. The hybrids are of course fertile which is to say that they are not really species hybrids at all. Winge, in his book "Inheritance in Dogs," describes a spontaneous cross between a male St. Bernard and a female Dachshund. A female product from this union herself bore a litter proving that no speciation had occurred during the long period of separation of the breeds. This female however inherited the large size from the St. Bernard male and the short legs from her Dachshund mother with the result that her belly dragged during her pregnancy and had to be protected with towels. (Winge 1950)

An even better example of the failure of selection is offered by the goldfish. Originally derived from the Asiatic carp, the Chinese and Japanese have produced some remarkably odd creatures such as those with telescopic eyes, some of which even look upward as in the variety "celestial." More important, they have produced varieties that depart from the fundamental vertebrate character of having two pairs of lateral appendages, the anterior or pectoral and the posterior or pelvic limbs corresponding to our arms and legs respectively. They managed to duplicate the anal fin thereby converting the fish from a quadruped to a hexapod and even duplicated the tail, a condition which does not exist in the natural world. They also removed the dorsal fin seriously impairing the animal's capacity to swim properly. None of this has produced any semblance of speciation and the animals are still Asiatic carp.

Why do these attempts fail? I think they fail because they represent changes in individual Mendelian genes from which one can draw the reasonable conclusion that such changes have little or nothing to do with evolution.

When I say that selection has failed, I am limiting that conclusion to the products of sexual reproduction. Of course, Nature selects what will be successful and what will fail. Natural selection, like "the survival of the fittest" is a tautology which explains nothing. The real issue is to determine how new life forms were produced..

Chromosome structure and evolution

Another way to look at the problem is to ask what has remained unchanged in the various varieties of dogs and goldfish. One obvious answer is their chromosomes. The karyotypes, or pictorial representations of the chromosomes, are invaluable in identifying species and relationships between species. One of the pioneers in the recognition of the importance of chromosome structure was the geneticist Richard B. Goldschmidt. In 1940, the same year as Huxley's "Evolution: The Modern Synthesis," Goldschmidt published "The Material Basis of Evolution." It is difficult to imagine two books more dissimilar than these while dealing with the same subject. Goldschmidt's book is divided into two sections, the first dealing with what he called microevolution, the second with macroevolution. His first section ends with this statement so reminiscent of Bateson:

"Subspecies are actually, therefore, neither incipient species nor models for the origin of species. They are more or less diversified blind alleys within the species. The decisive step in evolution, the first step toward macroevolution, the step from one species to another, requires another evolutionary method than the sheer accumulation of micromutations" (Goldschmidt 1940).

The other method to which Goldschmidt refers is the reordering of existing genetic information within the chromosome. Alterations in genetic expression resulting from such rearrangements are called position effects. In his words at the end of the section on macroevolution:

".... the fact remains that an unbiased analysis of a huge body of pertinent facts shows that macroevolution is linked to chromosomal repatterning and that the latter is a method of producing new organic reaction systems, a method which overcomes the great difficulties which the actual facts raise for the neo-Darwinian conception as applied to macroevolution" (Goldschmidt 1940).

There are several sorts of chromosome rearrangements that can occur. Two chromosomes can fuse together to form one. Two breaks can occur along a chromosome with the broken fragment undergoing a 180 degree rotation before reattaching . These are known as inversions. There are two types of inversions depending upon where in the chromosome they occur. Each chromosome has somewhere along its length a place where the spindle fibers attach when the chromosome is undergoing mitosis or meiosis. This structure, called the centromere, like the chromosome, contains DNA and is self-replicating. If the inversion does not include the centromere it is called a paracentric inversion. If the centromere is within the inverted segment, it is called a pericentric inversion. Another type of chromosome restructuring is reciprocal translocation in which two different chromosomes exchange parts. Other types of chromosome alterations include duplications, deficiencies and dissociations. Alterations can also occur in the number and position of nucleolar organizers as well as changes in the chromosome ends or telomeres.

Before proceeding, I want to mention the irreversibility of evolution as it relates to chromosome structure.

Why is evolution irreversible?

No amphibian has ever evolved into a fish and no bird into a reptile. There is not a single documented example of a reversible evolutionary event of any magnitude. Why?

First, point mutations in individual genes are reversible and as such that fact alone can be interpreted as indicating that such changes do not play an important role in evolution. Assume for purposes of discussion that evolutionary changes involve not individual gene mutations but chromosome restructurings such as those that I have mentioned. Consider an inversion as an example. If such an event should occur, the probability of it being reversed is virtually zero since the chromosome would have to break in exactly the same two places to return the chromosome to its original configuration. A similar argument applies to the improbable reversibility of chromosome fusion, dissociation or reciprocal translocation. Furthermore, these chromosomal structural changes are all-or-none events that have no intermediate states and cannot then be considered gradual. Accordingly, one might anticipate that their effects might be quite dramatic although unpredictable. This perspective also offers an explanation for the absence of intermediate forms in the fossil record.

The first meiotic division

The ideal situation might be for an organism to simultaneously reproduce its own genotype and produce trial balloons as well. It is in this ability that the significance of diploidy becomes apparent. When a haploid creature undergoes a heritable change, it loses its genetic identity. That is not the case for a diploid organism. To understand how that is possible it is only necessary to realize that there are three self-replicating elements involved in mitosis and meiosis. The first of these are the chromosomes, the second are the centromeres, and the third are the centrioles, the self-replicating structures at the ends of the spindle on which the chromosomes move. In mitosis the centromeres and the chromosomes duplicate in relative synchrony with the result that each daughter chromosome passes to opposite ends of the spindle independently. The result is that the two daughter cells receive identical genetic complements. In the first meiotic division, the centromeres do not duplicate when the chromosomes do with the result that the two sister strands must remain together. Consider an oocyte about to enter meiosis. Assume also that this cell has one chromosome that has undergone an inversion. When the tetrad is formed, it will consist of two daughter chromosomes of normal structure and two daughter chromosomes bearing the inversion. In the first meiotic division, half of the genetic information is discarded into the polar body leaving the other half behind. If the inverted pair is discarded, the egg remains as it was before the inversion took place. If the normal pair is discarded, the egg has instantly acquired a new karyotype and accordingly a new evolutionary potential.

All genetic changes occur originally in individual cells, and involve individual chromosomes. If the inversion in our example occurred in a cell destined to become part of the reproductive lineage then one half of the first meiotic division products of that lineage will be like the original organism and one half will be an organism with a new genotype. In this system, there is no requirement for the individual to be heterozygous for the chromosome change. The only requirement is for one or more of the oogonia to be heterozygous. Such an evolving system would be expected to produce a limited number of discrete products determined largely by the number of chromosome rearrangements involved in the series. I do not mean to suggest that all new chromosome homozygotes would result in new species. In fact we know that is not so. Nevertheless, this perspective is worthy of further attention.

Position effects and primate evolution

In Goldschmidt's day the internal structure of chromosomes was not well known and limited to a large extent to the giant salivary chromosomes of dipteran larvae especially those of the fruit fly Drosophila. Since then, new staining techniques have allowed a much more detailed analysis of the chromosome structure of many life forms. Of special interest are the chromosomes of the order Primates to which we belong. We are fortunate in having three living relatives with whom we can make comparisons, the chimpanzee, the gorilla and the orangutan. An analysis of the karyotypes of ourselves and our relatives provides invaluable information as to our evolutionary history.

In 1982, Yunis and Prakash published a paper entitled "The Origin of Man: A Chromosomal Pictorial Legacy." The paper compares the karyotypes of man, chimpanzee, gorilla and orangutan. Perhaps the most remarkable feature of the karyotypes is their similarity providing powerful evidence that we all four are related evolutionarily. The differences that do exist are largely structural rearrangements. For example, the three apes have 48 chromosomes while we have 46. This has apparently resulted from the fusion of two of the ape chromosomes to form a single human chromosome. Many of the chromosomal differences in the four species consist of inversions of homologous chromosome segments as well as variations in heterochromatin. Certain chromosomes demonstrate reciprocal translocations. Other differences include variations in telomeres or chromosome ends as well as differences in the position of nucleolar organizers (Yunis J.J., Prakash O. 1982). The important point is that the differences which are evident are precisely the kinds that Goldschmidt described, namely, the restructuring of existing genetic information. In examining the karyotypes, I note that the chromosome that seems to have undergone the least evolutionary change is the X chromosome. That is precisely what one might anticipate if the four species were linked gynogenetically.

Also, there are very small differences in both DNA and protein composition between ourselves and our living relatives (Andrews 1987), further supporting the view that point mutations are of relatively little significance in the evolutionary process .

The independent origin of sexual reproduction

In the Darwinian or sexual model, one might anticipate some universal sex-determining mechanism operating throughout evolutionary history. If, as I believe, the role of sexual reproduction is to limit evolution, one might anticipate a variety of sex-determining devices evolving independently. Such is the actual case. Again, I found that the idea of an independent sexual evolution had already been expounded. The Russian cytologist N.N.Vorontsov was one of the first to call attention to the independent evolution of sex determination.

"Just as the transition from isogamy to anisogamy and to oogamy took place independently of each other in the various phyla of plants so the formation of mechanisms of the cytogenetical sex determination with differentiated heterochromosomes follows the same pattern in various kingdoms and phyla and results in an independent occurrence of the XX-XY system in Melandrium as well as in many Insects and Mammals, whereas the ZW-ZZ system evolved independently in Trichoptera, Lepidoptera, Serpentes and Aves. Against the background of these facts it is unclear whether the male species of different groups are homologous to each other or not; they appear to be nonhomologous" (Vorontsov 1973).

In addition to the devices mentioned by Vorontsov, other mechanisms have also independently evolved. In the social insects, the female is diploid, the male haploid, a situation that also occurs in certain rotifers. In addition to these chromosomal mechanisms, the temperature during sensitive embryonic stages can serve to determine the sex as in some turtles and crocodilians. Sex reversal is common in certain animals as well as other forms of sex determination such as the age of the eggs when fertilized. This huge and varied literature has been reviewed by Bull (Bull l983). This is hardly the sort of situation one would anticipate if sexual reproduction were a requirement for evolutionary change. I think the most reasonable explanation for this great diversity is that the evolutionary steps involved in macroevolutionary events were made independently of sexual reproduction and accordingly were gynogenetic in nature, involving only the first meiotic division.

The importance of nonhomology

Homologous structures are structures that have a common origin. Any theory of evolution must recognize and incorporate nonhomology when that becomes evident. It is to the credit of Vorontsov that he did so when describing the various sex determining systems which have evolved. They have indeed evolved independently and accordingly are by definition nonhomologous. Another example of nonhomology which correlates beautifully with the various sex determining devices which have evolved is demonstrated b y the origins of the germ cells in contemporary vertebrates.

One of the most baffling features of vertebrate development is offered by the embryonic origins of the cells destined to become the reproductive cells. Again it is useful to place evolutionary theory in historical perspective. August Weismann is well known for having predicted meiosis and for his interesting aphorisms such as: "the protozoa are immortal" and "from eagles eggs come eagles." Each of these statements implies reproductive continuity, which of course is required for any theory of evolution. However, Weismann carried his theory further with his notion of the Continuity of the Germ Plasm (Weismann 1891). According to this concept there has been an unbroken chain of reproductive cells which have produced through modification the many life forms we see today. Evolution does not necessitate a homologous cell lineage but only reproductive continuity from one generation to the next. The simple facts are these. There is no way that the reproductive cells of mammals can be homologized with those of birds. Similarly, the eggs and sperm of frogs arise in an entirely different way than do those of salamanders and accordingly cannot be assumed to exhibit homology. In short, there is not a scintilla of evidence to support the notion of the continuity of the germ plasm. The details of these differences have been discussed elsewhere (Davison l984).The vertebrate gonad is a sterile organ unable to produce germ cells from its own epithelium (Nieuwkoop and Sutasurya l979). Instead, the testis or ovary receives its complement of potential gametes by a process of invasion from extragonadal sources during early development. Since the sources and modes of invasion cannot be homologized from group to group the continuity of the germ plasm is a myth. As someone so aptly put it: "hypotheses have to be reasonable- facts don't."

Fortunately, these nonhomologies correlate beautifully with the equally nonhomologous devices which have evolved to determine the sexes. Very simply, the cells now being used for reproduction are primarily sexual cells that, as has been suggested, are unable to support progressive evolution. I propose that the contemporary cell lineage is one that has independently replaced the semi-meiotic cell lineage. We may never know the original source of the semi-meiotic lineage but a very reasonable guess might be the gonad itself, a source which has since become sterile.

Semi-meiosis and the origin of diploidy

One of the most fundamental events in evolutionary history was the transformation from haploidy to diploidy since, as I have indicated, it allows the retention of the original karyotype at the same time that it permits new configurations to be produced by means of the first meiotic division. The transformation from haploidy to diploidy has probably taken place many times but one such transformation bears directly on the significance of the first meiotic division as an evolutionary device.

In 1947, L.R.Cleveland published a remarkable paper dealing with the origin and evolution of meiosis. His material was the various flagellate protozoa that live as commensals in the guts of termites and other wood-eating insects. Of particular interest here are his observations on the flagellate genus Spirotrichosoma which is found in three species of Stolotermes, a primitive termite with species in Australia, South Africa and New Zealand. The haploid number of chromosomes is 12 in Spirotrichosoma and haploids are found in all three locales. However, in addition, polyploids with 24,48 and 60 chromosomes are found only in the New Zealand populations of Spirotrichosoma. Clearly, within the confines of the genus we are observing the evolution from haploidy to diploidy in the New Zealand material. I quote Cleveland:

"Nuclear division of these polyploids can be seen very plainly, especially those with 4 rod-shaped chromosomes. Every division is exactly alike: synapsis in the prophase, followed by formation of tetrads, and movement of the chromosomes to the poles as dyads, i.e. every division is exactly like the first division in meiosis" (Cleveland 1947).

Following this single nuclear division the centromeres duplicate and the chromosomes separate. Then the cycle repeats itself. Note that these animals have no sexual phase since the second division never occurs. Accordingly, they present a living example of the semi-meiotic mechanism. In addition, Cleveland's observations suggest that the first meiotic division may be a more primitive form of reproduction than diploid mitosis (Davison 1984,1987).

Semi-meiosis in birds

A second example of semi-meiosis is offered by the Beltsville strain of small white turkey which produces a low percentage of parthenogenetic offspring (Olsen 1965). The proof that the mechanism is semi-meiotic is offered by the fact that all of these offspring are males. In birds, it is the female that is heterogametic (ZW) while the male is homogametic (ZZ). Before the first meiotic division, the tetrad consists of ZZ dyads and WW dyads. If the ZZ pair enters the polar body leaving the WW pair in the egg, the egg fails to develop since WW is apparently lethal in birds. If the ZZ pair remains in the egg it must develop as a male, which is the actual case. This example also demonstrates the instantaneous production of a chromosome homozygote from a single heterozygote precursor. If birds, like mammals, had homogametic females the parthenogenetic turkey would, in theory at least, be capable of progressive evolution and at the same time would be able to retain its original turkey genome.

There is another curious fact which supports the semi-meiotic hypothesis. Since sperm are universally haploid in their functional state, one might anticipate the same would be true for the mature ovum. Such is not the case. The vast majority of animal eggs are still unreduced at the time of penetration by the sperm. The egg at this time has often produced the first polar body and is arrested in metaphase of the second meiotic division. This is the case for most if not all vertebrates including man. I have suggested that this might represent an evolutionary relic from the time when the sperm either was not necessary for development or served only as a stimulus without necessarily contributing genetic information (Davison 1984). Clearly some other agency serves to activate the egg in the parthenogenetic turkey.

Semi-meiosis and genetic variability

While it is true that semi-meiosis can produce chromosome homozygosity, it is not true that it necessarily leads to gene homozygosity. Note that in sexual reproduction, in the absence of selection that heterozygosity can never exceed 50% even if one starts with two heterozygotes since it is immediately reduced to 50%. This limitation does not apply to the first meiotic division. Through experimental gynogenesis, one can detect heterozygosity by using heterozygous females and then inhibiting the second meiotic division. In the absence of crossing-over, only homozygous progeny will be produced. Lindsley et al (l956) found crossover frequencies of 0.688, 0.694 and 0.724 for three characters in the axolotl and Davison (1961) found a frequency of 0.78 for the "burnsi" locus in Rana pipiens. The only requirement for this result is that the genes in question be at an appropriate distance from the centromere, which apparently is the case for these four genetic loci. Accordingly, for some genes at least, heterozygosity can substantially exceed that possible through sexual reproduction. A second and enormous source of genetic variability is provided by the fact that it is a random matter which pair of dyads is eliminated into the polar body. For an organism with 10 pairs of chromosomes there are 2 raised to the power of 10 or 1024 possible genetic combinations exclusive of crossing over.

A critique of Darwinian speciation

Since the Darwinians have chosen to ignore the semi-meiotic hypothesis, I would like to introduce what I think their objections might be if they chose to offer them and then comment on the evidence for that perspective. The Darwinian or sexual model has restraints that are necessary for the hypothesis to succeed. Both genetic and chromosomal homozygosity require that the genetic alterations occur in small isolated populations. This is necessary because the probability of two heterozygotes mating would be very unlikely in a large population. No such restraint applies to the semi-meiotic model. The Darwinians might simply say that the sexual model could also produce chromosome and gene homozygosity through the inbreeding associated with small or insular populations. It is here that their hypothesis fails. For example, the biota of the Galapagos Islands is not unique but related to that of neighboring Equador. Darwin's finches are all members of the genus Geospiza. Since they are all remarkably similar, it is not surprising to discover that they produce spontaneous fertile hybrids ( Grant, Grant 1994). Accordingly, by a physiological definition of species, they are one and no significant evolution has really taken place.

One can also ask the direct question; what is the cytological evidence bearing on the question of the origin of chromosomal restructuring as it relates to speciation? Were species produced sexually by inbreeding as the Darwinian view demands, or were they produced by some other means? The Australian cytologist M.J.D.White has some very pertinent comments to make on this very issue. I quote one example from the conclusions section of his book "Animal Cytology and Evolution."

"The conclusion we must draw from these facts (and a great many more of the same kind in beetles, mammals and in fact in almost every group of animals whose chromosomes have been studied) is that, in certain groups at any rate, fusions and dissociations which exist as cytotaxonomic differences between species have not been preceded by a condition of balanced polymorphism in an ancestral population. In other words, the acquisition of most fusions and dissociations (and some other types of chromosomal rearrangements in particular groups) is not a normal and usual part of 'phyletic' evolution, but is somehow directly (and perhaps causally) related to the process of speciation itself" (my italics) (White l973, pg.765).

In short, White is saying that these chromosomal differences associated with speciation have not been produced sexually. I submit that if they were not produced sexually, then there is only one other way they could have been produced and that is semi-meiotically as I have suggested. If there is another way, I am unaware of it.

A hypothetical reconstruction of evolutionary history

The following represents an attempt to explain evolution in terms which recognize the preceding considerations. It is only necessary to accept the reality that contemporary germ cells cannot be ancestral but are secondary in origin. It then follows that there must have been a time in an evolving lineage when organisms possessed two sources for reproduction, the semi-meiotic lineage as I have indicated and the sexual lineage which now largely prevails. It is reasonable under those conditions that both modes of reproduction could occur simultaneously. Why then has the original or semi-meiotic mode disappeared? I will offer two possible explanations. First, while the semi-meiotic method can produce new trial balloons, that could become a disadvantage once a new and successful creature has been produced. Secondly, sexual reproduction has one great advantage in its capacity to produce virtually unlimited variation within a narrow range. The sexual mode then could be very useful in adapting the organism to minor environmental changes. Thus, the sexual lineage might be expected to out-compete the semi-meiotic lineage in a changing environment. However, severe changes might be beyond the capacities of the sexual mode, resulting in extinction. This would seem to be the situation at present when we are so drastically altering the environment and so many species are disappearing.

A second feature of evolution relates to the semi-meiotic mechanism as well. The vast majority of all species that have evolved have become extinct without undergoing substantial further change. Why? I suggest it is in part due to the fact that sexual reproduction is not well suited to the elimination of deleterious mutations which then will tend to accumulate, ultimately leading to the extinction of those forms. Particularly vulnerable to the accumulation of deleterious genes would be animals that produce relatively few offspring and accordingly offer fewer opportunities for natural selection to cull only the genetic defectives. In support of this view, it is interesting to note that giant animals, which typically leave few offspring, have been particularly vulnerable to extinction while many smaller and more prolific creatures have survived. Many living fossils, like the oyster, produce enormous numbers of progeny thus ensuring that some of the offspring will be genetically fit. Obversely, except for crossing over, semi-meiosis is admirably suited to the elimination of deleterious or semilethal genes and gene arrangements since these tend to be exposed as homozygotes and accordingly the semi-meiotic lineage could effectively purge itself of defective genes and gene arrangements (Davison l993).

Conclusion

Physiology is an experimental science. Embryology, genetics, biochemistry and ecology all have experimental aspects. Where is experimental evolution? I submit that in the past there was an enormous interest in experimental evolution .The findings proved to be negative and accordingly were not published. The semi-meiotic hypothesis is eminently testable in suitable material and thus, while evolution may or may not be finished, I am confident that we have the capacity to produce new life forms in the laboratory. I predict that goal will be reached through the experimental inhibition of the second meiotic division in female animals either heterozygous for chromosome rearrangements or bearing oogonia that are. Like all other aspects of scientific inquiry, the truth about evolution must ultimately be revealed at the laboratory bench.

References cited

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Broom R. 1951. Finding the missing link. Watts, London, p.107.

Bull JJ. 1983. Evolution of sex determining mechanisms. Benjamin Cummings, Menlo Park.

Burbank L. 1939. Partner of nature. D.Appleton-Century Co., New York, pp.97-98.

Cleveland LR. 1947.The origin and evolution of meiosis. Science 105,287-289.

Darwin C. 1896. The Origin of species by means of natural selection. D. Appleton, New York.

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