EUGENE M. MCCARTHY, PHD GENETICS
(Continued from the previous page)
An evolutionary theory should account, on a genetic basis, for the successive changes observed in the fossil record. That is, it should explain mechanisms that would allow ancient fossil forms to give rise to those of the present day. As we have seen in the previous section, stabilization theory in many respects provides a superior explanation of this process. But, one fact might seem to limit its applicability: In the very early stages of the fossil record bacteria are the only forms present. Stabilization processes are strongly linked with hybridization. In bacteria ordinary hybridization cannot occur because individuals of separate sexes do not exist. Bacteria reproduce by binary fission, a process in which the parent bacterial cell splits transversely into two daughter cells of equal size. So one might suppose hybridization could have played no role in the first stage of evolution. However, binary fission is not the only genetic mechanism in the bacterial repertoire. Though they lack sexes, bacteria do have a form of sex, a process known as conjugation. During conjugation, one bacterium injects DNA into a second, recipient bacterium. The amount of DNA transferred varies from one conjugation event to another. Conjugation occurs not only between individuals belonging to the same bacterial type, but also between types treated as separate species. Bacteria can therefore produce new offspring types via matings between two or more distinct types.
Conjugation is a form of hybridization, as we have seen. When one bacterium injects its DNA into some other type of bacterium, the recipient individual is altered genetically so that it differs from the parent bacterium that produced it by binary fission. It also differs from the bacterium that injected the DNA, which can be viewed as a second parent via conjugation. Because the resulting hybrid bacterium differs genetically from the types that produced it, it will usually differ also with respect to its traits. If it goes on to reproduce itself through binary fission, a distinct new type differing from both parental types will result. For instance, in a study of naturally occurring hybridization between two bacteria, Bacillus licheniformis and B. subtilis, Duncan et al. (1989: 1606) found that in such hybrids an average of 89 traits out 1,000 were altered from the original condition of the recipient, and that, on average, 56 of these 89 remained stable in the new line descended by binary fission from the initial hybrid individual. Presumably, such hybrid types will survive and stably reproduce if the new combination of traits specified by the new combination of genes is sufficiently favorable. Conjugation between even very distinct types of bacteria is a commonplace, well-documented phenomenon. For example, such exchanges have been repeatedly observed between Escherichia coli and Synechocystis PCC6803, a cyanobacterium.
A new gene arising through mutation in a single bacterium can be passed, then, not only to the descendants of that bacterium produced by binary fission, but also to ones produced via conjugation. Thus, under stabilization theory, binary fission plays the same role in the bacterial realm as does ordinary reproduction among multicellular organisms. It reproduces the parental type. Therefore, under that view, the stability over time of a bacterial type reflects the stability of reproduction inherent in binary fission, just as the temporal stability of a eukaryotic form reflects the stability of a genetically stable reproductive cycle (see discussion in Section 7). But conjugation between distinct types is equivalent to the disruptive processes associated with sexual hybridization. As with sexual hybridization, the offspring of conjugation will be of varying types. Some will be more viable than others. Those that are sufficiently viable to survive and stably reproduce will become new stable types. Among unicellular organisms reproducing by cell division, conjugation between distinct types produces new stable types. It is therefore a form of stabilization process. The word karyotype is not usually used in connection with bacteria. Note, however, that as it is defined under stabilization theory (see Section 3), karyotype can be applied even in the case of these non-eukaryotic microorganisms.
The ability to conjugate is widespread in bacteria today. In fact, it seems no bacterium is known to be incapable of it. Since conjugation has been observed in a wide variety of modern bacteria, it is likely even very early bacteria had this capability. Even in the very earliest fossil-bearing strata of the fossil record, dating back to the early Archaean (~3.5 billion years ago), multiple bacterial types are present. It is reasonable, then, to suppose conjugation between distinct types was a common way of producing new forms of life even at the earliest known stages of the evolutionary process. The early earth can thus be pictured as a bacterial playground, devoid, perhaps, of more complex organisms. This state apparently lasted some two billion years. These simple forms of life have since elaborated into the wide variety of organisms seen today. If conjugation between distinct bacterial types was prevalent even during this very early period, then the production of new types of organismss via this means was probably widespread as well. Such a mechanism could bring about large amounts of subsequent evolutionary change. This seems especially likely given that conjugation also occurs in a broad range of simple eukaryotes.
As has already been explained, hybridization of two types of organisms is not a process analogous to the averaging of two points on a geometric line. Although many traits in a hybrid will be intermediate to those seen in the parental types, other traits will not be (they will be heterotic or synergistic). In consequence, a new hybrid type produced from two interbreeding parental types is not bounded by the traits of the founding pair. Backcrossing, matings among the hybrids themselves, and interbreeding with additional parental types can produce an ever-increasing variety of types. Descendant types can become increasingly distinct. But they would not do so in a treelike fashion. A different topology applies in the case of hybridization and conjugation (see Figure 8.2). Since conjugation combines, in a single bacterium, the DNA of two different types, it is entirely plausible to suppose novel, synergistic traits would arise from time to time as a result of the interaction of genes newly combined in a single individual. In particular, such characteristics as true sexual reproduction, multicellularity, multiple chromosomes, and many other features typical of more complex organisms might well have arisen first as synergistic traits. This is the assumption under stabilization theory.
The theory also assumes (1) that various characteristic traits of eukaryotes evolved separately in different types of early bacteria; and (2) that these traits were later combined in single organisms via the sort of process outlined in Figure 8.2. For example, one can suppose that at some point hybridization among ancient bacteria produced forms with a membrane-bound nucleus. Such a process might also have yielded types with the ability to package genetic material into individual, linear chromosomes. Certain extant microsporan bacteria undergo meiosis during their life cycle, a characteristic usually seen only in eukaryotes. Thus, this trait, too, might have been present in early bacteria. The rudiments of multicellularity are seen in various types of extant bacteria with ancient origins, for example, in cyanobacteria, organisms as old as any in the fossil record. Again, this trait was probably present in some bacteria at a very early stage of evolution. Some types of cyanobacteria are simple unicells, but others are among the most elaborate of bacteria. The most complex are multicellular mosslike forms visible to the naked eye. In both unicellular and multicellular cyanobacteria structural and functional differentiation occurs in which different cells perform specialized functions.
Hybridization within an early bacterial similarity set containing separate forms in which these basic features of eukaryotes were separately present, then, could produce descendant sets, in which single forms combined these traits. As this process of assembling eukaryotic traits continued, presumably symbiogenesis also would play a role. Recall (see Section 4) that certain of the tiny organs ("organelles") of eukaryotic cells (e.g., mitochondria, chloroplasts) are now believed to be the descendants of ancient bacteria engulfed by single-celled precursors of eukaryotes (this is a form of symbiogenesis). With the occurrence of such events, and with further conjugation, larger and increasingly complex forms could make their debuts. Later, various similarity sets composed of relatively complex protoctists with plant, animal, or fungal characteristics would give rise to sets of simple plants, animals, and fungi, respectively. As one similarity set succeeded another over evolutionary time, this process would eventually produce the structurally complex forms characteristic of the more recent stages of evolution.
Note that at no point in this process is it necessary to suppose new forms arise via the gradual accumulation of distinctive traits in isolation. A colleague once claimed that the extensive hybridization posited by stabilization theory would prevent forms of life from increasing in number. This, he said, was the case because any pair of hybridizing populations would merge so that two preexisting forms would become one. An ongoing reduction in the number of existing forms would supposedly result. To make up for this deficit, he said, it would be necessary to assume that the sorts of processes described under neo-Darwinian theory also commonly produced new types of organisms. Otherwise the number of existing forms would steadily dwindle. This, however, is interpreting stabilization theory with a neo-Darwinian eye. In fact, such an argument is little more than a resuscitation of the medieval idea that hybridization must inevitably lead to a blending and confusion of forms (see Section 1). We have seen that people often suppose hybridization causes populations to merge, when in fact observation tells us that hybridizing populations usually remain morphologically and geographically discrete even when an active hybrid zone has long connected them.
Nothing in stabilization theory is inconsistent with the proliferation of new types of organisms. We have just seen how, under the theory, simple forms would have produced a wide array of more complex ones via conjugation and symbiogenesis. With the advent of eukaryotic sexual reproduction all the various stabilization processes discussed in previous sections would also come into play (some, such as the production of an autopolyploid via somatic chromosome multiplication, probably predated sex). These would only accelerate the production of new forms. Thus, with the sorts of processes posited in the theory, there is an unlimited potential to increase the number of stable forms of life. For example, two forms might hybridize to produce a third stable form, say an allopolyploid. Thousands of examples of this are known. This new, third type might go on to hybridize with some fourth form to produce a fifth form via recombinational stabilization. The fifth might produce on its own, a sixth, autopolyploid form. Obviously, such mechanisms can go on creating an ever-increasing number of forms, ad infinitum. So there is no need whatsoever to posit the sorts of gradual processes described in neo-Darwinian theory. NEXT PAGE >>
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