The Synthetic Assumption

I made the comment that there was still some discussion about whether the known mechanisms of mutation plus natural selection were enough to account for the abundance of life forms and the course of evolutionary history as detailed by the fossil record. The crux of this question is really contained in the Synthetic Assumption, which assumes that the same mechanisms found in microevolution (within species) also applies between species, i.e. macroevolution.

To support my earlier statement, I turn to the book Macroevolution – Pattern and Process by Steven M. Stanley (1979, 1998). In this paperback edition of the book, he added a new introduction, but other than that, the rest of the volume was more or less untouched from his earlier work of 1979. Indeed, another decade has passed and the question as to whether this still reflects the consensus is one that I can’t answer without further research of more recently published material.

I added the highlight pointing out that this evolutionist admits that there is no certain answer to the paradox of the modern synthesis. This is still far from answered. I think this should settle the debate about the idea that there has been some discussion in the field about the synthetic assumption. Indeed, you may make the assumption, but that does not resolve the question unless you can support the assumption with experimental results, i.e. show the actual generation of a new family. See the page 89 quote (”yet no such change has ever been recorded”).

I will also note that this author, who is a scientific researcher in the field, has no problem using the terms microevolution, macroevolution, and evolutionist. These terms are used routinely by those in the field and are no way regarded as denigrating.

In the final quoted material, Stanley attempts to harmonize the jumps in the fossil record with an apparent violation of the synthetic assumption. He may be right about his opinion on this matter but until we have some evidence to support it, it will continue to be no more than his educated guess.

Quotes:

Pg. xiv (Introduction 1998):
THE GREAT PARADOX OF THE MODERN SYNTHESIS
Ingrained in the Modern Synthesis of evolution, but never widely addressed or even acknowledged, was a remarkable paradox. In the laboratory, geneticists were orchestrating the evolution of fruit flies, an investigation that entailed selection coefficients so large that they would have transformed any radically different taxon in 104 to 105 generations. Paleontologists, on the other hand, had data in hand showing that fossils similar enough to be assigned to a single species encompassed very little change over millions of years; in fact, George Gaylord Simpson himself estimated that an average species of animals had survived for about 5 million years! Strangely, neither geneticists nor paleontologists blinked an eye when they encountered the other group’s data. No one attempted to reconcile the incompatible rates. The emergence of the punctuational model awakened researchers to the reality of evolutionary stability and led to research showing that this phenomenon is far more common than almost all modern evolutionists — paleontological or neontological — had previously envisioned.

[The author goes on to describe comprehensive studies revealing a prevalence of stasis, using a project to assess rates of evolution in strictly morphological terms.]

Pg. xxvii (Introduction 1998):
WHY ARE SPECIES SO STABLE
Why have so many populous, well-established species changed so little in the course of millions of years, and why has so much change occurred rapidly (presumably in small, localized populations)? These are complementary questions… At present, we have no certain answer, but part of the explanation must lie in the complexity of living organisms. A species is an incredibly intricate, self-regulating, self-replicating entity. Only very rare genetic accidents confer substantial phenotypic changes that are in some way useful to such an entity without disrupting another aspect of the development of its coadapted system. It seems likely that in small populations that occupy unusual habitats wherein competition and predation are relaxed, natural selection sometimes has the opportunity to eliminate potentially deleterious side effects that arise when an otherwise beneficial new trait evolves. Such settings must be the sites of fixation of distinctive traits that would have next to no chance of spreading throughout a very large population.

…What seems evident is that there is not generally an adequate supply of continuous variation to allow unidirectional natural selection to modify traits persistently. If such lability were normally present (if pleitropy and morphogenetic entanglements created no barriers), then changing biotic environments, as well as opportunities for improved adaptation without environmental change, would have produced much more conspicuous phyletic evolution than we observe in the fossil record.

Pg. 2 (Introduction):
There has, however, been some criticism of paleontology for its limited contribution to evolutionary theory (Kitts, 1974, Hecht, 1974). The basic role of the fossil record in Darwin’s general paradigm was simply to provide evidence of large-scale biotic turnover and long-term increase in the complexity and variety of life. More than a century transpired following Darwin’s contribution with little expansion of this role. While the broad outlines of the history of life fell into place, paleontologists did little to elucidate the underlying mechanisms and processes of evolutionary change. Even so, there have been highlights in the progress of paleontology within its traditional bounds, perhaps the brightest of these being the publication of G. G. Simpson’s Tempo and Mode in Evolution (1944) and, to a lesser extent because of overlap, its sequel, The Major Features of Evolution (1953). … In the first book, Simpson adopted the idea of Goldschmidt (1940) that evolutionary research could be divided into the study of microevolution, or changes within species, and the study of macroevolution, or evolution above the species level. Goldschmidt believed that a natural discontinuity actually exists within the evolution of life–that species and higher taxa arise only through sudden chromosomal changes, while the conventional process of natural selection acts upon genes to produce only lesser modifications within species.

Pg. 37 (summary of The Fabric of Evolution):
An important issue of evolutionary biology is whether most net evolutionary change in the history of life occurs in association with the branching of lineages (speciation) or by the gradual transformation of well-established species (phyletic evolution). The first alternative represents the punctuational model of evolution and the second, the gradualistic model. In subsequent chapters, evidence will be presented that the speciational model is valid. Phyletic evolution accounts for enough change in many lineages that intergrading chronospecies are recognized, but it will be argued, such evolution seldom produces major morphologic transitions.

Pg. 88
The invertebrate [fossil] record provides abundant evidence opposing the idea that gaps hide major phyletic trends. As Gould and Eldredge (1977) have put it, “stasis is data.” Omission of this message from most discussions of the gradualistic and puntuational views would seem to represent the forest not being seen for the trees. Attention has been paid largely to lineages that seem to exhibit measurable phyletic evolution, but to these must be added the much larger number of lineages that display so little phyletic evolution that none has yet been pointed out: most of the lineages are assigned to a single species.

Pg. 89
Myriads of invertebrate lineages have been traced through time by sequential sampling in the determination of stratigraphic ranges for species. … Any one of the thousands of statigraphic ranges of species that have been delimited could have been found to record suffient evolutionary change within 5 or 10 My (million years) to display the transition from one family to another, yet no such change has ever been recorded….

The examination of any well-fossilized invertebrate group will show that many family level transitions have occurred during intervals in the order of 50 My. Thus, documented rates of large-scale evolution are so high that, for phyletic evolution to have played a major role in large scale transformation, phyletic transitions from genus to genus within about 5 My would have to be commonplace in phylogeny.

In fact, only rarely has a lineage been found to yield what is considered to be a new genus. On the contrary, an average species of marine echinoids, bivalves, gastropods, or brachiopods has survived for at least 5 My without even evolving enough to be regarded as a new species. (Durham, 1969; Stanley 1975a).

Pg. 176
THE CONTINUITY OF EVOLUTION
Having argued that the unusual new features are normally fixed rapidly in association with phylogenetic branching, I feel compelled to address the question of continuity of both rate and process in evolution. To some workers, punctuational schemes seem inherently to imply a belief in discontinuity of rate and process, threatening the very foundations of the Modern Synthesis. Bock (1970, p 705) has seen as the the “fundamental cornerstone” of the Modern Synthesis what he has called the synthetic assumption, “that macroevolutionary changes can be explained completely by the known microevolutionary mechanisms and that no additional or special macroevolutionary mechanisms exist. Microevolution and macroevolution constitute a continuum of change.” Is this continuum challenged by the punctuational scheme advocated here?

The question of continuity is strongly linked to scale of observation. In the distant perspective of geologic time, phylogeny in the punctuational model takes on a disjointed appearance. This picture is misleading, however, in that as we magnify our view, almost any mode of transition, except special creation or the extreme hopeful monster concept, entails a continuum of descent–a graded series of generations from ancestral taxon to descendant taxon. For any rapid change of the sort proposed in this chapter, a minimum of three generations is required: A transition from parent to aberrant offspring to a generation in which the new feature is fixed by inbreeding. Usually, more than three generations and more than one genetic change will be required for quantum speciation.

Thus, I see the punctuational model as differing from the gradualistic Modern Synthesis largely in emphasis. It is convenient to divide this difference into two components: (1) that relating to the production of phenotypic variability, and (2) that relating to the factors that guide evolution by acting upon this variability.

In the production of variability, point and chromosomal mutations may be quite discrete features, yet point mutations must play a role in quantum speciation. Especially important here must be mutations of regulatory genes, but certainly also, to some degree, mutations of structural genes. If we look in the opposite direction, synthetic theorists were forced, by the very evidence of its occurrence, to accommodate chromosomal transformation, though they did so with little emphasis. (Among other things, it has long been accepted that new species of plants often arise in the form of single, polyploid individuals.) Thus, both chromosomal and point mutations are accepted in each model of evolution. The punctuational model simply stresses “high amplitude” sources of variability–ones, like changes in gene regulation, by which relatively pronounced morphologic modification issues from few genetic alterations. The Modern Synthesis laid greater importance to “low-amplitude” point mutations and sexual recombination, from which major transformations were thought to have been wrought gradually, over many generations, by the selective accumulation of infinitesimal steps.

Leave a Reply

You must be logged in to post a comment.