Tuesday, February 17, 2009

The Modern Synthesis

 
Most people do not understand current ideas about evolution. The following is a brief summary of the Modern Synthesis of Genetics and Evolution as put forth by evolutionary biologists in the late 1940s.

The idea that life on Earth has evolved was widely discussed in Europe in the late 1700s and the early part of the 1800s. In 1859 Charles Darwin supplied a mechanism—namely natural selection—that could explain how evolution occurred. Darwin's theory of natural selection helped to convince most people that life has evolved and this point has not been seriously challenged in the past one hundred and fifty years.

It is important to note that Darwin's book The Origin of Species by Means of Natural Selection did two things. It summarized all of the evidence in favor of the idea that organisms have descended with modification from a common ancestor. Darwin built a strong case for evolution. In addition, Darwin advocated natural selection as a mechanism of evolution.

Biologists no longer question whether evolution has occurred or is occurring. That part of Darwin's book is now considered to be so overwhelmingly demonstrated that is is often referred to as the FACT of evolution. However, the MECHANISM of evolution is still debated [Evolution Is a Fact and a Theory].

During the first part of this century the incorporation of genetics and population genetics into studies of evolution led to a Neo-Darwinian theory of evolution that recognized the importance of mutation and variation within a population. Natural selection then became a process that altered the frequency of genes in a population and this came to be the minimal definition evolution [What Is Evolution?].

The earliest version of this essay appears on the TalkOrigins Archive.

A later version is at Evolution by Accident.
This point of view held sway for many decades but by the 1940s the classic Neo-Darwinian view was replaced by a new concept that brought together field biology, paleontology, and population genetics. The new version took pains to exclude all mechanisms except natural selection and random genetic drift. This new version was called The Modern Synthesis after the title of a 1942 book by Julian Huxley.

We have learned much since Darwin's time and it is no longer appropriate to claim that natural selection is the only mechanism of evolution. I can understand why this point may not be appreciated by the average non-scientist because natural selection is easy to understand at a superficial level. It has been widely promoted in the popular press and the image of "survival of the fittest" is too powerful and too convenient.

One of the goals of the Modern Synthesis was to reach consensus on the importance of macroevolution. The founders of the Modern Synthesis insisted that macroevolution could be explained by microevolution and no additional mechanisms—such as the bogeyman of saltation—were required.

Ernst Mayr, one of the original founders of the Modern Synthesis, sums it up this way ...
The term "evolutionary synthesis" was introduced by Julian Huxley in Evolution: The Modern Synthesis (1942) to designate the general acceptance of two conclusions: gradual evolution can be explained in terms of small genetic changes ("mutations") and recombination, and the ordering of the genetic variation by natural selection; and the observed evolutionary phenomena, particularly macroevolutonary processes and speciation, can be explained in a manner that is consistent with the known genetic mechanisms.

Ernst Mayr (1980) "Some Thoughts on the History
of the Evolutionary Synthesis" in The Evolutionary Synthesis,
E. Mayr & W.B. Provine eds. Harvard University Press.
The original version of the Modern Synthesis included mechanisms other than natural selection, especially random genetic drift. Later on, there was a hardening of the synthesis so that natural selection became the predominant mechanism and drift was relegated to a bit part (see Mayr quotation, above). The original version is described by Douglas Futuyma as ....
The major tenets of the evolutionary synthesis, then, were that populations contain genetic variation that arises by random (ie. not adaptively directed) mutation and recombination; that populations evolve by changes in gene frequency brought about by random genetic drift, gene flow, and especially natural selection; that most adaptive genetic variants have individually slight phenotypic effects so that phenotypic changes are gradual (although some alleles with discrete effects may be advantageous, as in certain color polymorphisms); that diversification comes about by speciation, which normally entails the gradual evolution of reproductive isolation among populations; and that these processes, continued for sufficiently long, give rise to changes of such great magnitude as to warrant the designation of higher taxonomic levels (genera, families, and so forth).

Futuyma, D.J. in Evolutionary Biology,
Sinauer Associates, 1986; p.12
This description would be incomprehensible to Darwin since he was unaware of genes and genetic drift. The Modern Synthesis differed from Darwinism in four important ways:
  1. It defined evolution as a change in the frequency of alleles in a population; an idea based on population genetics.

  2. In addition to natural selection, it recognized random genetic drift as an important mechanism of evolution.

  3. It recognized that characteristics are inherited as discrete entities called genes. Variation within a population is due to the presence of multiple alleles of a gene. Variation is caused by mutation.

  4. It postulated that speciation is (usually) due to the gradual accumulation of small genetic changes. This is equivalent to saying that macroevolution is simply a lot of microevolution.
The Modern Synthesis was a theory about how evolution worked at the level of genes, phenotypes, and populations whereas Darwinism was concerned mainly with organisms, speciation and individuals. This was a major shift in emphasis and those who fail to appreciate it find themselves out of step with the thinking of evolutionary biologists.

The major controversies among evolutionary biologists today concern the validity of points #2 and #4 (above).

Following the centennial celebrations of the publication of Origin in 1959, there was a gradual hardening of the Modern Synthesis. The 1960s version concentrated almost exclusively on natural selection as a mechanism and random genetic drift was pretty much ignored. Today, there is debate about the relative importance of these two mechanisms and some are calling for an updating of the "hardened" Modern Synthesis.

This update would restore random genetic drift as an important mechanism, recognize neutral theory, and incorporate molecular phylogeny (and the molecular clock).

There are many who believe that the fossil record does not show gradual change but instead long periods of stasis followed by rapid speciation. This model is referred to as Punctuated Equilibrium and it is widely accepted as true, at least in some cases. The debate is over the relative contributions of gradual versus punctuated change, the average size of the punctuations, and the mechanism.

The Modern Synthesis is challenged over the emphasis on gradualism and over the claim that microevolution is sufficient to explain macroevolution. Some evolutionary biologists suggest that evolutionary theory be modified to incorporate mechanisms that occur at levels higher than the population (e.g. species sorting). These scientists advocate an extension called hierarchical theory.

There are other challenges to the Modern Synthesis. Some of them are valid and some of them are silly. But I think it's fair to say that the 50-year old version needs some serious updating to incorporate some of the new concepts.

Some scientists continue to refer to modern evolutionary theory as Neo-Darwinian. In some cases these scientists do not understand that the field has changed but in other cases they are referring to what I have called the Modern Synthesis, only they have retained an old name from the early 1900s.


20 comments :

  1. I've been trying to get a grip on this controversy, from a layman's point of view. I've noticed that your postings about adaptationists are generally complaints that they've ignored genetic drift, rather than attacks on their arguments against genetic drift. Do they ever make such arguments? If so, could you point me to where they've been made and refuted? To an outsider, following this topic mostly via your blog, it seems to be a one-sided conversation.

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  2. I am beginning to wonder if the tussle between the relative importance of natural selection and genetic drift is a fool's errand.

    Selection cannot act without the random mutations generated by drift and drift cannot produce organism-level phenotypic change without the consolidating effect of selection.

    Thus the disagreement is only on the emphasis: either natural selection is the creative power of evolution where mutations only act as a passive substrate, or genetic drift is the creative power of evolution where purifying selection only act passively to preserve key mutations.

    In order to tease apart the relative contributions of these two intertwined mechanisms on the biosphere in general, one would need to perform innumerable experiments on countless model organisms. No doubt the evolution of some features might be chiefly due to selection and others due to drift, but the vast majority of the changes will be somewhere in between.

    Even if all those experiments can be completed and one can assert with confidence that generally evolution is due to 80% drift + 20% selection, or 20% drift + 80% selection (or even 80% developmental structuralism + 10% drift + 10% selection), I don't understand how this data would advance evolutionary theory.

    As you have emphasized, Darwin's key contribution is the postulation of common descent, which has been strongly supported by evidence through the years and is no longer in dispute.

    In my view, debates over the mechanism of evolution have greater impact if they enlighten us to make more accurate testable predictions about the direction of organism-level phenotypic change.

    Unfortunately, selection acts via external contingencies of a multifactorial environment on a multifunctional organism, while drift acts via internal contingencies of unpredictable mutations on multifunctional, redundant genes and regulatory regions.

    If we can't do better than "Well, anything could happen!" then the debate over the relative importance of various mechanisms of evolution is really just getting mired in the noise.

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  3. 3. It recognized that characteristics are inherited as discrete entities called genes. Variation within a population is due to the presence of multiple alleles of a gene. Variation is caused by mutation.

    The truth of this one is also become less and less self-evident with time. What do you call the Let-7 miRNA in this context?

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  4. Selection cannot act without the random mutations generated by drift and drift cannot produce organism-level phenotypic change without the consolidating effect of selection.

    I don't think this is a correct interpretation of random genetic drift. Drift doesn't require selection to effect phenotypic change.

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  5. "I don't think this is a correct interpretation of random genetic drift. Drift doesn't require selection to effect phenotypic change."

    For drift to effect organism-level phenotypic change without purifying selection, you would have to invoke saltationism.

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  6. A nice summary, Larry. I’ll note that you seem to be using, in part of your discussion, a definition of “evolution” that differs from the “any change in the genome” molecular-level definition you normally argue from. (Otherwise, mere “stasis” in the appearance of fossils is no evidence at all that the pace of evolution varied; and on the flip side, an acceptance that there was varying rates of evolution at different times would be a serious challenge the view that most evolution occurs through neutral drift). But the context made it clear you were referring to evolutionary change as defined at the gross morphological level, so I could grasp the concepts you were referring to. (Similarly, when you referred to Darwin’s work as establishing the fact of evolution: clearly Darwin had nothing to say about random genetic drift).

    When I read your posts on evolution I find that I understand what you mean by assuming by default that you are referring to any change at the molecular level, and switch to a different meaning when your context (or an explicit indicator) tells me you mean something different. It’s kind of the opposite when I read something by Dawkins: there I assume he's referring to change at the phenotypic level, and I change if he gives me some indication he going molecular on me. I suspect you would react differently to many of Dawkins pieces if you read him the same way. It’s a shame that your only real reaction to his excellent article on Coyne’s book was to pounce on the reference to chance in the very last paragraph; a disagreement that existed mostly because you uncharitably imposed a definition on the term he used, knowing that that was not what he meant.

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  7. Nice article, Larry. It was fun to read about the history of modern evolutionary theory.


    @Lim Leng Hiong

    In my view, debates over the mechanism of evolution have greater impact if they enlighten us to make more accurate testable predictions about the direction of organism-level phenotypic change.

    Genetic drift and natural selection are differently affected by population size, population structure and gene flow. This is becoming a hot topic, because humans are causing deterioration and fragmentation of natural habitats all over the world. If you want to make long-term predictions on the fate of threatened species, you have to know about the relative importance of these evolutionary mechanisms.

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  8. Lim Leng Hiong says,

    Selection cannot act without the random mutations generated by drift and drift cannot produce organism-level phenotypic change without the consolidating effect of selection.

    In a single sentence you display the kind of ignorance that I'm trying to correct.

    Let's make two things perfectly clear. Try to remember them.

    1. Mutations are not produced by random genetic drift. Random genetic drift is a mechanism that fixes mutations in a population.

    2. Visible variation—which is probably what you mean by "phenotypic change"—may have no effect on the fitness on an organism. Such phenotypic variations become fixed in a population by random genetic drift.

    Even if all those experiments can be completed and one can assert with confidence that generally evolution is due to 80% drift + 20% selection, or 20% drift + 80% selection (or even 80% developmental structuralism + 10% drift + 10% selection), I don't understand how this data would advance evolutionary theory.

    Unfortunately, there are many people who act as though everything is due to natural selection. For example, you often see people make the claim that all visible phenotypes are subject to selection and random genetic drift only applies to the molecular level.

    Their understanding of evolutionary theory would be greatly enhanced if they learned that their view is incorrect.

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  9. anonymous asks,

    What do you call the Let-7 miRNA in this context?

    I call it the product of a gene. What do you call it?

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  10. divalent says,

    I’ll note that you seem to be using, in part of your discussion, a definition of “evolution” that differs from the “any change in the genome” molecular-level definition you normally argue from.

    No, this is incorrect. When I uses the word evolution I'm almost always referring to the underlying process that involves a change in the heritable characteristics of a population over time. That's the minimal definition of evolution.

    In some cases, this process is inferred by examining the fossil record. In some cases, I will discuss higher level processes, like species sorting or mass extinctions, but I always try to make this clear in my writing.

    But you are missing the important point. When I use the word "evolution" I am not making a commitment to a mechanism of change. The observed change could be due to selection, drift, or something else entirely.

    It’s a shame that your only real reaction to his excellent article on Coyne’s book was to pounce on the reference to chance in the very last paragraph; a disagreement that existed mostly because you uncharitably imposed a definition on the term he used, knowing that that was not what he meant.

    I did, indeed, know what he meant. He meant that "evolution" and "natural selection" are synonyms. In most cases when Richard Dawkins refers to "evolution" he is not to using it describe heritable change in a population without regard to the actual mechanism of change.

    Instead, he specifically means the kind of change that's due to natural selection. There are many problems with that usage, not the least of which is that he ignores the fact that some of the changes he's referring to might actually be due to drift and not selection.

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  11. A question; since you have mentioned random genetic drift so many times here, could you point us towards what you think is a good reference for the intelligent layman that expounds on this or includes a discussion about this? If you think there isn't any, then maybe you can write one yourself!

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  12. "In a single sentence you display the kind of ignorance that I'm trying to correct."

    My mistake.

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  13. Grateful thanks to Anonymous for getting me to re-read Dr. Moran's article on random genetic drift.

    I am probably being thick-headed as usual, but there is a paragraph in the article that escapes my understanding despite rereading, and I'd welcome any assistance. Here it is:

    The effect of population size is not important when you only consider overall change and not the effect on a single allele. This is because larger populations have more mutations and the probability of fixation by random genetic drift is identical to the rate of mutation for most alleles. Thus, large and small populations evolve at the same rate by random genetic drift.

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  14. Jud,

    The point Larry was making is not really that complicated. The probability that a new neutral mutation (initially present in just one individual of a popululation) becomes fixed is proportional to 1/N, where N is the population size. However, the rate of appearance of new mutations in the population is proportional to N.

    So, for a given population, the rate at which mutations get fixed is proportional to (rate of mutations) x (probability of fixation), or kN x 1/N = kN/N = k. In other words, it's independent of N.

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  15. Divalent,

    Your explanation is correct, but Larry's statement that

    ...large and small populations evolve at the same rate by random genetic drift

    is not. Mutation is not the same as genetic drift, but a distinct evolutionary mechanism. Large populations do not evolve by genetic drift.

    Please correct me if I'm wrong :-)

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  16. divalent explained exactly the part of the paragraph that was giving me problems, because I don't have the population-genetic background to know the equations involved. Thanks.

    corneel, I think the thrust of the paragraph is that random genetic drift contributes the same amount to the overall evolution of large vs. small populations, rather than the perhaps intuitive but incorrect conclusion that a greater rate of evolution-by-random-genetic-drift would take place in smaller populations.

    If you read the paragraph carefully (focus on the idea of overall rate of all changes that are fixed - same in small and large populations - vs. the degree to which any single characteristic might become fixed - greater in small populations) and look at the math divalent has helpfully provided, it should make sense. (At least I hope so, because I think I may understand it now and if I'm wrong I'd hate to have to go back and parse it again. :-)

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  17. Isn't this yesterday's battle?

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  18. @ Jud

    Ah yes, you are right. Larry was probably referring to the rate at which neutral mutations get fixed.

    It all makes sense now :-)

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  19. The probability that a new neutral mutation (initially present in just one individual of a popululation) becomes fixed is proportional to 1/N, where N is the population size. However, the rate of appearance of new mutations in the population is proportional to N.

    So, for a given population, the rate at which mutations get fixed is proportional to (rate of mutations) x (probability of fixation), or kN x 1/N = kN/N = k. In other words, it's independent of N.

    You can't get a rate by multiplying a probability times a rate.

    The number of mutations generated = kN. The number that get fixed is 1/N, so the number of mutations that get fixed (eventually) in the population = k. That doesn't say anything about the rate at which these mutations become fixed.

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