About

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  1. About me, Dr. Adam M. Goldstein.
  2. About the shifting balance theory.
  3. Licensing and permissions for this site.

Dr. Adam M. Goldstein

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Here is how Alex Olanow saw me in January 2006, 'Snice, 8th @ Jane St., Manhattan. The manuscript is my dissertation.

In March 2006, I defended my Ph.D. dissertation at Johns Hopkins, where I was a student in the Department of Philosophy, advised by Karen Neander, Peter Achinstein, and Alexander Rosenberg. In my dissertation, “Random Genetic Drift: Chance and Explanation in Evolutionary Biology,” I argue that chance events occurring due to random genetic drift can be explained, and I describe the strategy that evolutionary biologists use to do so. This is contrary to those who claim that only evolution due to natural selection can be explained, and in general, to those who believe that explanations require laws of nature. I am writing a book, tentatively entitled Chance Caught on the Wing, in which I extend and strengthen the argument of the dissertation.

I also hold a Masters in Library in Information Science, which I obtained from Pratt Institute’s School of Library and Information Science. My informatics research concerns the question, How can key word indexes and related tools such as ontologies improve resource and hypothesis discovery? To explore answers to this question, I am designing an ontology of evolutionary biology which will be used to classify a comprehensive database of bibliographic records of works about evolutionary biology, The Library of Evolution at the Darwin Manuscripts Project. The DMP, of which I am Associate Editor, is a project of the American Museum of Natural History. I am also working on ontological problems associated with information artifacts whose solutions are required to classify the many works we intend to list in the The Library.

I graduated from UC Santa Cruz, majoring in Biology and Philosophy. I am Assistant Professor of Philosophy at Iona College.

Yes, it is true: UCSC’s mascot is the banana slug (genus Ariolimax). We wouldn’t have it any other way.

This site is hosted at geekisp.com and is constructed using the Revolution Code Blue WordPress theme. I maintain and update the site myself. When I am not using the WordPress web-based utilities for editing or creating pages and posts, I use MarsEdit.

I receive no compensation for maintaining this site or from sites I link to. Sharing my work and calling attention to resources about evolution, bibliography, computers, and so on is its own reward and I would be happy if somone found it useful, engaging, or otherwise valuable.

The shifting balance theory

The most general conclusion is that evolution depends upon a certain balance among its factors. There must be gene mutation, but an excessive rate gives an array of freaks . . . ; there must be selection, but too severe a process destroys the field of variability, and thus the basis for further advance; prevalence of local inbreeding within a species has extremely important evolutionary consequences, but too close inbreeding leads merely to extinction. A certain amount of crossbreeding is favorable but not too much. In this dependence on balance the species is like a living organism. At all levels of organization life depends on the maintainance of a certain balance among its factors. (“Mutation, Inbreeding, Crossbreeding, and Selection in Evolution”)

The shifting balance theory was proposed by American geneticist and evolutionist Sewall Wright, one of the founders of population genetics. The theory is intended to explain the conditions under which it is most likely that a biological population increase its mean fitness. Whether or not it is true, is is among the most charming of theories, because it allows for an element of serendipity, and because populations undergoing the shifting balance process have the character of a collective in which differentiation among subgroups is the mechanism by which the whole is improved.

The process requires a large population, subdivided into local isolates among which there is nonetheless a steady interchange of small number organisms. Though spatially and biological distinct, the isolates are in more or less identical environments, so that what is fittest in one isolate will also be fittest in another.

The process has three stages.

  1. In the first stage of the process, the random genetic drift stage, random drift in each isolate gives rise to novel gene combinations which would have only a small chance of arising in the population as a whole. The isolates are each small enough so that genetic drift can exert a strong force within each, but not so small that extensive inbreeding will drive it to extinction. Diversity is maintained in each isolate by the migration of a small number of new individuals from other isolates. This is the element of serendipity I mentioned above.
  2. In some of the isolates, perhaps only one, the favorable novel gene combination spread by natural selection by natural selection. This is the intrademe selection stage, called this because selection occurs within the isolates, raising their mean fitness.
  3. In the final stage, interdeme selection, emigrants from populations in which novel gene combinations have spread carry them to other isolates, each of which incorporates the favorable gene combinations, the entire population reaching a maximum of mean fitness in this manner.
[A rugged fitness landscape]

An adaptive landscape.

The shifting balance theory provides the conceptual and theoretical backdrop to an especially influential and vivid visual metaphor for evolution, “rugged fitness landscapes” or “adaptive landscapes.” The “landscape” is an N-dimensional space in which N - 1 dimensions represent allele frequencies at each of N - 1 gene loci. The Nth dimension is the mean fitness of the population, W. A point in the space represents a possible genetic structure of a population, graded according to its mean fitness. Such a multidimensional space is most easily envisioned as a “landscape” in the highly idealized case of N = 3 dimensions, that is, for N - 1 = 2 gene loci. Each point along the X-axis represents the frequency of genes at one locus; each point along the Z-axis represents the frequency of genes at the other locus. The Y-axis—the “altitude” of the landscape—represents mean fitness W.

Each point in such a three-dimensional space may be described by an ordered triple  <x,y,z>. Each triple describes a possible state of the population, grading each set of allele frequencies according to its fitness. The first and third elements of this triple are the frequencies of alleles at each of the two loci, and the second element of the triple is the mean fitness of a population with those allele frequencies. Projected into three dimensions, a point described by a fitness-element that is greater than that of neighboring points is a fitness “peak,” while a point described by a fitness-element that is less than that of neighboring points is a fitness “valley.”

An adaptive landscape with three dimensions might have only one or two peaks. However, Wright suggests that adaptive landscapes with the number of dimensions required to represent an animal species in nature are quite “rugged” in the sense that they have many peaks and valleys. Selection, which, as a general rule, increases the mean fitness of a population, will cause the population to ascend to a local maximum: “in a rugged field of this character, selection will easily carry the species to the nearest peak.” However, the local maximum may not be a global maximum. “[T]here may be innumerable other peaks which are higher but which are separated by “valleys’.” This informs Wright’s provocative suggestion that “the problem of evolution is that of a mechanism by which the species may continually find its way from lower to higher peaks in such a field” (163 – 164).

The surfaces of selective value provide a powerful visual tool for understanding the shifting balance process. Representing a natural population on an adaptive landscape would require a large number of dimensions N - 1, corresponding to the large number of gene loci in the population. As I suggest above, Wright claims that such an adaptive landscape would have many peaks and valleys. Represented on such a surface, the shifting balance process would look as follows.

At the start of the process, the population might occupy a peak other than the tallest on the landscape, i.e., a local maximum of mean fitness that is not a global maximum; or, it might occupy a saddle between local maxima, or a valley beneath such local maxima. Because of the population’s proximity to local maxima and its separation by valleys from the global maximum, the latter is inaccessible by selection, which will take the population to the top of the local peak, causing it to remain there. Drift, in contrast, can carry a population across a valley to the slopes of a global peak, as drift is not constrained to increase the mean fitness of the population, as natural selection is.

This is what occurs in the first stage of the shifting balance process: organisms with a novel favorable gene combination created by drift in one of the isolates will break away from the rest of the population, moving to the slopes of a global maximum. If these organisms spread by natural selection in their isolate, they will cause that isolate to climb the global peak; this is intrademe selection. Next, suppose that migrants spread the favorable gene combination to other isolates. Then the other isolates will ascend the global peak as the favorable gene combination spreads throughout each. This is interdeme selection, the third stage of the process. Thus, the entire population can be made to ascend the global peak. In this manner, as Wright suggests, the shifting balance process provides “a trial and error mechanism by which in time the species may work its way to the highest peaks in the general field” (167)—trial and error, because random changes in the population due to drift provide the means for escaping local maxima and attaining the global.

References

All page references above are to Sewall Wright’s paper entitled “The Roles of Mutation, Inbreeding, Crossbreeding and Selection in Evolution, reprinted in the 1986 University of Chicago volume edited by William Provine, Evolution.

The section on rugged fitness landscapes is excerpted from my PhD dissertation, which can be found elsewhere on this site.

At the start of the process, the population might occupy a peak other than the tallest on the landscape, i.e., a local maximum of mean fitness that is not a global maximum; or, it might occupy a saddle between local maxima, or a valley beneath such local maxima. Because of the population’s proximity to local maxima and its separation by valleys from the global maximum, the latter is inaccessible by selection, which will take the population to the top of the local peak, causing it to remain there. Drift, in contrast, can carry a population across a valley to the slopes of a global peak, as drift is not constrained to increase the mean fitness of the population, as natural selection is.

This is what occurs in the first stage of the shifting balance process: organisms with a novel favorable gene combination created by drift in one of the isolates will break away from the rest of the population, moving to the slopes of a global maximum. If these organisms spread by natural selection in their isolate, they will cause that isolate to climb the global peak; this is intrademe selection. Next, suppose that migrants spread the favorable gene combination to other isolates. Then the other isolates will ascend the global peak as the favorable gene combination spreads throughout each. This is interdeme selection, the third stage of the process. Thus, the entire population can be made to ascend the global peak. In this manner, as Wright suggests, the shifting balance process provides “a trial and error mechanism by which in time the species may work its way to the highest peaks in the general field” (167)—trial and error, because random changes in the population due to drift provide the means for escaping local maxima and attaining the global.

References

All page references above are to Sewall Wright’s paper entitled “The Roles of Mutation, Inbreeding, Crossbreeding and Selection in Evolution, reprinted in the 1986 University of Chicago volume edited by William Provine, Evolution.

The section on rugged fitness landscapes is excerpted from my PhD dissertation, which can be found elsewhere on this site.

Licensing and permissions

All site content created by Adam M. Goldstein is copyright 2009-2010, Adam M. Goldstein. Permission is hereby granted for royalty-free use of any such content so long as (1) text copied verbatim is credited to Adam M. Goldstein, and annotated with the date of access, date of creation, and the URL at which the original text can be found, and the notice “Copyright 2009-2010 Adam M. Goldstein.” (2) All works derived from content created by Adam M. Goldstein are attributed according to the criteria described in (1) above, with the additional annotation, prominently displayed, that the text has been modified and that Adam M. Goldstein is not responsible for it. (3) No fee beyond the cost of distributing any content created by Adam M. Goldstein is be charged for its use, no registration is required to access it, and it remains available, if in digital form, to any Internet user with a web browser or similar tool such as ftp, etc. (4) This paragraph is placed at some location in the derived work which is prominent (though consistent with the derived work’s visual layout and design).

I can be contacted at z_calinfornianus <at> shiftingbalance.org. Replace “<at> with “@” in your mail client’s “To:” field. You will  receive an email from the spam filter asking you to confirm your email address—this helps keep robots from harvesting my address. I apologize for any inconvenience this may cause.