. . . scroll down for more about the shifting balance process and adaptive landscapes!
I am an historian and philosopher of science, librarian, and ontological engineer. My B.A. degree is from The University of California at Santa Cruz, where I majored (double) in philosophy and biology. Drs. David Hoy, Leo Laporte, Todd Newberry, and Emanuel Abraham made especially lasting and important contributions to my education; among teaching assistants, Christina Diaz and Christoph Cox were especially influential.
I attended The Johns Hopkins University, completing my PhD in Philosophy in 2006. I wrote my dissertation about chance and explanation in evolutionary biology. My work today extends my dissertation. Karen Neander, Peter Achinstein, and Steven Stanley advised my thesis work. I am grateful for their contribution. I continued my formal education at Pratt Institute, in Manhattan, from which I obtained a Master’s of Library and Information Science. I focused my efforts on learning about the organization of information. More information about my research can be found by browsing this site.
At present, my scholarly work is informed by the confluence of the historical-philosophical questions raised in my dissertation, and my interests in the organization of information. Much of my research has been in one way or another connected with the Darwin Manuscripts Project, of which I am associate editor. I am associate editor and reviews editor for Evolution: Education and Outreach. I consider this to be one of the most important contributions to knowledge and learning I have made.
I have developed a pedagogy constructed around student work in groups, exchange among them, and active work in class. I enjoy teaching a great deal. I feel privileged to have had the close attention my students, which I hope has served them well.
The shifting balance process
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.
- 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.
- 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.
- 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.
Despite the importance of chance in this process, it is a theory about adaptation. Stephen Jay Gould points this out in his paper “The Hardening of the Synthesis.” Like R. A. Fisher at the time, Wright was interested in the conditions under which a population as a whole would increase in fitness. Fisher claimed that mean fitness is most likely to increase in a large unstructured population in which there is random mating throughout. The dispute between these two animated the discussion of evolutionary biology in the 1930′s to the 1970′s, and continues to do so today. Wright formulated his idea of the landscape of selective values in order to explain the shifting balance process in terms he believed to be more intuitively clear terms.
There are good reasons to think that the shifting balance process explains the evolution of some populations. It does not appear to be a general theory of evolution, which probably does not exist. One way in which the theory is superior to others is its charm. The notion that the play of chance can re-work the genetics of a population from within to improve it responds to the sense that a species is a dynamic being that can improve itself by its own will. The shifting balance process parallels ideas of the Romantics and German Idealists about mechanisms of spontaneous self-improvement.
This is largely cribbed from my dissertation.
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 in this post are from Wright’s essay, “Mutation, Inbreeding, Crossbreeding, and Selection in Evolution,” collected in Evolution, edited by William Provine, Chicago, 1986.