I. Brief Thoughts
This book posits the "Zero-Force Evolutionary Law", or ZFEL (which the authors suggest pronouncing like "zeffle"). The book is not long, but it is dense, and the authors are what an old professor of mine would call "good philosophers" -- they tell you what they are doing as they do it. The lines of reasoning are easy to follow, and examples, objections, and distinctions are offered in due course to make sure readers keep the discussion well sorted in their minds. The ZFEL, briefly put, is a hierachical and probabilistic explanation for the widespread observation that organisms tend to diversify and become more complex over time. The authors argue the ZFEL is the best null hypothesis in biological situations, and they point to a change in how we formulate biological explanation. They discuss both relevant empirical and philosophical points in a wide-ranging but deceptively simple book.
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II. Further Comment
The ZFEL law is argued to be the fundamental background condition in any biological situation. Considering, as Dobzhansky put it, that nothing in biology makes sense except in the light of evolution, it will come as no surprise that "biology's first law" is an evolutionary law. It may come as a somewhat greater surprise that the ZFEL is distinct from natural selection. The ZFEL, though, is posited in a strongly neo-Darwinian context, and the authors argue strenuously that the ZFEL complements the principle of natural selection.
Here is how the authors articulate the ZFEL:
"In any evolutionary system in which there is variation and heredity, there is a tendency for diversity and complexity to increase, one that is always present but may be opposed or augmented by natural selection, other forces, or constraints acting on diversity and complexity."
The simplest example the authors use is the accumulation of variation on the pickets of a fence. As paint flakes off, lichen appears, etc., the variation is hereditary (retained through time), and so as time progresses the pickets become increasingly different. Importantly, We can say either that the population of pickets becomes more *diverse* or that the fence becomes more *complex*. Typically, we speak of complexity when examining "down" to a smaller scale, such as cellular machinery, whereas we speak of diversity when looking "up" to a wider scale, such as an ecosystem. Both, however, refer to the number of constituents of some entity -- both are hierarchical. And anyone who's ever seen (or made) a phylogeny should recognize that evolution is manifestly hierarchical.
Thus the authors point to evidence for the ZFEL, arguing "[t]he ZFEL is supported by an enormous amount of evidence, at every temporal and physical scale, at every level of organization, across biology." They specify that "[t]wo of the most compelling pieces of evidence" are "the increase in phenotypic diversity over the history of life and the rise in genomic complexity marked by the divergence of pseudogenes." Thus the ZFEL is a formalization of widespread patterns we are all familiar with. The authors argue that the pattern of increasing diversity/complexity is so pervasive that it constitutes the background condition, the null hypothesis of biology.
It is helpful to contrast the zero-force expectation of the ZFEL against the Newtonian zero-force expectation. The nature of inertia dictates that an unperturbed object maintains its trajectory -- that, absent some additional force, an entity at rest remains the same. The ZFEL says that we should expect an entity to be different (or differentiated) at some later time, as a null expectation, in the absence of any intervening forces.
It is also helpful to contrast the ZFEL against a pair of well regarded biological mechanisms: diversifying selection and drift. Both are already recognized as agents that increase diversity/complexity in the biosphere. The ZFEL subsumes these mechanisms with an important hierachical and probabilistic notion of respective randomness. Here is how the authors explain their position:
"Consider this. Take a snapshot of all of the people on a crowded city street corner at some moment in the middle of the day. Then find these same people 10 minutes later. Find them again 20 minutes later, and then 30 minutes later. With the passage of time, they will become increasingly dispersed, or in other words, the variance in their locations will increase. And this is true even if the trajectory of each person is the deterministic outcome of his or her plans for that afternoon. One is on her way to her office. Another is walking his dog. A third is going grocery shopping. And so on. To the extent those motivations and plans are different from and independent of each other, the individuals movements are random with respect to each other. And dispersion at the higher level - the greater variance in location of the group - is the expected outcome of randomness in the with-respect-to sense at the lower level. This is the principle underlying the ZFEL."
The ZFEL turns on this kind of hierarchial, probabilistic thinking. Thus although the ZFEL is a formalization of conventional data, the novelty of the ZFEL is in interpreting biological causation as essentially probabilistic and hierachical. Now, to be sure, biology has long grappled with both hierarchy and probability. But, the traditional norm has been to treat both notions rather differently than the ZFEL suggests. Discerning the novelty in the ZFEL is tricky, because, as the authors note:
"What we have offered here is a standard sort of scientific argument for a theoretical point of view. We have invoked the huge body of data represented by virtually everything we know about macroevolutionary divergence and argued that it best explained by the ZFEL. Further, we have argued that the standard explanations in the field are, implicitly, ZFEL explanations. So even though we are explicitly stating the ZFEL here for the first time, the data pre-exist and strongly support the ZFEL."
So, what is the value of dwelling on the obvious like this? The novelty of the ZFEL is not that it predicts diversification/complexification -- this is already the intuition of everyone from oncologists to ecologists. The novelty is in the roles of probability and hierarchy in explanation.
Probability is traditionally understood as a tool for verification (the "prob" in the word comes from the same root that gave us words like "prove" and "probity" and "proof"). In a complicated world, we can't expect even valid results to be perfectly self-evident, so probabiilty is used to reveal when research results are credible. McShea and Brandon, however, suggest that probability can become the logical basis of biological CAUSATION, like calculus did for classical mechanics. They are not precisely the first to suggest this, and point (for example) to another author's argument that probability is a sufficient explanatory tool for radio-decay.*
Hierarchical scales of biological activity delineate the traditioanl subdisciplines in biology; for example, biochemistry, cell biology, and physiology. Because biologists recognize that different scales are truly different -- there are different types of data to collect and different processess to explain -- the practice is to confine investigation to a single level. Rather less emphasis, however, has been put on the relationship between levels. The ZFEL explicitly states that biological explanation can be INTER-scale, not just INTRA-scale. At a time when interdisciplinary work is coming into its own, the ZFEL might be a helpful guiding tool in what inter-scale relationships investigators could work toward.
Somewhat more abstractly, the ZFEL challenges a person to try a kind of hierarchical thinking that is foreign and yet deeply gratifying to the intuition of anyone familiar with nature. One of the curious things about biota is that we can more fully distinguish two organisms that are more similar. That is, the more similar things are to each other, the more specific we can be about how they are different. If we want to compare toads to (other) frogs, we walk away feeling like we have said something directly relevant to what makes toads toads and frogs frogs. If we want to compare toads to porpoises, we will end up specifying a set of differences and similarities that differs substantially from the toad-frog set (e.g. how many heart chambers, rather than how water-permeable the skin). Comparing toads to sycamores will result in a still-different set of comparisons (e.g. the source of mechanical rigidity). McShea and Brandon recognize this situation in their explanation of how to paramaterize complexity/diversity -- by doing it ad hoc, with whatever characters could be useful for a given comparison -- and they make this situation explanatorily relevant in their formulation of the ZFEL. The slippery nature of inter-taxon comparison is manifestly a hierarchical phenomenon, and it is what makes it impossible to develop a single set of parameters that would describe all organisms fully without redundancy.**
This brings us to the crucial difference between the ZFEL and the 2nd Law of Thermodynamics: entropy can be compared in identical units between any two entities. Colloquially speaking, entropy is everything getting messier, while the ZFEL is everything getting more complicated. As the ZFEL pushes things to become more different, the nature of their difference itself becomes more complicated. This is not the case for entropy. To be sure, the ZFEL complements, rather than contradicts, the law of entropy.
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