Update on the Zero-Force Evolutionary Law (see blue text below for introduction):
1) In a 2019 paper in Evolution, Robert Brandon, Steve Wang, and I used random‐walk models to quantify the zero-force expectation, producing equations that give the probabilities of diversity or complexity increasing as a function of time, and that give the expected magnitude of the increase. We produced two sets of equations, one for the case in which variation occurs in discrete steps, the other for the case in which variation is continuous. The equations provide a way to decompose actual trajectories of diversity or complexity into two components, the portion due to the ZFEL and a remainder due to selection and constraint. Application of the equations was demonstrated using real and hypothetical data.
2) A follow-up book on the ZFEL is coming out soon, in 2020: Brandon & McShea, The Missing Two Thirds of Evolutionary Theory. Cambridge University Press.
In a 2010 book (Biology’s First Law) with philosopher Robert 
Brandon, we argued that complexity change in evolution is governed by what we call the Zero-Force Evolutionary Law (ZFEL). The law says that in the absence of selection and constraint, complexity (in the sense of differentiation among parts) will tend to increase. Further, even when forces and constraints are present, a tendency for complexity to increase is always present. The rationale is simply that when selection is absent, the parts of an organism should tend spontaneously to accumulate variation, and therefore to become more different from each other. In the same way, the pickets of a picket fence will tend to become different from each other as each picket accumulates its own unique features, one picket losing a chip of paint, another acquiring a sticky pollen grain, a third getting knocked and dented by a passing animal, and so on. In the same way, absent selection and constraint, the degree of differentiation among cells should increase with the accumulation of heritable accidents (e.g., mutation), leading eventually to an increase in the number of cell types.
The same argument applies to diversity. Organisms within a population and species within a higher taxon should tend to become more different from each other, in the absence of selection and constraints.
There is considerable evidence for the ZFEL, for the existence of a spontaneous increasing tendency in evolution. We discuss some of it in the book. For a recent empirical test, see this paper with Leonore Fleming on complexity in Drosophila.
As we argue in the book, the law applies at all hierarchical levels (molecules, organelles, cells, etc.). It also applies above the level of the organism, to differences among individuals in populations, and to differences among species and among higher taxa. In other words, the ZFEL says that diversity also tends spontaneously to increase. The ZFEL is universal, applying to all evolutionary lineages, at all times, in all places, everywhere life occurs. And any complete evolutionary explanation for change in complexity or diversity will necessarily include the ZFEL as one component.
Importantly, the ZFEL points to a tendency, not a result. My leaning against the corner of my house creates a “tendency” for the house to fall down even if my leaning doesn’t cause any actual movement. Likewise, the ZFEL produces a tendency for complexity to rise, but complexity may or may not actually rise, depending on the strength of opposing forces and constraints. So the conventional wisdom of evolutionary biology is that complexity has risen over the history of life, that there has been a trend. That could be right, and if it is, the ZFEL must be part of that story. On the other hand, the existence of an overall trend is mostly undemonstrated (many will be surprised to learn). Increases have occurred but complexity has often decreased as well. And if it turns out that the two have been roughly in balance, that no net trend has occurred, that finding would not refute the ZFEL. Rather, it would suggest that powerful forces have acted to block the ZFEL tendency.
A consequence of the ZFEL is that we do not need natural selection to explain complex tissues and organ in organisms. The ZFEL says that the complexity of all structures is expected to increase spontaneously, in the absence of selection. That is not to say that the structures we ordinarily think of as complex — such as eyes and brains — arise spontaneously. Eyes and brains are not just complex, they are also functional. They *do* something for the organism. And selection is the only mechanism known that can produce functionality in evolution. But functionality aside, we can ask why these structures are complex, why they are so differentiated, consisting of many different part types? (Undoubtedly at least some complex structures could have been simpler, performing their functions with fewer part types.) And the answer could well be spontaneous differentiation, the ZFEL. Indeed, if there is a puzzle to be solve about complexity in evolution, it is not why some structures are complex but why *any* are simple. The puzzle is not how mammals have been able to achieve 250 cell types but rather — given that we have billions of cells, every one of which could in principle be unique type — why we don’t have many, many more than 250. In principle, every cell could be its own unique type. The answer is natural selection, acting to maintain uniformity, to maintain simplicity, wherever simplicity is needed. That is, natural selection acting *against* complexity!
In a 2019 paper with Robert Brandon and Steve Wang, we offer a quantification of the ZFEL and a roadmap to deploying it in real datasets. The second book (with Robert Brandon, 2020), The Missing Two Thirds of Evolutionary Theory, covers the same ground and more, offering reasons to think the ZFEL has dominated the diversity history of life on Earth.
If I had to fit the point of the ZFEL on a bumper sticker, it would say this: Complexity is easy. Simplicity is hard.
