Drosophila mutants suggest a strong drive toward complexity in evolution

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2012

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Abstract

The view that complexity increases in evolution is uncontroversial, yet little is known about the possible causes of such a trend. One hypothesis, the Zero Force Evolutionary Law (ZFEL), predicts a strong drive toward complexity, although such a tendency can be overwhelmed by selection and constraints. In the absence of strong opposition, heritable variation accumulates and complexity increases. In order to investigate this claim, we evaluate the gross morphological complexity of laboratory mutants in Drosophila melanogaster, which represent organisms that arise in a context where selective forces are greatly reduced. Complexity was measured with respect to part types, shape, and color over two independent focal levels. Compared to the wild type, we find that D. melanogaster mutants are significantly more complex. When the parts of mutants are categorized by degree of constraint, we find that weakly constrained parts are significantly more complex than more constrained parts. These results support the ZFEL hypothesis. They also represent a first step in establishing the domain of application of the ZFEL and show one way in which a larger empirical investigation of the principle might proceed.

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10.1111/ede.12014

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Fleming, L, and DW McShea (2012). Drosophila mutants suggest a strong drive toward complexity in evolution. Evolution and Development, 15(1). pp. 53–62. 10.1111/ede.12014 Retrieved from https://hdl.handle.net/10161/29575.

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McShea

Daniel W. McShea

Professor of Biology

My main research interest is hierarchy theory, especially the causal relationship between higher-level wholes and their components (Spencer, Simon, Campbell, Salthe, Wimsatt). In biology, for example, we might want to know how large-scale processes within a multicellular organism act to control the smaller-scale processes within its component cells. Or, in the area of my current research, how do the emotions in mammals (and perhaps other animals) act to initiate and control conscious thought and behavior? It seems clear from the philosophical work of Hume (A Treatise of Human Nature) that the preferencing or valuing that motivates or drives conscious thought and behavior, and in particular conscious decision-making, must arise from the emotions. This is true because the only alternative, reason (in the sense of pure rationality), is value-neutral, and utterly incapable of motivating anything. As Hume put it, "Reason is and ought to be the slave of the passions and can never pretend to any other office than to serve and obey them."

But what is the nature of the causal process by which emotion drives thought and behavior? I argue that it is a form of downward causation, of a sort that occurs in many hierarchical systems. Consider a neutrally buoyant balloon filled with gas and hanging in a room. If the balloon as a whole is moved -- say 2 inches to the left -- this large-scale movement causes all of the gas molecules within it (as well as the molecules in the plastic skin of the balloon) to move, on average, 2 inches to the left. A similar sort of top-down causation occurs, it seems, in the emotion-behavior and emotion-thought relationship. The evidence is that these relationships seem to follow certain key principles of hierarchy theory. 1. Rates. Lower levels move quickly relative to the higher level. The gas molecules in a balloon typically move quickly relative to the balloon as a whole. Likewise, thought and behavior are fast relative to change in emotional state. 2. Causal asymmetry. Lower-level units cannot, as individuals, much affect the higher level. A single gas molecule cannot much affect a whole balloon. Likewise, individual thoughts and behaviors ordinarily do not much affect an emotion. Rather, an emotion hovers more or less unchanging, in the background, while thoughts and behaviors aimed at satisfying that emotion play out. 3. Vagueness. Lower-level units do not directly interact with higher levels and therefore "perceive" them only "vaguely." Thus, thoughts and behaviors are clear and distinct, but we perceive our emotions only vaguely. 4. Downward causation. Higher levels exert their causal influence on lower-level units via boundary conditions, and therefore higher-level control is not precise, with the result that lower-level units have considerable freedom. Consistent with this, in two similar higher-level systems, the sequence of behaviors of lower-level units could be very different. The movements of individual gas molecules in two very similar balloons will be very different. Likewise, the same emotion, the same motivation, in two different people is consistent with their thinking and behaving very differently. (Although presumably some very general similarities can be found. To the extent that the two share the same emotion, the goals they are pursuing are similar. Analogously, the movements of the gas molecules in the balloon share a general similarity, in that they all move two inches to the left on average.)

My past work has been mainly on large-scale evolutionary trends, that is, trends that include a number of higher taxa and that span a large portion of the history of life. Features that have been said to show such trends include complexity, size, fitness, and others. In my research, I worked mainly on developing operational measures of these features, devising methods for testing empirically whether trends have occurred, and studying the causes and correlates of trends. Most of this work so far has been on trends in complexity. In a recent book (Biology’s First Law 2010) with the philosopher Robert Brandon, we argue that complexity change in evolution is partly 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, we argue, even when forces and constraints are present, a tendency for complexity to increase is always present. The rationale is simply that in the absence of selection or constraint, the parts of an organism will tend spontaneously to accumulate variation, and therefore to become more different from each other. Thus, for example, in a multicellular organism, in the absence of selection and constraint, the degree of differentiation among cells should increase, leading eventually to an increase in the number of cell types. 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. A consequence is that any complete evolutionary explanation for change in complexity or diversity will necessarily include the ZFEL as one component.

Other interests include the philosophy of biology generally. (See my textbook coauthored with philosopher Alex Rosenberg, Philosophy Of Biology: A Contemporary Introduction 2009.) More specifically: 1. The connections among the various evolutionary forces acting on animal form -- functional, formal, and phylogenetic. 2. Animal psychology generally. 3. The relationship between morality and human nature.


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