Dr. Bonner aptly subtitles this book “An Essay on the Structure of Biology”. Indeed he attempts to outline the architecture of biological organisms by focusing on generalizing principles. Although first published in 1965, I found the content remains remarkably modern, and provides a constant harping for the experimentalist on the important problems in biology.

“Size and Cycle” predates modern literature on the topic of evolutionary developmental biology by a long shot, although is predated by several important works, notably from C.H. Waddington and De Beer. Nevertheless, this book provides a superb theoretical perspective on the relation between evolution and development, by relating the concepts of the life cycle, a sequence of steps of a developing organism; size, which allows a more powerful classifiction of these steps, compared to using the adult form alone; and evolution, the force which enables change within and between the steps.

Bonner’s introductory chapter is fantastic. To summarize, in his words…

With respect to content, Bonner first outlines the big picture by describing the major categories of organisms (unicellular, multinucleate, aggretive, and multicellular) and the major categories of the life cycle, common to each (size increase, size equilibrium, size decrease). Thus relating the title of the book, Bonner illustrates that all organic life follows a continuous life cycle, from a vegetative state following birth to a state of reproductive maturity at the pinnacle of the cycle. This cycle can be meaningfully, but arbitrarilly subdivided into major categories (aforementioned), using the notion of size.

Trying my best not to plagarize, here is the extremely condensed version of “Size and Cycle”, drawing mainly from the summary points at the end of each chapter.

Classification of organsisms in “standard” biology

• Grouping based on morphology
• An oranism is identified primarily by its adult form
• Emphasis on sexuality
• Architecture of classification is inherently phylogenetic, or based on the notion of homology, rather than on structure–function relationships

Bonner’s expanded view of classification

• Grouping based on morphology and size, as morphology describes an organism with qualitative descriptors of shape and form, while size provides a quantitative dimension.
• The organism is the whole life cycle, not merely the adult: genes act at all times.
• Variation control can be maintained by means other than sex. In addition asexual organisms have life cycles, and ploidy levels can be maniuplated without altering the life cycle drastically.
• Analogy over homology is emphasized in the classification of life cycles. The characteristics of a cycle are more relevant than the history of its progression.

The Method

• “Biology is in need of connecting laws that bring together molecular events, developmental (or life history) events, and evolutionary events.”
• “By use of the concept of steps it is possible to break down all biological processes to an abstract designation which refers to the sequential biochmical reactions. The sequence of these steps has a recurring pattern which involves separation and growth or size increase, providing points of maximum and minimum size.” Variations are introduced at the point of minimum size, and selectively eliminated at the point of maximum size, via differential reproduction.
• The life cycle is the sequence of steps between the points of minimum and maximum size, and, when analyzed in the light of size, can serve as a needed connecting principle for biology.

Size in the Cycle

• The life cycle can be meaningfully subdivided into periods of size increase, size decrease, and size equilibrium.
• Size itself is a key correlate of development, as well as a useful analytical tool.
• Size equilibrium can occur at any stage of the life cycle. If at the minimum, the stage is typically robust against external environmental perturbations. Other equilibria are typically not robust.

The (Nature of) Steps

• The cell, in its entirety, is the minimal unit of inheritance, and joins two life cycles. Thus the point of minimum size is the minimal unit of heredity.
• Steps can be affected by environmental or internal perturbations, and can be sensitive or robust to either. As an organism increases in size, it generally becomes more robust to perturbation.
• The steps in multicellular organisms are particularly interesting because although each cell shares an identical genome, the organism can display complex differentiation. Many solutions from this constraint exist as a function of cell motility (cell wall? cell membrane?)
• One step always leads to another: all life is connected seamlessly in this way.
• An individual life cycle can be viewed as a collection of chains linked serially and in parallel. Portions of chains can be moved, removed, or new chains added without destroying the cycle. This is referred to as heterochrony.
• Experimentalists should thus focus on understanding the architecture of chains for each organism.

Evolution

• Each and every component of an organism, at any point during the life cycle, can be affected through a positive, negative, or neutral adaptive process. (As Bonner focuses attention predominantly on the force of natural selection, here I suppose ‘neutral adaptive’ means ‘non-selected’ and thus the evolution of that component is due to one of the other forces).
• Each component’s evolutionary process is also affected by environmental changes (spatially, temporally, and dependent on the interaction with other organisms)
• Groups of selected traits may be correlated with each other with respect to measurments made on them (e.g. expression level, onset time, size, etc.)
• These correlations usually make it impossible to explain why any particular component is adaptive. It suffices to identify the correlations and understand that “the sum total must be adapative”.