Does diversity always grow?

Journal name:
Date published:
Published online

Samir Okasha is intrigued by a proposed universal law of biology: that complexity inevitably increases in the absence of other evolutionary forces.

Biology's First Law: The Tendency for Diversity and Complexity to Increase in Evolutionary Systems

by Daniel W. McShea and Robert N. Brandon University of Chicago Press: 2010. 184 pp. $20, £13 ISBN: 9780226562261

Buy this book: US UK Japan

The nineteenth-century astronomer John Herschel is said to have dismissed Charles Darwin's theory of evolution as “the law of higgledy-piggledy”. Whether he was referring to the role of chance in natural selection or the lack of definite predictions due to it, Herschel's criticism was misplaced: the selective preservation of the fittest variants makes a population more homogeneous, not less. Herschel's epithet does apply neatly to the 'zero-force evolutionary law' that palaeobiologist Daniel McShea and philosopher of science Robert Brandon propose in Biology's First Law.

McShea and Brandon state that diversity and complexity tend to increase over time in biological systems. It is, the authors argue, a universal law, applicable to all taxa, at all hierarchical levels and at all times. They use the analogy of Newton's law of inertia — just as it tells us that a body will move with a constant velocity if no forces act on it, this zero-force evolutionary law seeks to capture how a biological system will behave in the absence of other influences. Although the trend they describe may not manifest itself in cases when it is counteracted by constraints, it provides the background against which other evolutionary pressures should be understood, the authors contend.

McShea and Brandon use a standard definition of diversity: the amount of variation in a biological system. A taxon that contains many species, or a species that has many forms, is more diverse than one that has few. But the authors adopt a simplified measure of complexity that considers only the degree of differentiation among the parts of a biological system, not the various functions of those parts. Complexity thus becomes a matter of having many part types, irrespective of what the parts do or how they are organized. The authors argue persuasively that their simpler definition of complexity is more scientifically useful than the traditional one, because function is hard to quantify.


The eyes of cavefish vary between individuals despite never being used.

Diversity and complexity can be assessed at any level of the biological hierarchy — in clades, species, subspecies, populations and between and within organisms. But diversity and complexity are the same thing, the authors say, when viewed from adjacent levels. For example, an organism with a great diversity of cell types is also a complex organism. Thus, diversity at one level of the hierarchy equates to complexity one level higher. Both diversity and complexity will increase over time through the accumulation of mutations, they suggest.

“McShea and Brandon's law does not represent a wholly new evolutionary principle, rather a unifying one.”

McShea and Brandon do not claim that their law represents a wholly new evolutionary principle, rather that it is a unifying one. The tendency for increasing diversity has been recognized previously in specific situations. For example, molecular geneticists know that, in the absence of selection, populations will diverge genetically as neutral mutations accumulate. And evolutionary biologists have noticed that tissues and organs that are not subject to selection, such as the eyes of cave-dwelling fish, often show more variation between individuals. The authors aim to encompass these various findings in a single theory that covers all of the fields in which the principle has been seen — in molecular biology, population genetics, phylogenetics, palaeobiology and elsewhere. They make a good case for their argument that a single principle is at work.

Their theory suggests new research questions, such as whether the tendency for diversity to increase will usually be overcome by natural selection, and it advances our philosophical understanding of evolution. The law also makes testable predictions: for example, that diversity and complexity will increase fastest in ecological circumstances and taxa where selection is weak.

Biology's First Law is an original and unusual book. A hybrid of theoretical biology and philosophy of science, it addresses both conceptual and empirical problems. If there is a lacuna, it is that the authors do not attempt a mathematical formulation of their law, claiming only that it is reducible to probability theory. Such a formulation is essential if we are to investigate and integrate the law with other theories of evolutionary dynamics. Nevertheless, it is a thought-provoking study.

Author information


  1. Samir Okasha is a professor of the philosophy of science in the Department of Philosophy, University of Bristol, 9 Woodland Road, Bristol BS8 1TB, UK.

Author details


  1. Report this comment #12054

    Jeremy Field said:

    I'm troubled by the thesis that diversity increases most rapidly when selection is weak. The creatures inhabiting the volcanic Galapagos Islands are subject to fierce environmental and presumably selective pressures.

  2. Report this comment #12117

    oliver elbs said:

    The title of this review may even be better than the whole book reviewed: If "growth of diversity" is Biology's "First Law", then "Why is there any GROWTH at all?" may be Biology's thermodynamic "Zero Law".
    Already Ludwig van Bertalanffy used the last half of his life as a theoretical biologist to study cancer and the related question of "what is GROWTH"?
    That question still goes...

  3. Report this comment #12226

    Dalius Balciunas said:

    you hit the right nail on the head! Namely, zero law of thermodynamics probably can explain why biological complexity in general and biodiversity in particular increase in the course of evolution.

  4. Report this comment #12433

    peter borger said:

    "McShea and Brandon do not claim that their law represents a wholly new evolutionary principle, rather that it is a unifying one. The tendency for increasing diversity has been recognized previously in specific situations."

    The tendency to generate variation in offspring is a genetic trait. It is not just a blind accumulation of unpredictable mutations, rather the genome is frontloaded with variation-inducing genetic elements (VIGEs). VIGEs are well-known to science. McClintock was the first to identify them in plants: transposons. It appeared they are omnipresent, but they have been mistakenly interpreted as remnants of invasions of ancient retroviruses. Endogenous retrovrisuses qualifiy as VIGEs. TE elements in bacteria, too. The genomes of all organism were frontloaded with genetic elements in order to rapidly generate variation. The generation of increasing diversity is not the first, but rather the second law of biology. The first (unifying) law is: "The tendency to reproduce." (Nothing in biology makes sense, except in the light of reproduction)".



  5. Report this comment #12434

    peter borger said:

    I forgot to mention that RNA viruses, for instance RSV, forms from VIGEs (ERVs) in only two steps by picking up genes from the host.


    The VIGE hypothesis explains the origin of RNA viruses.

    NB: In bacteria IS elements qualify as VIGEs (not TE elements as I mentioned above...)


Subscribe to comments

Additional data