The so-called ‘Major Transitions’ framework is an attempt to explain the hierarchical structure of life on Earth: genes within chromosomes, chromosomes within cells, cells within cells (eukaryotic cells), individuals within sexual partnerships, cells within multicellular organisms, and organisms within societies. The best-known attempt to unify the origins of these relationships is a book by John Maynard Smith* and Eörs Szathmáry, The Major Transitions in Evolution.
First published in 1995, the book focused on the origins of these hierarchical levels, connecting them with the unifying theme that
…entities that were capable of independent replication before the transition can replicate only as part of a larger whole after it.
For example, after a transition from unicellular to multicellular organisms (there were several), cellular reproduction either contributes to the growth of the organism or to production of new multicellular organisms.
My Ph.D. advisor, Rick Michod, has revised this idea, focusing less on reproduction and more on units of fitness. An edited volume including chapters by Szathmáry and Michod was published in 2011.
More recently, Szathmáry himself has updated the framework he developed with Maynard Smith, removing sex from the list, adding endosymbiotic events, and folding in David Queller’s fraternal/egalitarian distinction.
Maureen O’Malley and Russell Powell think that the Major Transitions framework is due for an overhaul. In a new paper in Biology & Philosophy, they argue that
…not only is MTE [major transitions in evolution] in both its classic and revised forms incomplete, but that it represents a fundamentally problematic approach to major transitions.
While I agree with some of their criticisms, I think the proposed cure is not only worse than the disease, but less effective than some less invasive treatments. A bit of background will be necessary here: O’Malley and Powell discuss three versions of the transitions framework. By MTE, they mean the theory of major transitions essentially as described by Maynard Smith and Szathmáry. The framework developed by Michod, which might fairly be called his research program, they call ETI for ‘evolutionary transitions in individuality.’ The recent revision by Szathmáry, taken from his own title, is MTE 2.0. They summarize MTE (fairly, I think) this way:
Each transition in MTE is meant to exhibit a change from autonomous replication to some form of cooperative group replication. The loss of reproductive independence is the primary defining criterion for major transitions. There are also vague commitments to increased division of labour (1997: 210) and new modes of inheritance. But what most philosophical and theoretical biologists pay attention to in MTE is the emergence of new units of selection—the transitions in individuality that occur when one level of individuality gives way to another [Calcott and Sterelny 2011].
Most of this would also be true for Michod’s ETI framework, with the exception that few would call his commitment to division of labor ‘vague.’ So what’s the problem? O’Malley and Powell list several (my emphasis):
Some of these problems are general and theoretical, having to do with the lack of unity amongst events included in MTE, and the model’s propensity to encourage a fallacious ‘ladder-of-life’ or otherwise progressivist view of evolutionary history. Other problems are more specific and empirical, having to do with missing events.
The second of these is the easiest to deal with: major transitions theory is not and has never been progressivist. The Major Transitions book itself contains a section (1.2) entitled “The fallacy of progress”:
Empirically, the history of life is better visualized as a branching tree than as a single ascending line…On the theoretical side, there is no reason why evolution by natural selection should lead to an increase in complexity, if that is what we mean by progress.
The major transitions framework describes the increases in hierarchical complexity (number of nested levels, such as cells within multicellular organisms) that have happened in some lineages. As the first paragraph in The Major Transitions puts it:
This increase has been neither universal nor inevitable. Bacteria, for example, are probably no more complex today than their ancestors 2000 million years ago. The most that we can say is that some lineages have become more complex in the course of time. Complexity is hard to define or to measure, but there is surely some sense in which elephants or oak trees are more complex than bacteria, and bacteria than the first replicating molecules.
There is nothing progressivist in claiming that some lineages have increased in complexity; given any reasonable definition of complexity, this is simply true. A claim that such increases are universal or inevitable would be progressivist, but I’m fairly familiar with the literature on major transitions, and I can’t remember ever seeing such a claim. If major transitions theory encourages a ‘ladder of life’ view, it is in spite of rather than because of what the major players have written.
On the other two shortcomings, lack of unity and missing events, I’m mostly in agreement with O’Malley and Powell. I’ll get into that in part 2 and discuss their proposed solution in part 3.
* First name John, last name Maynard Smith. I have occasionally seen the book cited as Smith, JM and Szathmáry, E, and in fact I just noticed that Google Books lists the first edition as written by “John M. Smith.”
Calcot G, Sterelney K (eds.) (2011) The Major Transitions in Evolution Revisited. MIT Press, Cambridge, MA.
Grosberg RK, Strathmann RR (2007) The evolution of multicellularity: a minor major transition? Annual Review of Ecology, Evolution, and Systematics, 38, 621–654.
Michod RE (2000) Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality. Princeton University Press, Princeton, NJ.
O ’Malley MA, Powell R (2016) Major problems in evolutionary transitions: how a metabolic perspective can enrich our understanding of macroevolution. Biology & Philosophy, 31, 159–189.
Queller DC (1997) Cooperators since life began. Quarterly Review of Biology, 72, 184–188.
Szathmáry E (2015) Toward major evolutionary transitions theory 2.0. Proceedings of the National Academy of Sciences, 201421398.
Nick Gotts says
One event (actually, several in different lines) which I felt was missing from MTE (I haven’t read the other two – now added to must-read list) was post-birth/hatching parental care (mostly maternal of course, but there are exceptions), which enables cultural transmission.