Major Transitions in Evolution

In 1995, a highly-influential Nature paper was published: The Major Evolutionary Transitions, by Eörs Szathmáry and John Maynard-Smith, who then expanded it to an excellent book. They identified events in which the nature of evolution itself changed.

Evolution can be simplified as a combination of mutation, selection, and drift. This isn’t the way it has always been, and is not the way it will always be. Evolution doesn’t conform to a uniformitarian principle, and this is what the paper and book outlined.

The Cambrian Radiation, when animals originated in a bewilderingly fast and psychedelic 30-50 million years, was an event in which evolution pushed itself to greater heights. The evolutions of sight, of burrowing, of active predation, all these innovations allowed animals to explore evolutionary avenues that were until then closed off.

That is only one example from within one lineage (the animals), and it does not count as a major evolutionary transition. The transitions they discussed go much deeper than that.

The evolution of genes, a system of heredity and information flow and storage, fundamentally changed how evolution worked.

The evolution of multicellularity blew open the concept of individuality. One organism is made up of many individual cells that develop together as one individual organism, and this set-up must function both during lifetime and during evolutionary time. Evolution in unicellular organisms is radically different than in multicellular organisms.

Similarly, the evolutions of language and culture, the last of the originally-identified major evolutionary transitions, are representative of new domains for evolution to grow into, where it will work in new, heretofore impossible, ways.

To make a technological analogy, the Cambrian Radiation was the evolution of the internal combustion engine from the steam engine. Same principle, just many more possibilities. The major transitions in evolution were the invention of the wheel, of engines, the harvesting of electricity. Each of these completely changed the way society worked, just as genes, multicellularity, and language changed the way evolution worked.

Of course, the originally-listed major evolutionary transitions were just recommendations, and further research will continuously make additions to and removals from the list. For example, while the Cambrian Radiation could just be seen as a development in biomechanics, the origin of centralised nervous systems that we know took place then could be argued to be a major evolutionary transition.

Major transitions represent systematic shifts in the mode of evolution, and these shifts are merely consequences of novelties at the organismal level. Language evolved as a result of the evolution of our facial musculature, of our brains, and of our sociality, each of which involved novelties of their own.

The origin of such novelties is the topic of much research, and varies depending on the case. Sometimes it’s genes, sometimes development, sometimes anatomy, behaviour, or environment. Most of the time, there is no single cause, and individual researchers will prioritise their own hobby-horses.

However, the fact remains that not every evolutionary novelty will lead to a major transition in evolution. Flight was a hugely important novelty and definitely a major transition for insects and for ecology, but not so much for evolution, so the question of how a novelty can result in a whole new avenue of evolutionary change remains.

Personally, I like to define major transitions as anything that dramatically increase evolvability. The first major evolutionary transition – the origin of life – was simply a transition that enabled evolvability, and each major transition since then increased it. However, this definition is very impractical since we cannot quantify evolvability.

Major transitions include the evolution of genomes, of eukaryotes, of multicellularity, and of eusociality. The one thread linking them all is incorporation. Genes coming together into a cooperative genome. The endosymbiosis of mitochondria was basically the subordination of an external organism into a larger one to make the eukaryotic cell. Multicellularity shows many of these formerly independent individual cells foregoing their selfishness to divide themselves into labour groups for the sake of the organism. Ditto eusociality for a superorganism.

(Super)organisms are like Matryoshka dolls, and when a larger doll is added, that counts as a major evolutionary transition. In other words, the key characteristic of a major evolutionary transition is the coming together of individuals into a single reproductive cooperative group: from genes to genome, genomes to cell, cells to multicellular organism, multicellular organisms to eusocial colony. This is the most functional definition of major evolutionary transitions.

The biggest advantage of this definition is that we can examine major evolutionary transitions under the same framework as the evolution and maintenance of cooperation and division of labour, and the minimisation of selfishness, topics that are already subjects of much research. The same problems we have with the evolution of cooperation, we have with identifying major evolutionary transitions: beneath the surface of a well-oiled group, there are always exceptions, from selfish genes to selfish ants. Volvocalean algae show unicellularity, multicellularity, and many intermediates in between, and they don’t count as having undergone a major evolutionary transition (you can read about them in Nedelcu & Michod’s chapter 21 in Modularity in Development and Evolution). There are thousands of symbioses, but only a few (mitochondria, plastids) turned to true major transitions.

When there are so many grey zones, a diversity of conditions may be at play, and you cannot assign a single universal cause for all major transitions. Major evolutionary transitions could also be defined by an increase in complexity. As nebulous as “complexity” is, it is obvious that a multicellular eukaryote is more “complex” than a unicellular bacterium. Some transitions are characterised by new inheritance mechanisms – this unites the origins of life, genes, and language.

In any case, one cannot make the mistake of considering these transitions teleologically, and they are certainly not golden tickets to megadiversity and evolutionary success; the analogy I made earlier with technology is somewhat fallacious. For example, only one species underwent a major transition to language. Beetles, most likely the most diverse group of animals, only show the major transitions up to multicellularity, none of the “higher” transitions. Most of biodiversity is made up of unicellular prokaryotes.

From my point of view, the first discussion of major evolutionary transitions in 1995 was a significant milestone in evolutionary biology. An abridged history of modern evolutionary biology can be written as a narrative of increasing holism. We begin with the Modern Synthesis of the 1950s, when genetics and Darwinism were finally fused together to paint a picture where evolution is reduced to allele frequency changes, and these changes lead directly to phenotypic evolution.

Although repeatedly disputed, this remained the consensus view until the 1980s, when Stephen J. Gould and Niles Eldredge started rocking the boat with punctuated equilibrium, which prompted leading evolutionary biologist John Maynard Smith to welcome palaeobiology to the “high table”. In so doing, the gates were opened and palaeobiology flooded the prevailing view with more and more challenges: historical contingency, morphological stasis, evolutionary trends, and other such macroevolutionary topics became hot debates that could simply not be explained with such a simple view of evolution as the sum of gene frequency changes.

All this was soon followed by the received wisdom from George Williams’s 1966 Adaptation and Natural Selection beginning to be questioned. For one thing, the concept of genes or individuals as units of selections was becoming murkier with the discoveries of endosymbiosis and of widespread horizontal gene transfer, throwing doubts on many of the fundamental concepts of the multicellular-based Modern Synthesis. A push towards a more fluid view of evolution as capable of acting on many levels was spearheaded by work on group selection, all of which led to a re-emergence of the interest in cooperation as a driver of evolution. Basically, when we can’t even say what an “individual” is anymore, how can we say what the one unit of selection is? We can’t, and we have to accept that evolution is no longer as simple as gene frequency changes between populations.

Finally, developmental biology was brought into the fold as evo-devo, thanks in no small part to Gould’s earlier book, Ontogeny and Phylogeny.

The cap on this clear trend of evolutionary biology’s philosophical expansion came with the major evolutionary transitions paper in 1995. The next milestone, a new Synthesis that acknowledges and explicitly includes all of these developments,  has not yet been reached.

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