Major Transitions
A minor evolutionary transition is for example the transition from fins to legs. A major evolutionary transition (a 'landmark transition') in the sense of Eörs Szathmáry and John Maynard Smith is a change in the way information is stored and transmitted. Often this requires a new code (i.e. a new evolutionary system). The largest evolutionary transitions are the transitions from one evolutionary system (or CAS) to another.
- cosmic (physical) evolution
- stellar (chemical) evolution
- mineral (geological) evolution
Then we have an evolutionary phase where inanimate matter evolved into living matter, and the "real" biological evolution began:
- genetic (biological) evolution
- memetic (cultural) evolution
- ..
In the book The Major Transitions in Evolution from John Maynard Smith and Eörs Szathmáry (Oxford University Press, 1995), the authors define a major transition as a major change in the way (genetic) information is organized and transmitted from one generation to the next. The authors focus on biological evolution. The book description says:
- "Over the history of life there have been several major changes in the way genetic information is organized and transmitted from one generation to the next. These transitions include the origin of life itself, the first eukaryotic cells, reproduction by sexual means, the appearance of multicellular plants and animals, the emergence of cooperation and of animal societies, and the unique language ability of humans. This ambitious book provides the first unified discussion of the full range of these transitions. The authors highlight the similarities between different transitions--between the union of replicating molecules to form chromosomes and of cells to form multicellular organisms, for example--and show how understanding one transition sheds light on others. They trace a common theme throughout the history of evolution: after a major transition some entities lose the ability to replicate independently, becoming able to reproduce only as part of a larger whole. The authors investigate this pattern and why selection between entities at a lower level does not disrupt selection at more complex levels. Their explanation encompasses a compelling theory of the evolution of cooperation at all levels of complexity. Engagingly written and filled with numerous illustrations, this book can be read with enjoyment by anyone with an undergraduate training in biology. It is ideal for advanced discussion groups on evolution and includes accessible discussions of a wide range of topics, from molecular biology and linguistics to insect societies."
| Transitions described in the book | ||
|---|---|---|
| Transition from: | Transition to: | Notes |
| Replicating molecules | "Populations" of molecules in compartments | Can't observe |
| Independent replicators (probably RNA) | Chromosomes | RNA world hypothesis |
| RNA as both genes and enzymes | DNA as genes; proteins as enzymes | |
| Prokaryotes | Eukaryotes | Can observe |
| Asexual clones | Sexual populations | Evolution of sex |
| Protists | Multicellular organisms — animals, plants, fungi | Evolution of multicellularity |
| Solitary individuals | Colonies with non-reproductive castes | |
| Primate societies | Human societies with language, enabling memes | Sociocultural evolution |
Maynard Smith and Szathmary identified several properties common to the transitions:
- Smaller entities have often come about together to form larger entities. e.g. Chromosomes, eukaryotes, sex multicellular colonies.
- Smaller entities often become differentiated as part of a larger entity. e.g. DNA & protein, organelles, anisogamy, tissues, castes
- The smaller entities are often unable to replicate in the absence of the larger entity. e.g. Organelles, tissues, castes
- The smaller entities can sometimes disrupt the development of the larger entity e.g. Meiotic drive (selfish non-Mendelian genes), parthenogenesis, cancers, coup d’état
- New ways of transmitting information have arisen.e.g. DNA-protein, cell heredity, epigenesis, universal grammar.
Links
- Nature article from Szathmary and Smith, Szathmary and Smith, The major evolutionary transitions, Nature Vol. 374 (1995) 227-232
Books
- Richard E. Michod, "Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality", Princeton University Press, 1999
- John Maynard Smith and Eörs Szathmary, "The Major Transitions in Evolution", Oxford University Press, 1997
