Evolution without accidents
Despite advances in molecular genetics, too many biologists think that natural selection is driven by random mutations
James A Shapiro is professor of microbiology in the Department of Biochemistry and Molecular Biology at the University of Chicago. His books include Bacteria as Multicellular Organisms (1997), co-edited with Martin Dworkin, and Evolution: A View from the 21st Century, Fortified (2nd ed, 2022).
Edited by Cameron Allan McKean
5,000 words
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Since 1859, when Charles Darwin’s On the Origin of Species was first published, the theory of natural selection has dominated our conceptions of evolution. As Darwin understood it, natural selection is a slow and gradual process that takes place across multiple generations through successive random hereditary variations. In the short term, a small variation might confer a slight advantage to an organism and its offspring, such as a longer beak or better camouflage, allowing it to outcompete similar organisms lacking that variation. Over longer periods of time, Darwin postulated, an accumulation of advantageous variations might produce more significant novel adaptations – or even the emergence of an entirely new species.
Natural selection is not a fast process. It takes place gradually through random variations, or ‘mutations’ as we call them today, which accumulate over decades, centuries, or millions of years. Initially, Darwin believed that natural selection was the only process that led to evolution, and he made this explicit in On the Origin of Species:
If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find out no such case.
A lot has changed since 1859. We now know that Darwin’s ‘gradualist’ view of evolution, exclusively driven by natural selection, is no longer compatible with contemporary science. It’s not just that random mutations are one of many evolutionary processes that produce new species; they have nothing to do with the major evolutionary transformations of macroevolution. Species do not emerge from an accumulation of random genetic changes. This has been confirmed by 21st-century genome sequencing, but the idea that natural selection inadequately explains evolutionary change goes back 151 years – to Darwin himself. In the 6th edition of On the Origin of Species, published in 1872, he acknowledged forms of variations that seemed to arise spontaneously, without successive, slight modifications:
It appears that I formerly underrated the frequency and value of these latter forms of variation, as leading to permanent modifications of structure independently of natural selection.
– from Chapter 15, p395, emphasis added
Today, we know in exquisite detail how these larger-scale ‘spontaneous’ variations come about without the intervention of random mutations. And yet, even in the age of genome sequencing, many evolutionary scientists still cling stubbornly to a view of evolution fuelled by a gradual accumulation of random mutations. They insist on the accuracy of the mid-20th-century ‘updated’ version of Darwin’s ideas – the ‘Modern Synthesis’ of Darwinian evolution (through natural selection) and Mendelian genetics – and have consistently failed to integrate evidence for other genetic processes. As Ernst Mayr, a major figure in the Modern Synthesis, wrote in Populations, Species and Evolution (1970):
The proponents of the synthetic theory maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection, and that transpecific evolution [ie, the origins of new species and taxonomic groups] is nothing but an extrapolation and magnification of the events that take place within populations and species.
This failure to take account of alternative modes of change has been foundational to popular and scientific misconceptions of evolution. It continues to impact the study of antibiotic and pesticide resistance, the breeding of new crops for agriculture, the mitigation of climate change, and our understanding of humanity’s impacts on biodiversity.
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