Barbara McClintock in her laboratory in 1947 (Image from the Smithsonian Institution collection via Wikimedia Commons)

Barbara McClintock (1902-1992) was an American geneticist and a pioneer in the field of cytogenetics, a branch of genetics that focuses on the function of chromosomes in individual cells. McClintock’s work on chromosomes in maize revolutionised this field. In her time, she was widely respected, receiving a number of prestigious awards and fellowships, not least the National Medal of Science. However, it is her discovery of transposons – which revolutionised our understanding of genetics – that was entirely radical. Yet it took thirty years for the scale of her discovery to be fully acknowledged, with the award of the Nobel Prize in Medicine or Physiology “for her discovery of mobile genetic elements” in 1983 (NobelPrize.org).

A trademark of McClintock’s research was her unwavering attention to detail. Meticulous data analysis remains essential if one is to have confidence in one’s scientific hypotheses, and all the more vital if such assertions are novel and so question established ideas. Challenging the status quo is precisely what McClintock’s discovery of transposons achieved.

It was her thorough interrogation of the data that enabled her to have first confidence and eventually conviction as to what at that time seemed to be an unlikely – even implausible – scenario. In effect, her data showed that genes could be mobile, moving around the chromosomes, hopping from one place and re-inserting themselves elsewhere. McClintock discovered these peculiarly behaving genes whilst working on maize plants in the late 1940s. We now call these genes ‘transposons’, often aptly referred to as ‘jumping genes’. In November 1953 McClintock published her findings in the journal Genetics, with a paper entitled Induction of instability at selected loci in maize.

Mechanism of transposition
(Image: Wikimedia Commons)

McClintock’s discovery did not sit comfortably with the twentieth century consensus as to how genes should behave. At the time, geneticists considered that genes were stable entities and the notion that there could be ‘renegade’ strands of DNA moving about the genome was treated with extreme caution. But if one places McClintock’s discovery in a broader social and historical perspective, given the nature of the proposal – that is, the idea that something which was perceived as stable, was actually subject to change – one could almost see the skepticism with which it was initially received as less surprising, maybe even predictable.

In some broad sense this might say something about a human desire for stability. If one looks back over the history of science it is not unreasonable to suggest that the scientific theories which met with the most vehement, almost visceral resistance were often those which are not only ‘big ideas’, but those which have championed ‘change’ in a previously accepted framework of ‘permanency’.

This is all the more true when it comes to natural phenomena that an individual cannot observe with the naked eye and/or witness over the course of their own short life-span. Notable examples include the idea of the Big Bang, and nearer to home that of continental drift; a proposal by the meteorologist Alfred Wegener who suggested (quite rightly) that Earth’s continents were once joined together and slowly drifted apart (and continue to move) over millions of years. From the time it was first proposed in 1912 the idea of continental drift found little favour and until the late 1960s the consensus remained that the Earth’s continents were static.

Another example is of course Charles Darwin’s theory of evolution by natural selection, which was proffered at a time when society at large was still committed to the notion of the permanency of species (i.e. that each species on Earth was created in its current form and was not the result of evolution). Here too, Darwin was correct, but some of the resistance stemmed from people who saw weaknesses in the theory, and ones that Darwin lost little time in addressing.

Pigmentation on corn kernels reveals the activity transposons (Image: Damon Lisch via Wikimedia Commons)

In the case of McClintock her work perhaps reflects more than a hint of our collective human condition inasmuch as although the existence and behaviour of transposons was relatively swiftly accepted by other geneticists who conducted research on maize, when it came to applying these ideas to other forms of life, not least humans, the implications were far more slowly accepted. In the case of transposons, interest in these small pieces of mobile DNA remained largely dormant until the 1970s when they were found in viruses and bacteria. From there, interest was ignited and further research revealed that transposons are found in most life-forms, including of course humans.

Let us leave the last words to McClintock. After receiving the Nobel Prize in 1983 she remarked: “You just know sooner or later, it will come out in the wash, but you may have to wait some time.” (quoted in McGrayne 2001, p.144)

Text copyright © 2015 Victoria Ling. All rights reserved.

References

McClintock, B. (1953)  Induction of instability at selected loci in maize.  Genetics 38, 579–99.

McGrayne, S.B. (2001)  Nobel Prize Women in Science.  Carol Publishing Group, Secaucus, NJ.

Pray, L. and Zhaurova, K. (2008)  Barbara McClintock and the discovery of jumping genes (Transposons). Nature Education 1, 169.

Ravindran, S. (2012)  Barbara McClintock and the discovery of jumping genes.  Proceedings of the National Academy of Sciences, USA 109, 20198-20199.

George Gaylord Simpson in 1965 (Image from Concession to the Improbable: An Unconventional Autobiography via Wikimedia Commons)

George Gaylord Simpson (1902-1984) was an American palaeontologist. He published extensively on the taxonomy of extinct and extant animal species, the intercontinental migration of animals, and helped to reconcile the fields of palaeontology and genetics.

One of Simpson’s greatest contributions to evolutionary studies was as one of the founding fathers of the ‘modern synthesis’. In the early twentieth century the study of evolution experienced a renaissance with the work of Gregor Mendel. Even so, some considered that Mendel’s law of genetic inheritance was irreconcilable with the Darwinian theory of evolution. That is, evolution by natural selection asserted that new traits can appear in a species that were not previously present. This conflicted with Mendelian genetics which stated that genes – although passed on to offspring in different combinations – remained unchanged through the generations.

However, in the early 20^th century, an increasing amount of research was carried out on how genes ‘mutate’. Mutation creates permanent changes to genes and can lead to new traits in a given species which have never previously been seen; totally harmonious with evolution by natural selection. The modern synthesis argued that Darwin’s theory and Mendel’s Law were complimentary pieces of the same jigsaw puzzle. It stressed that whilst Darwinian evolution identified the importance of natural selection in evolution, changes in the gene pool are also caused by genetic drift, mutation and gene flow.

Simpson was especially keen to integrate the fields of palaeontology and genetics, and this was elegantly demonstrated in his 1944 publication Tempo and Mode in Evolution. He used palaeontology to show that evolution was not ‘goal-oriented’ (as was being argued at the time by those who thought Darwinian theory could not be reconciled with Mendelian inheritance). In particular, he used the evolutionary history of the horse to show that evolution is not a straightforward, linear process, but one that is littered with extinctions. Broadly speaking, the modern synthesis shows how different academic disciplines – in this case, initially palaeontology and genetics – come together to answer scientific questions common to them both. As the decades have passed, academia in general and evolutionary studies in particular, have become increasingly multi-disciplinary; it takes information from a broad range of disciplines to build a thorough understanding of the evolutionary process.

Simpson can also be noted for his forceful opposition to Alfred Wegener’s theory of Continental Drift. Continental drift – which, in the form of plate tectonics, is now an accepted fact –is the idea that the Earth’s continents have moved over millions of years. This in turn explains why we find very similar fossils on continents that are today separated by vast oceans. Simpson however, argued that the continents were fixed, arguing that the distribution of extinct creatures could be explained by processes other than moving landmasses. Such was the respect for Simpson, together with the  influence of his publications that this led to the theory of Continental Drift losing credibility in the scientific world.

However, by the 1960’s geophysical evidence began to support – and ultimately validated – Wegener’s theory of Continental Drift, and this in turn prompted Simpson to revise his stance. In doing so, Simpson demonstrated what the scientific method is all about; testing a hypothesis, holding it up to scrutiny and, if need be, revising one’s own theory in the face of overwhelming facts. As a point of interest, it should be noted that Simpson’s idea of continental immobility, although wrong in the broad sense, had several fruitful aspects, such as the idea of “island hopping”, “rafting” and that Antarctica once served as a land corridor from South America to Australia. So too our understanding of the evolution of South America’s biotas very largely hinges on first its isolation, with occasional episodes of rafting from Africa, before connectivity was made via the Isthmus of Panama, first as island hopping and then a corridor. Animals moved in either direction and that explains why we see opposums in Toronto and llamas in Chile.

Text copyright © 2015 Victoria Ling. All rights reserved.

References

Eldredge, N. (1985)  Unfinished Synthesis: Biological Hierarchies and Modern Evolutionary Thought.  Oxford University Press.

Frankel, H. (1987)  The continental drift debate.  In Scientific Controversies: Case Studies in the Resolution and Closure of Disputes in Science and Technology (H.T. Engelhardt Jr and A.L. Caplan, eds), pp. 203-248.  Cambridge University Press.

Laporte, L.O.F. (1994)  Simpson on species.  Journal of the History of Biology 27, 141–159.

Olson, E.C. (1991)  George Gaylord Simpson. 1902-1984.  A biographical memoir by Everett Olson. Biographical Memoirs of the National Academy of Sciences 60, 331–353.

Simpson, G.G. (1943)  Mammals and the nature of continents.  American Journal of Science 241, 1-31.

Simpson, G.G. (1944)  Tempo and Mode in Evolution.  Columbia University Press.