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Charles Darwin – where he was right and wrong

by Tim Harding, B.Sc.

(An edited version of this essay was published in The Skeptic magazine, June 2015, Vol 35 No 2, under the title ‘Darwin’s Missing Link’.  The essay is based on a talk presented to the Mordi Skeptics on Tuesday 5 May 2015).

Charles Darwin (1809-1882) is best known for his major contributions to evolutionary theory. In 1859, Darwin published his theory of natural selection as the mechanism of evolution in his revolutionary book On the Origin of Species. This book provided compelling evidence overcoming the scientific rejection of earlier concepts of transmutation of species. The basic principles of his theory have been shown to be correct and are now widely accepted as the basis of mainstream zoology, botany and ecology.

On the other hand, in a later book Darwin got it wrong with the mechanisms of inheritance.  The empirical rules of genetics, based solely on observational results, were largely understood since Gregor Mendel’s ‘wrinkled pea’ experiments in the 1860s. The postulated units of inheritance were called genes, but in Charles Darwin’s time it was not understood where genes were located in the body or what they physically consisted of. Darwin knew that there must have been a physical mechanism for inheritance, but his speculations about it – called pangenesis – were incorrect. Fortunately for the credibility of his theory of evolution by natural selection, he published these speculations later in a separate 1868 book titled Variation of Animals and Plants Under Domestication.

Darwin’s early career

Charles Robert Darwin was born in Shrewsbury, England, on 12 February 1809 at his family home, The Mount. He was the fifth of six children of wealthy society doctor and financier Robert Darwin, and Susannah Darwin (née Wedgwood).

Darwin went to Edinburgh University in 1825 to study medicine. In his second year he neglected his medical studies for natural history and spent four months assisting Robert Grant’s research into marine invertebrates. Grant revealed his enthusiasm for the concept of transmutation of species (the altering of one species into another), but Darwin initially rejected this concept (probably for religious reasons).

Ideas about the transmutation of species were controversial as they conflicted with theological beliefs that species were unchanging parts of a designed hierarchy and that humans were unique, unrelated to other animals. The political and religious implications were intensely debated, but transmutation was not accepted by the scientific mainstream until Darwin’s theory.

In December 1831, Darwin had joined the Beagle ship voyage as a gentleman naturalist and geologist.  In South America, he discovered fossils resembling huge armadillos, and noted the geographical distribution of modern species in hope of finding their ‘centre of creation’.  As the Beagle neared England in 1836, he began to think that species might not be immutable after all.

In March 1837, ornithologist John Gould announced that mockingbirds collected on the Galápagos Islands represented three separate species each unique to a particular island, and that several distinct birds from those islands were all classified as finches. Darwin began speculating, in a series of notebooks, on the possibility that ‘one species does change into another’ to explain these findings, and around July of that year sketched a genealogical branching of a single evolutionary tree.  Unconventionally, Darwin asked questions of fancy pigeon and animal breeders as well as established scientists.

Charles Darwin

Charles Darwin in 1860, aged 51

In late September 1838, Darwin started reading Thomas Malthus’s An Essay on the Principle of Population with its statistical argument that human populations, if unrestrained, breed beyond their means and struggle to survive. Darwin related this to the struggle for existence among wildlife and plants, so that the survivors would pass on their form and abilities, and unfavourable variations would be destroyed.  By December 1838, he had noted a similarity between the act of breeders selecting traits and a Malthusian nature selecting among variants thrown up by chance.

Darwin now had the framework of his theory of natural selection, but he was fully occupied with his career as a geologist and held off writing a sketch of his theory until his book on The Structure and Distribution of Coral Reefs was completed in May 1842.

Evolution by natural selection

Darwin continued to research and extensively revise his theory of natural selection while focusing on his main work of publishing the scientific results of the Beagle voyage.  He tentatively wrote of his ideas to the famous Scottish geologist Charles Lyell in January 1842; then in June he roughed out a 35-page pencil sketch of his theory. Darwin began correspondence about his theorising with the botanist Joseph Dalton Hooker in January 1844, and by July had rounded out his sketch into a 230-page essay, to be expanded with his research results and published if he died prematurely.

His famous 1859 book On the Origin of Species was written for non-specialist readers and attracted widespread interest upon its publication. As Darwin was already an eminent scientist, his findings were taken seriously.  The evidence he presented generated scientific, philosophical, and religious discussion. The debate over the book contributed to the campaign by Thomas Huxley and his fellow members of the X Club to secularise science by promoting scientific naturalism. Within two decades there was widespread scientific agreement that evolution, with a branching pattern of common descent, had occurred, but scientists were slow to give the mechanism of natural selection the significance that it deserved.

species divergence

Diagram representing the divergence of species, from Darwin’s Origin of Species

Darwin’s theory of evolution is based on some key facts (based on wild populations without human interference), which biologist Ernst Mayr has summarised as follows:

  • Every species is fertile enough that if all offspring survived to reproduce the population would grow.
  • Despite periodic fluctuations, populations remain roughly the same size.
  • Resources such as food are limited and are relatively stable over time.
  • Individuals in a population vary significantly from one another.
  • Much of this variation is heritable.

From these key facts, the following important inferences may be made, once again summarised by Ernst May:

  • A struggle for survival ensues.
  • Individuals less suited to the environment are less likely to survive and less likely to reproduce.
  • Individuals more suited to the environment are more likely to survive and more likely to reproduce and leave their heritable traits to future generations, which produces the process of natural selection.
  • This slow process gradually results in populations changing to adapt to their environments, and ultimately, these variations accumulate over time to form new species.

Natural selection provided a mechanism for variation and eventual speciation, but it did not explain the inheritance of variation.  Without some way to explain the inheritance of characteristics acted on by natural selection, his theory would be incomplete.

Mechanisms of inheritance

Before the advent of genetics, Hippokratic theories attempted to explain inheritance in terms of a blending of fluids extracted from all parts of both male and female bodies during intercourse.  It was thought that the characteristics of the offspring are determined by the relative amounts and strength of fluids from each part of the body of each parent.

On the other hand, ‘preformationist’ theories held that the new mammalian offspring is already preformed in miniature, either within the egg of its mother or in the semen of its father.  Both of these types of theories incorporated ‘encasement’, which was the thesis that God created all future organisms in miniature, and that reproduction was just the growth and development of these miniatures.

Hippokratic theories were very good at explaining inheritance but very bad at explaining growth and development; whilst preformationist theories were the opposite – very good at explaining growth and development but very bad at explaining inheritance.  To give some examples, Hippokratic theories were unable to adequately explain phenomena such as the regeneration of freshwater polyps; while preformationist theories were unable to adequately explain how the mating of a mare with a donkey produces a mule.

Darwin came to his hypothesis of pangenesis, from a different direction – to fill a gap left in his theory of evolution.  Darwin’s breeding experiments on domestic animals (mainly pigeons) in the 1850s and 60s were part of his attempts to complete his evolution theory.  He was attempting in these experiments to show just how quickly varying characteristics can be amplified by domestic breeding, and therefore how natural selection can operate.

Darwin called his explanation of inheritance ‘the hypothesis of Pangenesis’, which he published in 1868.  However, he provides a more succinct description of this hypothesis in an earlier unpublished manuscript on pangenesis sent to Thomas Huxley in 1865:

“Furthermore, I am led to believe from analogies immediately to be given that protoplasm or formative matter which is throughout the whole organisation, is generated by each different tissue and cell or aggregate of similar cells; – that as each tissue or cell becomes developed, a superabundant atom or gemmule as may be called of the formative matter is thrown off; – that these almost infinitely numerous and infinitely minute gemmules unite together in due proportion to form the true germ; – that they have the power of self-increase or propagation; and that they here run through the same course of development, as that which the true germ, of which they are to constitute elements, has to run through, before they can be developed into their parent tissues or cells. This may be called the hypothesis of Pangenesis”.

pangenesis

The Laws of Inheritance & Pangenesis

Darwin further proposed that his hypothesis would not only account for inheritance, but also for development:

“The development of each being, including all the forms of metamorphosis and metagenesis, as well as the so-called growth of the higher animals, in which structure changes, though not in a striking manner, depends on the presence of gemmules thrown off at each period of life, and on their development, at a corresponding period, in union with the preceding cells”.

Through these mechanisms, Darwin proposed that inheritance and development were tied together – not only in the generation of offspring and early stages of embryonic life, but throughout the life of the organism.  By giving ‘gemmules’ the power to be modified throughout the life of an organism and then be transferred to the next generation, he argued that inheritance should be viewed as a form of growth.

By means of this single hypothesis, Darwin attempted to not only fill a gap in his theory of evolution, but whether he meant to or not, he created an apparent synthesis between the then competing paradigms relating to inheritance and development.

After reading Variation Under Domestication, Francis Galton (a cousin of Darwin’s) arranged for a series of experiments to be conducted on rabbits initially housed in the Zoological Gardens of London and later at his Kensington home.  His intention was to demonstrate the transmission of ‘gemmules’ to succeeding generations via blood injected from one rabbit to another, using coat colour as a marker.  Galton ultimately found that not a single instance of induced variation of coat colour occurred in a total of 88 offspring from blood transfused parents, and in 1871 published his results in Nature.

In later editions of Variation Under Domestication, Darwin admitted in a footnote that he would have expected to find ‘gemmules’ in the blood, although their presence was not absolutely necessary to his hypothesis.  Darwin’s response is unconvincing, as he provides no alternative explanation as to how the ‘gemmules’ are transmitted from the parents’ somatic cells to the germ cells.  He made no real attempt to modify his hypothesis in response to Galton’s falsification of it, indicating a possible abandonment of commitment to his hypothesis.

After the rediscovery of Mendel’s work in the 1890s, scientists tried to determine which molecules in the cell were responsible for inheritance.  In 1910, Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies.  In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome.  It was soon discovered that chromosomes consisted of DNA and proteins, but DNA was not identified as the gene carrier until 1944. Watson and Crick’s breakthrough discovery of the chemical structure of DNA in 1953 finally revealed how genetic instructions are stored inside organisms and passed from generation to generation.

In view of the fact that it took another 85 years after Darwin’s book Variation Under Domestication before the molecular mechanisms of inheritance to be discovered, Darwin can hardly be blamed for getting it wrong way back in 1868.  This was before even chromosomes had been discovered, let alone DNA.

On the plus side, Darwin’s theory of evolution by natural selection, with its tree-like model of branching common descent, has become the unifying theory of the life sciences. The theory explains the diversity of living organisms and their adaptation to the environment. It makes sense of the geologic record, biogeography, parallels in embryonic development, biological homologies, vestigiality, cladistics, phylogenetics and other fields, with unrivalled explanatory power; it has also become essential to applied sciences such as medicine, agriculture, conservation and environmental sciences.

References

Darwin, Charles (1859) The Origin Of Species. 6th ed. 1873. London: John Murray.

Darwin, Charles (1875) The Variation of Animals and Plants Under Domestication, Vol II London: John Murray.

Mayr, Ernst (1982) The Growth of Biological Thought: Diversity, Evolution, and Inheritance Harvard University Press.

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Turning the tables: using genetic mutations to fix nature’s problems

The Conversation

Merlin Crossley, UNSW Australia

Everyone is different. That’s a simple truism, but it’s is also true when it comes to how people respond to diseases; some people are laid low and others shrug off the same ailment.

And it’s true of genetic diseases. Even when two individuals carry the same mutation, the severity of the disease may vary between them.

Sometimes this is due to environmental variation, but in other cases it reflects additional genetic changes that also influence how the disease affects that person. Some people will have other harmful mutations that combine with the main disease gene to make the condition worse, while more fortunate people may have inherited other variations, actual beneficial mutations, that reduce or even eliminate symptoms.

One of the best known illustrations of this phenomenon centres around the inherited blood disorder sickle cell anaemia. This lifelong condition is due to mutations in the adult globin gene – a point mutation in that gene renders it defective and patients suffer from anaemia throughout their lives. The symptoms can be severe. Damaged blood cells can block blood vessels leading to intense pain and even loss of life.

In the blood

But, as mentioned above, symptoms vary between individuals. Environmental variability also influences symptoms, so affected individuals may be advised to avoid high altitude and oxygen stress, for example. But genetic variations also exist. Some individuals carry a second mutation in the regulatory region of another globin gene that alleviates symptoms of sickle cell anaemia.

These individuals have a benign condition called Hereditary Persistence of Foetal Haemoglobin (HPFH). They have an “up-mutation” in the control region of a separate globin, the foetal globin gene, which boosts expression of that gene. The extra foetal globin can replace the defective beta globin.

I realise that is fairly complicated. But, put simply: humans have several globin genes. The foetal globins are turned on before birth and have a high affinity for oxygen; they enable the baby to snatch oxygen from its mother’s blood. After we are born the adult, or beta globin, gene comes on and the foetal globin gene is shut off.

But in a few people with HPFH the foetal globin gene stays on throughout life. Interestingly, this doesn’t seem to cause any health problems. Individuals with HPFH can even have normal pregnancies. They just have extra foetal globin in their blood.

The crux of the matter is this: if an individual inherits the sickle cell mutation and an HPFH mutation, they have few if any symptoms, because the extra foetal globin does the work of the defective adult globin gene.

So could one effectively “cure” sickle cell anaemia by introducing the HPFH mutation into blood cells affected by the defective adult globin gene?

Switching on the backup

Well, this is precisely the approach we have taken. Using the new technique of “genome editing”, we have introduced one of the best characterised HPFH mutations, and we find that we can successfully turn on the sleeping foetal globin gene.

At this stage we have only done this in cell lines in the laboratory. To turn this into a therapy, one would have to do it in haematopoietic stem cells – i.e. blood-forming stem cells – from the patient. It would be necessary to achieve a high frequency of editing in enough stem cells to enable repopulation of the patient’s blood with genetically enhanced cells.

Gene repair

But if it is so easy to edit the genome now, why don’t we just correct the sickle cell mutation rather than introducing a new mutation, albeit a beneficial and benign mutation?

Well, that is certainly a good strategy in the case of sickle cell anaemia, and many people are working on just that. But it may be a less ideal strategy for other blood diseases and various genetic diseases where large genes or regions of the genome are deleted.

In the case of the thalassaemias, for example, many different gene deletions occur. It may not be practical to edit in large gene replacement cassettes, and one would have to design a different insert for each mutation. In contrast, building in the foetal globin activating mutation should provide additional globin and work to compensate in many of these conditions.

Towards gene therapy using genome editing

A new age of genetic engineering is beginning, due to the ability to edit the genome using new DNA-cutting tools, with the technical names: CRISPRs, TALENs and ZFNs.

Gene correction or the introduction of beneficial mutations may be important in treatments in the future.

In agriculture they may also be important. Many genome wide association studies have identified beneficial mutations associated with particular prised qualities. Genome editing can also be used to introduce beneficial mutations in this context and may give rise to a new generation of crops and livestock.

The techniques are also interesting because no new or artificial material need be introduced. All one is doing is mimicking a naturally occurring beneficial mutation. The introduction of artificial transgenes has alarmed some parts of society.

Additionally, transgenes are recognised as foreign by some organisms and are shut down by epigenetic silencing, just as computer viruses are recognised and shut down by anti-virus software.

Beneficial mutations are unlikely to be subject to the same limitations. They are already known to work in nature and introducing them to improve human health or in agriculture may have many advantages.

The ConversationMerlin Crossley is Dean of Science and Professor of Molecular Biology at UNSW Australia.

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.


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Bird tree of life shows ‘explosive evolution’: studies

The Conversation

By Bryonie Scott, The Conversation and Tessa Evans, The Conversation

Today’s land birds, from ducks to eagles, shared a common ancestor after dinosaurs went extinct – just one finding from bird gene studies published in journals, including Science and GigaScience, today.

Genetic data of 48 bird species were sequenced in a massive international collaboration to create a new and detailed version of the avian tree of life.

“Birds have always been very good for this kind of work because we have a greater understanding about the world’s birds than we do about any of the other vertebrate groups,” Simon Griffith, Associate Professor of Avian Behavioural Ecology at Macquarie University, said.

“We are familiar with almost all the birds in the world – around 10,000 species – and we know how they differ, in important characteristics such as how long they live, how many offspring they have each year, how old they are when they breed.”

About 66 million years ago, a mass extinction event wiped out around 80% of Earth’s plant and animal population, but opened the door for the rapid expansion of birds.

Giant terror birds are similar to a common ancestor of all land birds. Marcelo Braga/Flickr, CC BY

Only a few bird lineages survived the mass extinction, and most modern land birds such as songbirds, owls and woodpeckers share a common apex predator ancestor.

This top-of-the-food-chain brute was similar to the giant terror birds which stalked the Americas between 27 million and 15,000 years ago.

“The main problem in resolving the relationships among birds is that they diversified very quickly,” Research Director and the Curator of the Australian National Wildlife Collection, CSIRO, Leo Joseph said.

Extreme adaptations

Birds are among the most widespread land animals and have experienced evolutionary adaptations to extreme environments. Penguins live in some of the harshest conditions on Earth, and DNA analysis, published in GigaScience, confirmed fossil evidence that penguins first appeared in Antarctica around 60 million years ago.

“If you have a complete genome, you can compare the variations between the chromosomes and get a picture of the population history” Sankar Subramanian, a research fellow at Griffith University and who worked on the project, said.

“By comparing the complete genome of penguins living today we can track when evolutionary changes occurred, up to 200,000 years ago.”

This aspect of the research, Emperor penguins were found to have a stable population, but Adélie penguins present a very different story, showing fluctuations in population matching extreme climactic periods.

In a warmer period 150,000 years ago there was a large population explosion, but in a more recent glacial period the population declined dramatically.

Adélie penguins struggle in glacial periods as they require ice-free land for nesting. Dominique Génin/Flickr, CC BY-NC-ND

Rather than being evolutionary in origin, Dr Subramanian explained the fluctuations depended on “how much ice-free land is available for nesting and breeding”.

Compared to other bird species, penguins had more genes for lipid metabolism, which is essential for forming layers of blubber to withstand the cold.

This subgroup of the project hopes to look at penguins which live in tropical and temperate waters such as the Galápagos Islands and New Zealand to see how more recent evolutionary adaptations have affected genes of modern penguins.

Now that these hallmark studies have been completed, and the bird tree of life established with some degree of authority, it provides a scaffold for further research.

“This new study helps tease apart the rapid diversification of birds, which has been a long-standing problem,” Dr Joseph said.

“We can now address really interesting questions,” Associate Professor Griffith said. “When did song learning evolve? When did birds evolve different patterns of parental care?”

These insights provide sought-after answers about how species can diversify so quickly to fill the ecological niches.


This articled was edited on Friday December 12, 2014, to clarify some quotes by Dr Leo Joseph.

The ConversationThis article was originally published on The Conversation. (Republished with permission). Read the original article.

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Theories of Generation

by Tim Harding

Does Darwin successfully combine the two explanatory paradigms of inheritance, and growth and development?  If he does, is this a problem for Kuhn’s view of scientific change?

In this essay, I intend to show how Darwin’s hypothesis of pangenesis created a synthesis between the then competing paradigms relating to inheritance and development.  I think that this synthesis does present a problem for Kuhn’s view of scientific change, and that Popper’s account may actually be more relevant than Kuhn’s to certain aspects of this particular case study.

Before discussing Darwin’s theories, it is necessary to briefly describe the two paradigms that attempted to explain inheritance (how characteristics are transferred to offspring from parents), and growth and development (how the growth of offspring is organised).  These two paradigms were characterised by competing Hippocratic theories and preformationist theories (Verdnik, 1998:126, 157).

Before the advent of genetics, Hippocratic theories attempted to explain inheritance in terms of a blending of fluids extracted from all parts of both male and female bodies during intercourse.  It was thought that the characteristics of the offspring are determined by the relative amounts and strength of fluids from each part of the body of each parent (Verdnik, 1998:126).

On the other hand, preformationist theories held that the new mammalian offspring is already preformed in miniature, either within the egg of its mother (ovist preformationism) or in the semen of its father (animalculist preformationism).  Both of these types of theories incorporated emboîtment (encasement), which was the thesis that God created all future organisms in miniature, and that reproduction was just the growth and development of these miniatures (Verdnik, 1998:128).

Hippocratic theories were very good at explaining inheritance but very bad at explaining growth and development; whilst preformationist theories were the opposite – very good at explaining growth and development but very bad at explaining inheritance (Verdnik, 1998:126).  To give some examples, Hippocratic theories were unable to adequately explain phenomena such as the regeneration of freshwater polyps as observed by Trembley (Trembley 1744: 148); while preformationist theories were unable to adequately explain how the mating of a mare with a donkey produces a mule (Maupertuis 1745: 172).

Darwin came to his hypothesis of pangenesis, from a different direction – to fill a gap left in his theory of evolution, as published in his 1859 book The Origin of Species (Darwin, 1859).  Natural selection provided a mechanism for variation and eventual speciation, but it did not explain the inheritance of variation.  Without some way to explain the inheritance of characteristics acted on by natural selection, his theory would be incomplete.  Darwin’s breeding experiments on domestic animals (mainly pigeons) in the 1850s and 60s[1] were part of his attempts to complete his evolution theory (Bartley, 1992: 308).  He was attempting in these experiments to show just how quickly varying characteristics can be amplified by domestic breeding, and therefore how natural selection can operate (Verdnik, 1998:156).

evolution_peas

The Laws Of Inheritance & Pangenesis

Darwin called his explanation of inheritance ‘the hypothesis of Pangenesis’, which he published in Part II of Chapter XXVII of Variation Under Domestication (Darwin, 1875: 374-404).  However, he provides a more succinct description of this hypothesis in an earlier unpublished manuscript on pangenesis sent to Huxley in 1865:

‘Furthermore, I am led to believe from analogies immediately to be given that protoplasm or formative matter which is throughout the whole organisation, is generated by each different tissue and cell or aggregate of similar cells; – that as each tissue or cell becomes developed, a superabundant atom or gemmule as may be called of the formative matter is thrown off; – that these almost infinitely numerous and infinitely minute gemmules unite together in due proportion to form the true germ; – that they have the power of self-increase or propagation; and that they here run through the same course of development, as that which the true germ, of which they are to constitute elements, has to run through, before they can be developed into their parent tissues or cells. This may be called the hypothesis of Pangenesis’ (Olby 1963: 259).

Darwin further proposed that his hypothesis would not only account for inheritance, but also for development:

‘The development of each being, including all the forms of metamorphosis and metagenesis, as well as the so-called growth of the higher animals, in which structure changes, though not in a striking manner, depends on the presence of gemmules thrown off at each period of life, and on their development, at a corresponding period, in union with the preceding cells ‘ (Darwin, 1875: 403-404).

Through these mechanisms, Darwin proposed that inheritance and development were tied together – not only in the generation of offspring and early stages of embryonic life, but throughout the life of the organism (Bartley, 1992: 310).  By giving ‘gemmules’ the power to be modified throughout the life of an organism and then be transferred to the next generation, he argued that inheritance should be viewed as a form of growth (Bartley, 1992: 331).

By means of this single hypothesis, Darwin not only filled a gap in his theory of evolution, but whether he meant to or not, he created a synthesis between the then competing paradigms relating to inheritance and development.

The preceding discussion raises some significant philosophical issues.  How does it compare with Kuhn’s view of scientific change; and in particular, is Darwin’s paradigm synthesis a problem for the Kuhnian account?

According to Kuhn, ‘normal science’ operates within scientific paradigms[2] that not only determine which scientific theories are acceptable, but define scientific communities and even the areas of research undertaken (Kuhn, 1962: 10-11).

Kuhn says that in so far as he is engaged in normal science, the researcher is a solver of puzzles, not a tester of paradigms.

Paradigm-testing occurs only after persistent failure to solve a noteworthy puzzle has given rise to crisis.  And even then it occurs only after the sense of crisis has evoked an alternative candidate for a paradigm.  In the sciences, the testing situation never consists, as puzzle-solving does, simply in the comparison of a single paradigm with nature.  Instead, testing occurs as part of the competition between two rival paradigms for the allegiance of the scientific community (Kuhn, 1962: 144-145).

Kuhn notes that the schools guided by different paradigms are ‘always slightly at cross-purposes’ and that they ‘fail to make complete contact with each other’s viewpoints’, a phenomenon which describes as ‘incommensurability’ (Kuhn, 1962: 112, 148-150).  The failure of the adherents of Hippocratic and preformationist theories to focus on the strengths and weaknesses of both paradigms, and to engage in the testing of both paradigms, tends to support Kuhn’s notion of incommensurability.  No crisis occurred within each paradigm, because their adherents failed to focus on how well each paradigm explained both inheritance and development.

The formulation of Darwin’s hypothesis does not accord with the Kuhnian account.  He did not conduct his breeding experiments on domestic animals to test either the Hippocratic or the preformationist paradigms, but to fill a gap in his quite separate theory of evolution.  Similarly, his synthesis between the then competing paradigms relating to inheritance and development, was not done to resolve any crisis in these paradigms, but for a separate purpose.  A further problem for the Kuhnian account is that the scientific method Darwin used to formulate his hypothesis was essentially inductive, although this may also be a problem for the Popperian account.

Another aspect of the Kuhnian account is that the failure of experimental results to conform to the prevailing paradigm is seen as an ‘anomaly’, rather than as an instance of Popperian falsification.[3]  If scientists are confronted by anomalies, they will often devise ad hoc modifications in order to eliminate any apparent conflict.  Anomalies do not cause the abandonment of paradigm or theory unless and until the level of anomalies builds to a crisis where the prevailing paradigm or theory is replaced by a new one (Kuhn, 1962: 77-78).

After reading Variation Under Domestication, Francis Galton (a cousin of Darwin’s) arranged for a series of experiments to be conducted on rabbits initially housed in the Zoological Gardens of London and later at his Kensington home.  His intention was to demonstrate the transmission of ‘gemmules’ to succeeding generations via blood injected from one rabbit to another, using coat colour as a marker.  Galton ultimately found that not a single instance of induced variation of coat colour occurred in a total of 88 offspring from blood transfused parents, and in 1871 published his results in Nature (Brown, 2002: 290-291).  Although intended to verify Darwin’s hypothesis, it seems that Galton’s experiments provided a Popperian falsification instance.  Whether the purpose was verification or falsification, Galton’s experiments do not accord with Kuhn’s account of paradigm-testing discussed above.

In later editions of Variation Under Domestication, Darwin admitted in a footnote that he would have expected to find ‘gemmules’ in the blood, although their presence was not absolutely necessary to his hypothesis (Brown, 2002: 292).  I find Darwin’s response unconvincing, as he provides no alternative explanation as to how the ‘gemmules’ are transmitted from the parents’ somatic cells to the germ cells.  He made no real attempt to modify his hypothesis in response to Galton’s falsification of it.

I therefore conclude that whilst the Hippocratic and preformationist paradigms accord with Kuhn’s notion of incommensurability, Darwin’s synthesis of these two paradigms does not accord with the Kuhnian account of science.  Furthermore, Darwin’s failure to make an ad hoc modification to his hypothesis after the discovery by Galton of an anomaly, as would be expected under the Kuhnian account, supports a Popperian rather than a Kuhnian account of the scientific process in this case.

References:

Bartley, M.M. (1992) Darwin and Domestication: Studies on Inheritance. Journal of the History of Biology, Vol 25, No. 2 pp.307-333.

Browne, J. (2002) Charles Darwin – The Power of Place, Volume II of a Biography. London: Pimlico.

Darwin, C. (1859) The Origin Of Species By Means Of Natural Selection, Or The Preservation Of Favoured Races In The Struggle For Life. 6th ed. 1873. London: John Murray.

Darwin, C. (1875) The Variation of Animals and Plants Under Domestication, Vol II London: John Murray.

Kuhn, T.S. (1962) The Structure of Scientific Revolutions 3rd ed. Chicago: University of Chicago Press.

Maupertuis, P. (1745) The Earthly Venus in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Olby, R.C. (1963) Charles Darwin’s Manuscript of Pangenesis, British Journal for Philosophy of Science 1: 251-263.

Popper, K. (1959) The Logic of Scientific Discovery. rev. ed. 1968, in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Trembley, A. (1744) Memoires pour server a l’histoire d’un genre de polypes d’eau douce in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

 Endnotes:

[1] Darwin reported the results of these experiments in both The Origin Of Species and The Variation of Animals and Plants Under Domestication.

[2] Kuhn does not provide a concise definition of a scientific paradigm, but the Merriam-Webster Online dictionary defines it as ‘a philosophical and theoretical framework of a scientific school or discipline within which theories, laws, and generalizations and the experiments performed in support of them are formulated; broadly: a philosophical or theoretical framework of any kind.’

[3] Popper proposes falsifiability as the criterion of demarcation between scientific and non-scientific statements.  In other words, a theory is scientific if it is falsifiable (Verdnik, 1998:76).

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