By Mano Singham
In Darwin's own time, there was a three-way dispute concerning the theory of evolution. Strange as it may sound these days in the US where so many question whether evolution even occurs at all, the idea that evolution had occurred and new species were being created and old ones dying out was not such a major problem in the mid-to-late 19th century. Elite opinion of that time had been exposed to that idea and had accepted it even before Darwin because of all the fossil records that were being discovered all the time. Even Darwin's own grandfather Erasmus Darwin, a freethinker, had around 1795 published a book Zoonomia that had floated the idea that species had evolved, but he used a Lamarckian model. What religious people mostly shied away from was the idea that human beings were also part of the evolutionary process and shared common ancestors with other species, a reluctance that still persists.
In Darwin's time what the dispute mainly centered on was the mechanism of evolution and how it operated.
Apart from Biblical literalists and believers in special creation, there were those of a religious bent who argued that god had to intervene somehow at least occasionally to create new species (especially humans) and this view persists down to this day among people who seek a tangible role for religion. At the very least, believers in an immortal soul needed a god to insert it into humans at some point in that person's development.
But the more interesting dispute was among those scientists who were not invoking religious ideas. Their dispute centered on the scale of the mutations necessary for natural selection to work.
Due to all the buffeting that Darwin's theory had received from those who argued that the age of the Earth was too short for evolution to have occurred and that mutations would get blended away, in the later editions of his book, Darwin himself started qualifying some of the more ambitious claims that he had forthrightly stated in the 1859 edition. As a result, his later editions lost some of the directness and clarity of his first edition, and scholars now recommend reading the first edition as being the best. I personally found it a fascinating book, remarkably accessible to the layperson.
For example, his first edition contained a rough estimate by him, based on geological phenomena he observed in England, that the Earth was about 300 million years old, which was in his view sufficient time for evolution to have occurred. He arrived at this by assuming that the Weald, a valley in the south of England, had been created by erosion that had always occurred at the same rate it was occurring now. He removed this claim in later editions, presumably due to unease over physicist William Thomson's calculations that the Earth was only 30 million years old. As it turns out, Darwin had no need to be worried since the current age of the Earth is calculated to be more than ten times his own estimate.
But while willing to give ground on some peripheral issues, Darwin steadfastly stuck until his death (in 1882) to one central idea, and that was that natural selection was able to act on even extremely small advantages in the fitness of some organisms, causing them to grow in the population, and that it was the cumulative effect of these minute changes that led to new species.
Contrasted with Darwin's continuous model of change were those scientists (including even Darwin's staunch defender and ally Thomas Huxley) who argued that natural selection could not really work with very tiny changes because they would get washed away because of blending inheritance. These people argued in favor of a discontinuous model which only valued those mutations that produced significant changes in the organism that represented a new and stable phenotype whose qualities were robust enough that they would not get blended away by breeding.
To better understand the difference, compare a sphere and (say) a twenty-sided die, which is almost a sphere, both resting on a table. The sphere can be shifted by any small amount and would stay in that new position. The die on the other hand, if tilted slightly and released, would revert to its original position unless the tilt were sufficient to topple it to rest on the adjacent flat face. Then it would be stable in the new position and would resist any further shift, even back to its original state. One faction led by Darwin was arguing that natural selection could act on the continuous changes represented by the sphere while others said that only the changes beyond a certain critical amount and represented by the die were stable enough for selection to work on.
It must be emphasized that both sides supported the mechanism of natural selection for driving evolution. They simply disagreed on its ability to act on very small changes. While we may think that this was a small issue to disagree on, in actual fact the debate was fierce and very acrimonious, with both sides trying to marshal evidence for their side and picking holes in the evidence of their opponents. William B. Provine in his book The Origins of Theoretical Population Genetics (2001) gives a fascinating account of this controversy, the personalities involved, and the heated nature of the exchanges, which grew increasingly bitter by the time 1900 rolled around.
The rediscovery of Mendel's work on genetics (he was a monk who lived from 1822-1884 and published his major work in 1865, but it remained obscure until it was rediscovered in 1900) provided new fuel to the controversy. Scientists quickly recognized the significance and importance of Mendel's work. While Mendel's model was accepted as having finally produced the correct theory of how inheritance works, this did not immediately resolve the dispute because there was still disagreement about what Mendel's theory actually implied and how it fitted into Darwin's theory.
Next in this series: The synthesis of Mendelian genetics and natural selection.
(Please see here for previous posts in this series.)