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The Eclipse Of Darwin

Like many revolutionary scientific theories, Darwin's ideas were not accepted immediately. His theory suggested that the origins of life are both cruel and blind - relying on death and competition - not everyone's cup of tea! There were also plenty of rival theories around at the time. Read on to see what all of the arguments were about - and how Darwin was proved to be right!

Darwinism Is Delayed

Darwinism is evolution by natural selection. Before Darwin published On the Origin of the Species, many scientists, mainly inspired by the fossil record, were starting to consider that one species might give rise to another. This is evolution. The alternative is continuous creation by a supernatural agency. Darwin succeeded in persuading most of his contemporaries that evolution can and does happen. However his mechanism of natural selection was not so well received. This was mainly due to the sheer unpleasantness of the theory. Natural selection works only if better-adapted individuals have more surviving offspring. The characteristics that make them more adapted thus spread through the population. The flip side of this is that less well-adapted individuals have fewer offspring, perhaps none. For example, the gazelle is fast because lions have been eating the slowest gazelles for thousands of generations.

Additionally, natural selection is not only cruel, it is blind. There is no guiding purpose, no divine will, only the endless early deaths of the least fit. There were also some practical problems. One was the mechanism of hereditary. Darwin believed that variation was continuous. The problem with this, if it were true, is that any new variation would always be diluted. Suppose we had a population of white animals and it becomes advantageous to be black. A black mutant will be successful and have more offspring. However, the offspring will have a white parent also and so will be grey. Their children will be light grey and eventually the mutation is diluted to nothing.

Rival Theories Of Evolution

Scientists who were reluctant to accept natural selection were forced to look for an alternative theory. They found it in the earlier ideas of Jean-Baptiste Lamarck: the inheritance of acquired characteristics. This is the idea that changes which take place during life - like the hardening of the hands of a workman - are inherited by the offspring. Neo-Lamarkism was preferable to many, as it meant that living organisms could direct their own evolution; it was a more hopeful vision. In addition, as the whole species might be working towards the same goal, the required new characteristic would appear repeatedly, rather than having to spread through the population froma single individual. Dilution was thus not a problem.

Darwin had believed that Lamarckism might have a role in evolution. Weismann, a later follower of Darwin, argued that natural selection alone was sufficient and plausible as a driving force for evolution. For Lamarckism to work, in order for changes to be inherited, every part of the body must be involved in making sperm and eggs. This was widely assumed to be the case. Weismann believed that the body was constructed according to information stored in the ‘germ plasm’, which was then replicated to produce offspring. We now know that this is the case; the ‘germ plasm’ is the DNA within the reproductive organs. When we are conceived, the genetic material from our parents is combined to produce our genome, which then directs the construction of our bodies. This DNA cannot be changed in our lifetime.

Weismann demonstrated the failure of Lamarckism in a famous experiment. He cut off the tails of mice and allowed them to reproduce. He repeated this for many generations. There was no change in the average length of tail. This showed that tailless mice still had the information to produce tails, meaning that the tails were not involved in making new mice. This implied that Weismann’s theory was correct and that Lamarckism could not occur. Unfortunately, this did not convince many neo-Lamarckians. Forced to choose, most instead rejected natural selection. By the start of the 20th century, Darwinists were in a minority.

Another rival theory developed at around this time. This was orthogenesis, which proposed that evolution was directed by internal forces. This predicted that new variations appear non-randomly and are directed towards a particular end. Orthogenesis, however, could not explain how some traits exist that are not fully adaptive, or that in the long run end up driving a species to extinction. One example was the Irish Elk, which is thought to have gone extinct because its antlers grew too large, as they imposed a heavy energetic cost and made the species more vulnerable to predators. The theory doesn’t explain why traits which are costly would exist. It also did not explain how complex structures appeared in the first place, just how they changed. Orthogenesis lingered on for a while, complicating the evolution debate even further.

Genetics And Mutation

In the 1860s, Gregor Mendel, an Austrian monk, had proposed that characteristics are inherited as discrete units, what we would now call genes. For example, human eyes come in a set variety of colours. These characteristics are not altered by the environment of the parent, which rules out Lamarckism. Mendel’s work was ignored in his lifetime, as people still believed in the blending theory of heriditary. In 1900, Carl Correns and Hugo De Vries rediscovered the Mendelian theory of genetics and promoted it. It soon achieved widespread acceptance among laboratory biologists, thanks to more experiments.

Mendelian genetics was the saviour of Darwinism. If characteristics are inherited as genes, natural selection can occur, as beneficial new variation cannot be diluted into nothing. Unfortunately, the geneticists did not attempt to reconcile their work with Darwinism. They believed that nature produced new genes by random mutations, giving rise to new species. They thought that selection was not involved in creating new characteristics, apart from eliminating the unviable mutants. If this were true, a new species would appear in a single generation from a single new gene. The Mendelians had finished off Lamarckism, by showing how genes are inherited regardless of environment. However, they were equally scornful towards natural selection, seeing it as another old theory. On the other hand, the Darwinists could see that many characteristics, such as height in humans, are continuous. They insisted that complex organs must be assembled gradually by small changes, as a single mutation would be too improbable. The result was that Mendelian genetics was established as a rival to Darwinism, rather than another part of the same puzzle.

But who was right and why?

Written by Joe Walmswell

References & Further Reading

Evolution: The History of an Idea
by Peter Bowler, University of California Press: 1983

The Evolution of Darwinism
by Timothy Shanahan, CUP: 2004

This Is Biology
by Ernst Mayr, Harvard University Press: 1997