(This series of three posts is taken from a review article I am planning to submit for publication. I have removed references from the posts, but will be happy to supply them on request.)
The idea that the source of variation in individuals of a species is random and not in any way directed did not come from Darwin. In the Origin of Species, he states:
“I have hitherto sometimes spoken as if the variations…were due to chance. This, of course is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation.”
As the quote demonstrates, Darwin simply had no idea, and more importantly, the distinction between chance and purpose really had no direct consequence on the general theory.
The issue of randomness or chance is closely tied in with one of the most essential questions in biology, is there a purpose or direction to evolution? While the standard answer among most biologists is no, there is an increasing amount of evidence that the standard dogma might be wrong. The existence of teleology in nature, and in evolution has not been ruled out, and the work of Simon Conway Morris on convergence, and his demonstration that evolution in fact follows fairly narrow pathways, restricted by biological constraints supports the idea of reexamining this question. Others, such as Francisco Ayala have found evidence for teleology in the very nature of adaptive change.
The Neo Darwinian Synthesis, and Alternatives
In the middle of the 20th century, even before the discovery of DNA as the genetic molecule, biologists were examining mutations in experimental systems of bacteria, to answer questions about purpose and chance in mutation production. Do bacteria tend to specifically mutate those genes that would help them survive an environmental stress, such as starvation or exposure to toxic drugs, or do they simply generate random mutations, and then selection chooses which ones allow for increased fitness? Luria and Delbruck seemed to have answered this question in the 1940s with an elegant system called fluctuation analysis . The results of these experiments were clear: mutations were random, and then selected for their relative fitness. This finding contributed to the emerging neo-Darwinian synthesis, with molecular genetics playing the key role in the production of phenotypic variation, and with a confirmation of the idea that purpose is replaced by chance in the mechanism of the first stage of evolution.
This idea became engrained in the biological dogma, and as more and more data regarding the nature of genes and how they operate and change became available, the prevailing consensus grew stronger. Evolution became a theory that neither required, nor admitted to any degree of purpose or design. . The proponents of Neo-Darwinism maintain that evolution is a slow, continuous process wherein the effects of small scale point mutations accumulate gradually over vast stretches of time to produce changes in the phenotype of species, as well as lead to the emergence of new species and even new genera and phyla.
But there now exist a great deal of new data and new theoretical constructs that are chipping away at the standard neo-Darwinian paradigm. The eventual acceptance of the neutral drift theory led to a modification of the role of positive adaptationist mutations as the only drivers of evolutionary change, especially as related to population genetics and microevolution.
The ideas of Mayr and Gould on punctuated equilibrium, first roundly rejected by dogmatic neo-Darwinians such as Richard Dawkins, have been debated for decades, but there is now emerging a body of solid evidence that provides strong molecular mechanisms for them. The fossil record seems to show long periods (on the scale of hundreds of millions of years) of very little change, “punctuated” by remarkable brief “moments” (in geological time) of explosions of new forms. The Cambrian explosion is the best known of these, but there are many other examples. While the paleontological data is consistent with brief moments of very dramatic changes, no molecular mechanisms were put forward to explain how this could have happened. That has now changed.