Darwin, Then and Now

Posts Tagged ‘genetics’

In 1943, published in a paper entitled “Mutations of Bacteria from Virus Sensitivity to Virus Resistance,” microbiologist Salvador Luria, biophysicist Max Delbrück, and bacteriologist and geneticist Alfred Hershey discovered that mutations occur at a constant rate. In 1969, they were awarded the Nobel Prize in Physiology and Medicine “for their discoveries concerning the replication mechanism and genetic structure of virus.”

The Luria-Delbrück Experiment opened the question, are mutations inherent to microbes for the purpose of adaption to rapidly changinging environments and not for evolution? While microbe resistance through mutation is a logical mechanism for evolution, the reality is the bacteria have remained a bacteria and the virus has remained a virus. Preexistent genetic variants determine the range of mutations. Pierre-Paul Grassé, president of the French Academy of Sciences, observed, “bacteria, the study of which has formed a great part of the foundation of genetics and molecular biology … stabilized a billion years ago.”

The question is whether the mutations are the “raw material for evolution” or nature’s means for the microbes to adapt to the environment. In a 2009 review article by entitled “Darwinian evolution in the light of genomics”, published in Nucleic Acid Research, Eugene V Koonin concluded, “There is no consistent tendency of evolution towards increased genomic complexity” through mutation as expected with current evolutionary theories. Mechanisms of evolution remain beyond any known natural law.

Reflecting on the role of mutations, Grassé questioned, “What is the use of their unceasing mutations if they do not change?” Grassé concludes, “the mutations of bacteria and viruses are merely hereditary fluctuations around a median position; a swing to the right, a swing to the left, but no final evolutionary effect.” Microbes undergo constant mutations, but do not evolve – mutation stasis.

*Nucleic Acids Res. 2009 March; 37(4): 1011–1034 

Pangenesis was Darwin’s hypothetical mechanism for the origin of variation and inheritance through particles called gemmules. This “provisional hypothesis” on the origin of variation was presented in his 1868 work The Variation of Animals and Plants under Domestication through gemmules acquiring new variations that brings “together a multitude of facts which are at present left disconnected by any efficient cause”.

The etymology of pangenesis comes from the Greek words pan (a prefix meaning “whole”, “encompassing”) and genesis (birth) or genos (origin). Gemmules were thought to learn from experiences.

The origin of new variations was critical for Darwin’s theory since the “slight, successive” changes in evolution requires a constant stream of new variations for the actions of natural selection. Gemmules were imagined particles. These learned gemmules particles sent from every cell (pan) in the body with new variations (genos) accumulated in the germ cells and had a ‘vote’ in the constitution of the offspring (genesis).

This hypothesis provided a possible mechanism for the inheritance of acquired characteristics, as proposed by Jean-Baptiste Lamarck, which Darwin believed to be the origin of new variations in living organisms.

Little did Darwin know that even before the publication of the fourth edition of The Origin of Species in 1866, Gregor Mendel had presented the now-famous paper entitled “Experiments on Plant Hybridization,” laying the foundations of modern genetics.

Although, Mendel’s discovery went unnoticed until the turn of the twentieth century, German biologist August Weismann, at the University of Freiburg, launched the first scientific evidence directly challenging Darwin’s theory. Now known as the “Weisman Barrier,” in 1883, Weismann cut off the tails of mice from 21 generations. Seeing that the 22^nd generation still had tails, Weismann concluded that the evidence contradicted Darwin’s theory of pangenesis despite obvious reasons for change in the mice, “continuity” was observed, not new variations.

Ernst Mayr, Darwin’s twentieth-century bull-dag, stated Weismann as “The second most notable evolutionary theorist of the 19th century.” What is still unresolved now 150 years later is—what is the origin of variation?

Charles Darwin used the term species more than any other term in The Origin of Species— 1,926 times. Defining the term species, however, has been a problem. Darwin wrote, there “is no possible test but individual opinion to determine which of them shall be considered as species”.

Naturalist Henry Alleyne Nicholson explains, “No term is more difficult to define than ‘species,’ and on no point are zoologists more divided than as to what should be understood by this word.”

Geneticist Laura M. Zahn in looking for the genetic distinction between species, published there results in paper entitled “Background Matters” in the October 2009 edition of Evolution and abstracted in Science. Much to their surprise, Zahn and colleagues discovered that no single genetic allele is known to exist that can define the difference between species.

After 150 years, Darwin’s statement continues to be dead on, “It is all-important to remember that naturalists have no golden rule by which to distinguish species.”

Ironically, a book on the origins of an indefinable term, then and now, ascended into a historical phenomenon.