Posts Tagged ‘origin of variation’

Charles Darwin envisioned evolution proceeding by an accumulation of changes, “I do believe that natural selection will generally act very slowly, only over long periods of time…. natural selection acts slowly by accumulating slight, successive, favorable variations.”

Over the past 150 years since the publication of The Origin of Species, the focus of research has centered on discovering the origin of the variation. Darwin clearly acknowledged ignorance on this subject, our “ignorance of the laws of variation is profound. Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied.”

Neo-Darwinism, otherwise known as the Modern Synthesis proposed that mutations originated the variations required for natural selection to act.

Now in the aftermath of the gene-centric Modern Synthesis meltdown, evolutionary scientists have been forced to explore new avenues beyond mutations to account for the origin of new and novel variations. At the top of the evolution industry list of potential candidates are self-organizational theories. Continue Reading

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?