The Luria's 1943 Fluctuation Test

1
THE
LURIA-­‐DELBRÜCK
EXPERIMENT
The Luria-Delbrück Experiment
Henry Paige
University College Freiburg
2
THE
LURIA-­‐DELBRÜCK
EXPERIMENT
Abstract
This paper discusses Luria and Delbrück’s 1943 Fluctuation Test. Historical background is first addressed to contextualize the experiment and define the theories, ideas, and discussions relevant at the time of the experiment. A brief summary of biologists’ knowledge and attitudes towards bacteria is explained through contemporary quotes. The two main competitive theories of evolution, (mutation/natural selection and acquired inheritance) are described and evaluated.. Luria and Delbrücks two hypotheses (induction and mutation) are identified and explained with reference to the two theories. These
3
THE
LURIA-­‐DELBRÜCK
EXPERIMENT
Introduction
What were the most important scientific experiments ever conducted? There are famous and familiar ones such as Pavlov’s dog or Jenner’s cowpox vaccine, but perhaps the answer is better understood contextually. When one considers a field more closely, several important steps in understanding become apparent. In the study of living things, the Luria and
Delbrück experiment stands out for its unique role in opening the door to genetics and microbiology. Though it initially made little impact on the natural sciences, it would come to be recognized as a cornerstone in the development of genetics and microbiology. Luria and
Delbrücks fluctuation test showed that mutations arise absent from selection, consequently establishing that Darwinian natural selection acted on bacteria.
Background
To fully recognize the significance of Luria and Delbrück’s experiment, it is important to first understand the historical context. By the 20th century most scientists had come to accept the ideas of inheritance and evolution, drawing heavily on the work of Gregor
Mendel and Charles Darwin (“Random Mutations,” 2014). There remained however
Interpretation and Legacy
Experiment Inferences
The fluctuation test had identified several key traits of bacteria as well as their theoretical significance. They had proven that bacteria did indeed undergo mutation, and that these mutations could be passed on to future generations. This meant that bacteria had genes at least somewhat analogous to other creatures, and obeyed similar hereditary rules. More importantly, they showed that mutations occurred randomly and separately from the environment, rather than being induced by the environment. This effectively obsoleted
Lamarckian theories of inheritance and showed that Darwinian natural selection also applied to microscopic life. This insight into the vehicle of evolution was a critical point in the understanding of biodiversity.
Luria and Delbrück’s Legacy
Luria and Delbrück also helped in part to start the new era of microbiology. In 1969 they shared the Nobel Prize in Physiology or Medicine with Alfred Hershey as a result of
11
THE
LURIA-­‐DELBRÜCK
EXPERIMENT their work with phages (Karolinska Instutute, 1969), particularly the fluctuation test. They further collaborated at the Cold Springs Harbor laboratory, where they instituted a