of microbes (a mutation) or if the conditions caused the new characters to appear in the
cells of the population (the Lamarckian explanation). In 1934 I. M. Lewis summarized
reports of experiments on Bacillus coli mutabile and concluded:
The subject of bacterial variation and heredity has reached an almost hopeless state of
confusion. Almost every possible view has been set forth and there seems no reason to hope
that any uniform consensus of opinion may be reached in the immediate future. There are
many advocates of a Lamarckian mode of bacterial inheritance while others hold to the view
that it is essentially Darwinian. The early workers regarded variation in so-called mutabile
strains as mutation in the sense of DeVries. Some more recent workers have explained this
behavior as due to Mendellian segregation, while others have regarded it as evidence of a
cyclogenic life history. (Lewis 1934)
Salvador Luria and Max Delbrück solved the dilemma in 1943 by publishing a study
that showed that mutations occurred in populations of Escherichia coli before
selection was applied to the cultures, thus demonstrating that Lamarckian evolution
was not occurring. This work spurred a modest increase in interest in examining the
genetic aspects of bacterial evolution through laboratory culturing (Atwood et al.
1951; Novick and Szilard 1950). This ironically occurred at nearly the same time
that those who had long-standing interests in bacterial evolution were giving up hope
of ever finding characters that would allow one to trace evolutionary events.
Lab evolution of bacteria was used for many years to evolve strains to improve or
create enzyme activities or to enhance the ability of cells to catabolize new substrates
or detoxify environmental pollutants (Hegeman and Rosenberg 1970; Liu and Suflita
1993; Campbell et al. 1973). Those studies, however, did not attempt to test or apply
Darwinian principles.
Dallinger’s challenge was finally taken up by Richard Lenski (Fig. 2.10). Lenski
conducted his postdoctoral research examining genetic diversity of aphids (Service
and Lenski 1982). Later, he looked for an experimental system that would allow
more direct examination of evolutionary principles, and he hit upon using bacteria,
specifically E. coli. He published several papers about the genetic changes necessary
to elicit the appearance of phage-resistant mutants in a population, evolution in
response to thermal stress, and gene stability. He developed an interest in the course
of evolution in a population. How quickly do traits change, are the patterns of change
repeatable, what is the effect of contingency on the mode of evolution? He realized
that by maintaining cultures of E. coli for many generations, he could perhaps
observe changes in the population and, so, address these questions (Lenski 2011).
In February 1988, he started 12 cultures growing in a glucose-limited defined
medium that he transferred daily after each had exhausted its glucose. Each daily
cultivation provided six to seven generations, and he originally planned to grow the
cultures for about 2000 generations, or less than a year. This Long-Term Evolution
Experiment (LTEE) has now been running for over 32 years and has produced over
70,000 generations (Good et al. 2017; Lamrabet et al. 2019).
Cultures in the LTEE have unexpectedly shown great similarity with one another
in their increases in fitness (defined as ability to outgrow the parent culture) and have
generally improved quickly and then slowed in later generations (Lenski 2011). The
size and shape of cells changed along with their preference for carbon sources.
2 Darwin’s Science’s Impact on the Evolution of the Microbiological Sciences
47
cells of the population (the Lamarckian explanation). In 1934 I. M. Lewis summarized
reports of experiments on Bacillus coli mutabile and concluded:
The subject of bacterial variation and heredity has reached an almost hopeless state of
confusion. Almost every possible view has been set forth and there seems no reason to hope
that any uniform consensus of opinion may be reached in the immediate future. There are
many advocates of a Lamarckian mode of bacterial inheritance while others hold to the view
that it is essentially Darwinian. The early workers regarded variation in so-called mutabile
strains as mutation in the sense of DeVries. Some more recent workers have explained this
behavior as due to Mendellian segregation, while others have regarded it as evidence of a
cyclogenic life history. (Lewis 1934)
Salvador Luria and Max Delbrück solved the dilemma in 1943 by publishing a study
that showed that mutations occurred in populations of Escherichia coli before
selection was applied to the cultures, thus demonstrating that Lamarckian evolution
was not occurring. This work spurred a modest increase in interest in examining the
genetic aspects of bacterial evolution through laboratory culturing (Atwood et al.
1951; Novick and Szilard 1950). This ironically occurred at nearly the same time
that those who had long-standing interests in bacterial evolution were giving up hope
of ever finding characters that would allow one to trace evolutionary events.
Lab evolution of bacteria was used for many years to evolve strains to improve or
create enzyme activities or to enhance the ability of cells to catabolize new substrates
or detoxify environmental pollutants (Hegeman and Rosenberg 1970; Liu and Suflita
1993; Campbell et al. 1973). Those studies, however, did not attempt to test or apply
Darwinian principles.
Dallinger’s challenge was finally taken up by Richard Lenski (Fig. 2.10). Lenski
conducted his postdoctoral research examining genetic diversity of aphids (Service
and Lenski 1982). Later, he looked for an experimental system that would allow
more direct examination of evolutionary principles, and he hit upon using bacteria,
specifically E. coli. He published several papers about the genetic changes necessary
to elicit the appearance of phage-resistant mutants in a population, evolution in
response to thermal stress, and gene stability. He developed an interest in the course
of evolution in a population. How quickly do traits change, are the patterns of change
repeatable, what is the effect of contingency on the mode of evolution? He realized
that by maintaining cultures of E. coli for many generations, he could perhaps
observe changes in the population and, so, address these questions (Lenski 2011).
In February 1988, he started 12 cultures growing in a glucose-limited defined
medium that he transferred daily after each had exhausted its glucose. Each daily
cultivation provided six to seven generations, and he originally planned to grow the
cultures for about 2000 generations, or less than a year. This Long-Term Evolution
Experiment (LTEE) has now been running for over 32 years and has produced over
70,000 generations (Good et al. 2017; Lamrabet et al. 2019).
Cultures in the LTEE have unexpectedly shown great similarity with one another
in their increases in fitness (defined as ability to outgrow the parent culture) and have
generally improved quickly and then slowed in later generations (Lenski 2011). The
size and shape of cells changed along with their preference for carbon sources.
2 Darwin’s Science’s Impact on the Evolution of the Microbiological Sciences
47