932 THE STRUCTURE OF EVOLUTIONARY THEORY
over a sufficient number of generations to claim potential linkage with scales of
substantial evolutionary change in nature: well-controlled experimental studies of
bacterial lineages.
This field is now developing so rapidly that any particular study, as discussed
here, will, no doubt, seem quite rudimentary by the time this book reaches the
presses. But, as I write in 1999, an impressive case may be taken as indicative of
possibilities and directions. By using strains of E. coli that pass through six
generations in a single day, Elena, Cooper, and Lenski (1996; see also Lenski and
Travisano, 1994) were able to study evolution in cell size for 10,000 continuous
generations. By imposing constancy of environment (to limits of experimental
perceptibility of course), and using a strain lacking any mechanism for genetic
exchange (Elena et al., 1996), mutation becomes the sole, and experimentally well
isolated, source of genetic variations.
The experimenters have followed 12 replicate populations, each founded by a
single cell from an asexual clone, and each grown under the same regimen (of
daily serial transfer, with growth for 24 hours in 10 ml of a glucose-limited
minimal salts medium that can support ca. 5 x 10^7 cells per ml). At an average of
6.6 bacterial generations per cycle, the population undergoes a daily transition
from lag phase following transfer, to sustained increase, to depletion of limiting
glucose and subsequent starvation. At each serial transfer, a 1:100 dilution begins
the next daily cycle with a minimal bacterial population of ca. 5 X 10^6 cells.
Samples of the common ancestral population, and of selected stages in the history
of each population, were stored at —80° C, and can be revived for competition
experiments with the continually evolving populations—a situation that can only
fill a paleontologist with envy, and with thoughts of beautiful and utterly undoable
experiments from life's multi-cellular history (neanderthals or australopithecines
released in New York City; tyrannosaurs revived to compete against lions in a field
of zebras, etc.).
In each of the 12 populations, both fitness and cell volume increased in a
punctuational manner through the 10,000 generations of the experiment. (The
experimenters measured cell volume by displacement (Lenski and Travisano,
1994, p. 6809), and mean fitness of populations by the Malthusian parameter of
realized rate of increase in competition against resuscitated populations of the
common ancestor.) The general path of increase followed the same trajectory in all
populations, but with fascinating differences of both form and genetics in each
case—a remarkable commentary, at such a small and well-controlled scale, of the
roles of detailed contingency and broad predictability in evolution (see the explicit
discussion of Lenski and Travisano, 1994, on this point). *
*Soon after I wrote this section, Science published a special issue on evolution (25 June
1999), featuring the work of Lenski's lab in a news article entitled, "Test tube evolution
catches time in a bottle." The twelve populations have now been evolving for 24,000 gen-
erations. Although all have shown similar increases in cell size and fitness, the genetic bases
of change have been highly varied and unpredictable. Moreover, alteration in environmental
and adaptive regimes yields no common response. When, after 2000 generations of growth
on glucose (with similar evolutionary responses), the 12 populations were switched