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Critics have sometimes alleged as evidence against Darwin's
theory of evolution examples showing that random processes cannot
yield meaningful, organized outcomes. It is thus pointed out that
a series of monkeys randomly striking letters on a typewriter
would never write The Origin of Species, even if we allow for
millions of years and many generations of monkeys pounding at
typewriters.
This criticism would be valid if evolution would depend only
on random processes. But natural selection is a nonrandom process
that promotes adaptation by selecting combinations that "make
sense," i.e., that are useful to the organisms. The analogy
of the monkeys would be more appropriate if a process existed
by which, first, meaningful words would be chosen every time they
appeared on the typewriter; and then we would also have typewriters
with previously selected words rather than just letters in the
keys, and again there would be a process to select meaningful
sentences every time they appeared in this second typewriter.
If every time words such as "the," "origin,"
"species," and so on, appeared in the first kind of
typewriter, they each became a key in the second kind of typewriter,
meaningful sentences would occasionally be produced in this second
typewriter. If such sentences became incorporated into keys of
a third type of typewriter, in which meaningful paragraphs were
selected whenever they appeared, it is clear that pages and even
chapters "making sense" would eventually be produced.
We need not carry the analogy too far, since the analogy is
not fully satisfactory, but the point is clear. Evolution is not
the outcome of purely random processes, but rather there is a
"selecting" process, which picks up adaptive combinations
because these reproduce more effectively and thus become established
in populations. These adaptive combinations constitute, in turn,
new levels of organization upon which the mutation (random) plus
selection (nonrandom or directional) process again operates.
The manner in which natural selection can generate novelty
in the form of accumulated hereditary information may be illustrated
by the following example. Some strains of the colon bacterium,
Escherichia coli, in order to be able to reproduce in a culture
medium, require that a certain substance, the amino acid histidine,
be provided in the medium. When a few such bacteria are added
to a cubic centimeter of liquid culture medium, they multiply
rapidly and produce between two and three billion bacteria in
a few hours. Spontaneous mutations to streptomycin resistance
occur in normal (i.e., sensitive) bacteria at rates of the order
of one in one hundred million (1 x 10-8) cells. In
our bacterial culture we expect between twenty and thirty bacteria
to be resistant to streptomycin due to spontaneous mutation. If
a proper concentration of the antibiotic is added to the culture,
only the resistant cells survive. The twenty or thirty surviving
bacteria will start reproducing, however, and allowing a few hours
for the necessary number of cell divisions, several billion bacteria
are produced, all resistant to streptomycin. Among cells requiring
histidine as a growth factor, spontaneous mutants able to reproduce
in the absence of histidine arise at rates of about four in one
hundred million (4 x 10-8) bacteria. The streptomycin
resistant cells may now be transferred to a culture with streptomycin
but with no histidine. Most of them will not be able to reproduce,
but about a hundred will start reproducing until the available
medium is saturated.
Natural selection has produced in two steps bacterial cells
resistant to streptomycin and not requiring histidine for growth.
The probability of the two mutational events happening in the
same bacterium is of about four in ten million billion (1 x 10-8
x 4 x 10-8 = 4 x 10-16) cells. An event
of such low probability is unlikely to occur even in a large laboratory
culture of bacterial cells. With natural selection, cells having
both properties are the common result.
As illustrated by the bacterial example, natural selection
produces combinations of genes that would otherwise be highly
improbable because natural selection proceeds stepwise. The vertebrate
eye did not appear suddenly in all its present perfection. Its
formation requires the appropriate integration of many genetic
units, and thus the eye could not have resulted from random processes
alone. The ancestors of today's vertebrates had for more than
half a billion years some kind of organs sensitive to light. Perception
of light, and later vision, were important for these organisms'
survival and reproductive success. Accordingly, natural selection
favored genes and gene combinations increasing the functional
efficiency of the eye. Such genetic units gradually accumulated,
eventually leading to the highly complex and efficient vertebrate
eye. Natural selection can account for the rise and spread of
genetic constitutions, and therefore of types of organisms, that
would never have existed under the uncontrolled action of random
mutation. In this sense, natural selection is a creative process,
although it does not create the raw materials—the genes—upon
which it acts.
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| Contributed by: Dr. Francisco Ayala
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