Encyclopedia of Evolution.pdf - Online Reading Center

chemical spraying the weaker members of the insect populations
are being weeded out. Now, in many areas and
among many species only the strong and fit remain to defy
our efforts to control them.”
Antibiotics are chemicals (often produced by fungi) that
kill bacteria. Pesticides kill animals, and herbicides kill weeds.
Within populations of bacteria, animals, and weeds, individual
organisms may be resistant to the chemicals that kill most
of the other organisms.
• Within a population of bacteria, penicillin may be deadly
to most of the individuals, but a few of them have a genetically
based ability to survive exposure to penicillin. This
resistance may result from an altered mechanism of bacterial
cell wall formation.
• Within a population of insects, certain individuals may
have a genetically based ability to resist pesticides that will
kill most members of their species. This resistance may
result from an increased production of defensive enzymes,
the production of an improved defensive enzyme, or a
behavioral tendency to detect and avoid the pesticide. For
example, some cockroach populations have evolved an
aversion to sugar—not a good thing for them, under normal
conditions, but it does cause them to avoid sugar-baited
pesticide traps.
• Within a population of weeds, certain individuals may have
a genetically based ability to resist herbicides that would
kill most members of their species. This resistance may
result from a mutation that alters the membranes of the
chloroplast, the structure within plant cells that carries out
photosynthetic food production.
Under normal conditions (in the absence of antibiotics,
pesticides, and herbicides), the mutant (resistant) individuals
have inferior reproduction:
• Insects that expend much of their metabolic reserves producing
resistance enzymes have less energy left over for
reproduction than normal insects.
• Rats that are resistant to the rat poison warfarin suffer a
high incidence of bleeding and require higher levels of vitamin
K in their diets; therefore, under normal conditions,
they bleed to death and die of vitamin deficiency more frequently
than normal rats.
• Weeds with mutated chloroplasts have inferior photosynthetic
food production compared to normal weeds.
Under these conditions, natural selection operates against the
resistant individuals (see figure). However, when antibiotics,
pesticides, or herbicides are common and persistent in the
environment of the populations, natural selection operates in
favor of these very same resistant individuals. The heavy and
persistent use of chemicals, therefore, creates the perfect environment
for the evolution of bacteria, animals, and weeds
that resist the very chemicals that humans use to control their
populations.
Individual bacteria, animals, and weeds generally cannot
change their resistance. They either are resistant or are
not. The individuals survive and reproduce, or they do not
resistance, evolution of
survive; therefore only the populations and species can evolve
resistance (see natural selection).
Antibiotic resistance. Antibiotic resistance can be
detected by measuring the zone of inhibition, in which bacteria
will not grow near the antibiotic on a culture plate.
The overuse of antibiotics has resulted from the tendency of
doctors to prescribe antibiotics for mild bacterial infections,
and for viral infections that cannot be controlled by antibiotics.
This evolution can occur very rapidly. Consider these
examples:
• Intestinal infections. A study of Swiss hospitals showed
that between 1983 and 1990, when only 1.4 percent of
the patients were receiving routine antibiotic administrations,
there were no samples of the bacterium E. coli
that could resist any of the five kinds of fluoroquinolone
antibiotics. (E. coli is a bacterium that lives in human
intestines and is usually harmless, but it can cause infections
in people with impaired immune systems.) However,
between 1991 and 1993, during which time 45 percent of
the patients were receiving routine antibiotic administrations,
28 percent of the E. coli samples were resistant to
all five of the antibiotics.
• Sexually transmitted diseases (STDs). From 1993 to 2001
Hawaii experienced a rapid increase in the incidence of
gonorrhea bacteria (Neisseria gonorrhoeae) that resist fluoroquinolones.
Gonorrhea had originally been treated with
penicillin, but when the bacteria evolved resistance to penicillin,
doctors substituted tetracycline, which also became
Natural selection of resistance. In A and B, the top line of boxes is one
generation, the bottom line of boxes is the next generation, and arrows
represent reproduction. (A) In the absence of chemicals, the resistant
organisms (shaded boxes) reproduce less often than the nonresistant
organisms (white boxes); natural selection operates against them. In
this example, the proportion of resistant organisms in the population
declines from 0 percent to 0 percent. (B) In the presence of chemicals,
the resistant organisms reproduce more often than the nonresistant
organisms; natural selection operates in their favor. By chance, a few
organisms that are not resistant to the antibiotics will reproduce. In
this example, the proportion of resistant organisms in the population
increases from 0 percent to 0 percent.