Selection WORLD OF MICROBIOLOGY AND IMMUNOLOGY506•
likely. At the time Schick embarked on his research, scientists
had already isolated the microbe or toxin that caused diphthe-
ria. A horse serum had also been developed that could prevent
or even cure the disease. But the serum had so many side
effects that doctors were unwilling to prescribe it unless they
knew a patient was seriously in danger of catching diphtheria.
Thus, Schick’s discovery made it easier for them to treat those
who were the most vulnerable.
In 1923, an antitoxin without side effects was developed
and was then given to babies during their first year of life.
Later on, the Schick test would show whether immunityper-
sisted. Schick’s test technique was also used years later to treat
people with allergies, using the same technique of injecting
small doses of an antitoxin.
Schick left Vienna in 1923 to become pediatrician-in-
chief at Mt. Sinai Hospital in New York City. Schick became
an American citizen that same year and two years later mar-
ried his wife, Catherine C. Fries. He held his post at Mt. Sinai
Hospital until his retirement in May 1943, when he became a
consulting pediatrician. During his career, he also worked
simultaneously at other hospitals, acting as director of pedi-
atrics at Sea View Hospital in Staten Island, New York and
consulting pediatrician at the Willard Parker Hospital, the
New York Infirmary for Women and Children, and Beth Israel
Hospital. He also taught as a professor of the diseases of chil-
dren at Columbia University College of Physicians and
Surgeons, starting in 1936.
Schick directed a private practice in New York City as
well. His office held a collection of dolls and animals that he
had acquired in travels throughout the world. He would often
play the piano in his office, or take out one of his doll or ani-
mal figures to calm a child. He never displayed a stethoscope
until he made sure a child was relaxed. At one time, he esti-
mated that he had treated over a million children.
Childless himself, he had a great fondness for children
and in 1932 authored a popular book titled Child Care Today
that contained his firm beliefs about how children should be
raised. Many of his ideas were advanced for his time. He
advocated little punishment for children and no corporal pun-
ishment. He also said that trauma in a child’s early life often
had a lasting effect.
Schick and his wife lived in a large apartment in New
York City and were frequent travelers around the world. On a
cruise to South America with his wife during his later years,
Schick fell ill with pleurisy. Eventually brought back to the
United States to Mt. Sinai Hospital, he died on Dec. 6, 1967.See alsoAllergies; History of immunology; History of micro-
biology; History of public health; Immune system;
Immunology; Medical training and careers in microbiologySCID•seeSEVERE COMBINED IMMUNODEFICIENCY
(SCID)SECONDARY IMMUNE RESPONSE•see
IMMUNITY, ACTIVE, PASSIVE, AND DELAYEDSSelectionELECTION
Evolutionary selection pressures act on all living organisms,
regardless whether they are prokaryotic or higher eukaryotes.
Selection refers to an evolutionary pressure that is the result of
a combination of environmental and genetic pressures that
affect the ability of an organism to live and, equally impor-
tantly, to produce reproductively successful offspring (includ-
ing prokaryotic strains of cells).
As implied, natural selection involves the natural (but
often complex) pressures present in an organism’s environ-
ment. Artificial selection is the conscious manipulation of
mating, manipulation, and fusion of genetic material to pro-
duce a desired result.
Evolutionrequires genetic variation, and these varia-
tions or changes (mutations) are usually deleterious because
environmental factors already support the extent genetic dis-
tribution within a population.
Natural selection is based upon expressed differences in
the ability of organisms to thrive and produce biologically suc-
cessful offspring. Importantly, selection can only act to exert
influence (drive) on those differences in genotypethat appear
as phenotypic differences. In a very real sense, evolutionary
pressures act blindly.
There are three basic types of natural selection: direc-
tional selection favoring an extreme phenotype; stabilizing
selection favoring a phenotypewith characteristics intermedi-
ate to an extreme phenotype (i.e., normalizing selection); and
disruptive selection that favors extreme phenotypes over inter-
mediate genotypes.
The evolution of pesticide resistance provides a vivid
example of directional selection, wherein the selective agent
(in this case DDT) creates an apparent force in one direction,
producing a corresponding change (improved resistance) in
the affected organisms. Directional selection is also evident in
the efforts of human beings to produce desired traits in many
organisms ranging from bacteriato plants and animals.
Not all selective effects are directional, however.
Selection can also produce results that are stabilizing or dis-
ruptive. Stabilizing selection occurs when significant changes
in the traits of organisms are selected against. An example of
this is birth weight in humans. Babies that are much heavier or
lighter than average do not survive as well as those that are
nearer the mean (average) weight.
On the other hand, selection is said to be disruptive if
the extremes of some trait become favored over the interme-
diate values. Although not a factor for microorganisms, sexual
selection and sexual dimorphism can influence the immuno-
logic traits and capacity of a population.
Sometimes the fitness of a phenotype in some environ-
ment depends on how common (or rare) it is; this is known as
frequency-dependent selection. Perhaps an animal enjoys an
increased advantage if it conforms to the majority phenotype
in the population. Conversely, a phenotype could be favored if
it is rare, and its alternatives are in the majority. Frequency-
dependent selection provides an interesting case in which the
genefrequency itself alters the selective environment in which
the genotype exists.womi_S 5/7/03 8:20 AM Page 506