Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Semmelweis, Ignaz Philipp

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Many people attribute the phrase “survival of the fittest”

to Darwin, but in fact, it originated from another

naturalist/philosopher, Herbert Spencer (1820–1903).

Recently, many recent evolutionary biologists have asked:

Survival of the fittest what? At what organismal level is selec-

tion most powerful? What is the biological unit of natural

selection-the species, the individual, or even the gene?

Selection can provide interesting consequences for bac-

teria and viruses. For example, reduced virulence in parasites,

who depend on the survival of their hosts for their own sur-

vival may increase the reproductive success of the invading

parasite. The myxomavirus, introduced in Australia to control

imported European rabbits (Oryctolagus cuniculus), at first

caused the deaths of many individuals. However, within a few

years, the mortality rate was much lower, partly because the

rabbits became resistant to the pathogen, but also partly

because the virus had evolved a lower virulence. The reduc-

tion in the virulence is thought to have been aided because the

virus is transmitted by a mosquito, from one living rabbit to

another. The less deadly viral strain is maintained in the rabbit

host population because rabbits afflicted with the more viru-

lent strain would die before passing on the virus. Thus, the

viral genes for reduced virulence could spread by group selec-

tion. Of course, reduced virulence is also in the interest of

every individual virus, if it is to persist in its host. Scientists

argue that one would not expect to observe evolution by group

selection when individual selection is acting strongly in an

opposing direction.

Some biologists, most notably Richard Dawkins (1941–),

have argued that the gene itself is the true unit of selection. If

one genetic alternative, or allele, provides its bearer with an

adaptive advantage over some other individual who carries a

different allele then the more beneficial allele will be replicated

more times, as its bearer enjoys greater fitness. In his book The

Selfish Gene, Dawkins argues that genes help to build the bod-

ies that aid in their transmission; individual organisms are

merely the “survival machines” that genes require to make more

copies of themselves.

This argument has been criticized because natural selec-

tion cannot “see” the individual genes that reside in an organ-

ism’s genome, but rather selects among phenotypes, the

outward manifestation of all the genes that organisms possess.

Some genetic combinations may confer very high fitness, but

they may reside with genes having negative effects in the same

individual. When an individual reproduces, its “bad” genes are

replicated along with its “good” genes; if it fails to do so, even

its most advantageous genes will not be transmitted into the

next generation. Although the focus among most evolutionary

biologists has been on selection at the level of the individual,

this example raises the possibility that individual genes in

genomes are under a kind of group selection. The success of

single genes in being transmitted to subsequent generations

will depend on their functioning well together, collectively

building the best possible organism in a given environment.

When selective change is brought about by human

effort, it is known as artificial selection. By allowing only a

selected minority of individuals or specimen to reproduce,

breeders can produce new generations of organisms (e.g. a

particular virus or bacterium) that feature desired traits.

See alsoEpidemiology; Evolution and evolutionary mecha-

nisms; Evolutionary origin of bacteria and viruses; Rare geno-

type advantage

SELECTIVE MEDIA•seeGROWTH AND GROWTH

MEDIA

SEM• seeELECTRON MICROSCOPE, TRANSMISSION AND

SCANNING

SEMMELWEIS, IGNAZPHILIPP

(1818-1865)Semmelweis, Ignaz Philipp

Hungarian physician

Along with American physician Oliver Wendell Holmes

(1809–1894), Ignaz Semmelweis was one of the first two doc-

tors worldwide to recognize the contagious nature of puerperal

fever and promote steps to eliminate it, thereby dramatically

reducing maternal deaths.

Semmelweis was born in Ofen, or Tabàn, then near

Buda, now part of Budapest, Hungary, on July 1, 1818, the son

of a Roman Catholic shopkeeper of German descent. After

graduating from the Catholic Gymnasium of Buda in 1835 and

the University of Pest in 1837, he went to the University of

Vienna to study law, but immediately switched to medicine.

He studied at Vienna until 1839, then again at Pest until 1841,

then again at Vienna, earning his M.D. in 1844. Among his

teachers were Karl von Rokitansky (1804–1878), Josef Skoda

(1805–1881), and Ferdinand von Hebra (1816–1880). He did

postgraduate work in Vienna hospitals in obstetrics, surgery,

and, under Skoda, diagnostic methods. In 1846, he became

assistant physician, tantamount to senior resident, at the

obstetrical clinic of the Vienna General Hospital.

In the mid-nineteenth century, the maternal death rate

for hospital births attended by physicians was much higher

than for either home births or births attended by midwives.

The principal killer was puerperal fever, or childbed fever,

whose etiology was then unknown, but which Louis Pasteur

(1822–1895) learned in 1879 was caused by a streptococcal

infection of the open wound at the site of the placenta in

women who had recently given birth. The infection could

remain topical or it could pass through the uterus into the

bloodstream and quickly become fatal. Before Semmelweis

and Holmes, physicians generally assumed that puerperal

fever was an unpreventable and natural consequence of some

childbirths, and accepted the terrifying mortality statistics.

Witnessing so many healthy young mothers sicken and

die greatly affected Semmelweis, and he grew determined to

discover the cause and prevention of puerperal fever. Using

Rokitanksy’s pathological methods, he began a comparative

study of autopsies of puerperal fever victims. The break-

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