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Insect Evidence
of the
Evolutionary
Process
Gene Kritsky
C
harles Darwin never wrote a book
devoted exclusively to insects. Yet his
first contribution to science involved
new records of beetles, and he used insect
examples liberally in his books and papers.
Indeed, insects illustrate all aspects of his
concept of the origin of species by natural
selection. Those points include population
potential, variation within a species, limits
to population growth resulting in a natural
selection, population differentiation, and the
origin of species.
Population Potential
All populations produce more individuals than can survive. Many pest
populations illustrate this point, and
the periodical cicadas provide a natural example of this requirement of the
evolutionary process.
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American Entomologist • Spring 2006
Variation within a Species
No two individuals are exactly alike
and much of that variation is inherited.
Insect taxonomists will quickly point
out that this is true with insects. The
variations within Cicindela denverensis
are clearly shown in this collection from
Theodore Roosevelt National Park.
Natural Selection
Limits to population growth, when combined with natural variation, result in a differential survivorship. Darwin
called this “natural selection.” A natural example can be
found in the oviposition of periodical cicadas. As optimal
oviposition sites become scarce, some cicadas lay their eggs
in thin branches, damaging the vascular system of the tree
and killing the tip of the branch. Approximately 50% of
the cicadas’ eggs laid in these dying branches will perish,
compared to the 15% mortality of eggs laid in branches
that did not wither and die.
Differentiation within a Species
Natural selection differs over a species’ range and may
produce changes in a species as its populations adapt to
local conditions. Such clinal variation has been documented within many insects. The clinal variation of
Cicindela tranquebarica from New Jersey to Virginia to
Tennessee can be seen in this photograph. If species will
change across their range because of changing selection
pressures, species should also change through time as
conditions change.
Vestigial Structures
Vestigial structures challenge the notion
of design in nature. For example, if wings
were designed for flight, then why do flightless birds have wings? If design fails for one
structure, it fails as an explanation for all
structures. To Darwin, vestigial structures
were also evidence of common descent.
Hemiptera do not possess labial palpi, but
the vestigial buds of the lost labial palpi
of enicocephalids show that hemipterans
were once related to insects with fully
developed labial palpi.
American Entomologist • Volume 52, Number 1
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