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Charles Darwin’s theory of natural selection explains the
general laws by which any given species transforms into other varieties
and species. Darwin extends the application of his theory to the
entire hierarchy of classification and states that all forms of life have
descended from one incredibly remote ancestor. The process of natural
selection entails the divergence of character of specific varieties and the
subsequent classification of once-related living forms as distinct entities
on one or many levels of classification. The process occurs as a species
varies slightly over the course of numerous generations. Through
inheritance, natural selection preserves each variation that proves
advantageous to that species in its present circumstances of living,
which include its interaction with closely related species in the “struggle
for existence” (Darwin 62). Darwin states that this struggle need not be
competitive in nature and also entails a species’ efficiency at producing
offspring. Natural selection works not as an active entity that seeks and
exterminates species that are not suited for their environment; instead,
it retains variations that heighten a species’ ability to dominate in the
struggle for existence and discards those that are detrimental or useless
to that species.
Stephen J. Gould explains the case of r-selection in which a
species’ chances of survival are most reliant on its ability to reproduce
rapidly and not on its structure being ideally suited for its environment.
Gould’s example shows the beneficial results of perceiving natural
selection not as something that changes a species in accordance with its
environment but as something that preserves characteristics beneficial
in the struggle for existence. Gould states, “The best illustrations of
adaptation by evolution are the ones that strike our intuition as peculiar
or bizarre” (91). Gould goes on to explain that those specific and
unusual examples force one to define the general concepts in terms
outside of one’s predetermined and limited sensibilities.
The case of r-selection provides one with an accurate
comprehension of natural selection because it emphasizes success in
a metaphorical struggle for existence instead of success in a literal and
immediate struggle between two species or between a species and its
environment. The main obstacle in the struggle of cecidomyian gall
midges is the fact that these flies find the mushrooms they feed on only
intermittently. In the instances in which they have no food source, the
mobile type of this fly seeks resources. Once food becomes plentiful
for a short duration, the female flies begin to reproduce asexually by
parthenogenesis at stages of their life so early that they are devoured
from the inside out by their offspring. The sacrifice of structural
development for the benefit of rapid reproduction is an example
of natural selection retaining a beneficial variation and discarding a
detrimental or useless one. Gould states, “Evolutionists have learned
that organisms adapt not only by altering their size and shape but also
by adjusting the timing of their lives and the energy invested in different
activities (feeding, growth, and reproduction, for example)” (94). The
theory of natural selection can be more effectively grasped when
considering this example because it proves that variations retained need
not drastically alter the structure of a species in relation to its physical
environment; variations may instead act to give a species a better
strategy for dealing with the circumstances of its environment and
therefore reproducing effectively. As the term “struggle” is called into
question by the case of rapid reproduction, so, too, is the concept of a
general progress towards some ideal of perfection “through constant
selection of better adapted shapes” (Gould 94). Natural selection
becomes understandable not as a force that transforms the structure of
a species so that it may better contend with its closest competitors, but
as one that preserves variations in any way favorable to a species’ success
at existing for the present and subsequent generations.
The case of k-selection provides another framework
for understanding natural selection that accords better with the
preconceived notions of structural change and competition among
species. This kind of selection, however, still adheres to the general
principles of retention of beneficial variations and disposal of those
that are harmful or useless. The environment of k-strategists is
relatively stable compared with that of r-strategists. Gould states,
“Species that live in stable environments, near the maximum population
size that the environment can support, will gain nothing by producing
hordes of poorly adjusted progeny. Better to raise a few, finely tuned
offspring” (94-95). In these instances, instead of preserving rapid
methods of reproduction, natural selection acts on the structural
variations of a species. Since species that are k-strategists exist “near the
maximum population size that the environment can support,” “the more
diversified the descendants from any one species become in structure,
constitution, and habits, by so much will they be better enabled to seize
on many and widely diversified places in the polity of nature, and so be
enabled to increase in numbers” (Darwin 112). Variations that give a
species some advantage over other species are preserved by k-selection
because these variations enable a species’ carrying capacity to be
increased at the expense of exterminated species and varieties.
Darwin illustrates a hypothetical case of natural selection that
falls within the category of k-selection because it demonstrates how
a preserved beneficial variation may increase the number of a variety
allowed by nature through means other than rapid reproduction. This
example also conforms to popular ideas about natural selection because
it involves progressive physical changes and the extermination of certain
varieties of a species. Darwin describes an instance in which a species
of wolf varies in the characteristics of fleetness, craftiness, and strength.
Each individual member of the species may use all three of these
characteristics for the purpose of hunting; however, certain varieties of
this wolf are naturally better endowed with one or other of the traits.
Darwin then supposes that during a season in which the wolf species is
in dire need of food, all of its prey, except for fast-moving deer, become
extremely rare. “I can under such circumstances see no reason to doubt
that the swiftest and slimmest wolves would have the best chance of
surviving, and so be preserved or selected” (Darwin 90). The varieties
of wolf best endowed with the trait of speed would in this case gain
an advantage over their closely allied varieties and therefore emerge
dominant in the struggle for existence, possibly causing the extinction
of the other varieties. The selected variation of speed would then be
passed down through generations causing a species of wolf that is
structurally adapted for speed to descend from a variety of the previous
wolf species.
In the hypothetical case of the wolf species, one must not be
deceived into supposing that the struggle for existence occurred merely
between the different varieties of the species. The struggle in this and
all cases of natural selection involves the complex interaction of many
different living forms. The varieties of wolf that were not endowed
with speed did not become extinct because the faster variety won a
competition between the two, but because some complex interaction
reaching far into the past eventually caused a limited amount of food to
be available to these particular varieties.
The variations accumulated in a species by natural selection,
however, need not be of a nature that allows them to be more effective
predators. In some cases, naturally selected variations enhance a species’
defense mechanisms that help it safeguard itself from predators. The
example of “predator satiation” provides an unusual example of
natural selection that, like that of the cecidomyian gall midge, aids
one in an understanding of the elusive term “struggle for existence.”
The complexity of the natural environment is demonstrated by this
example of the measures taken by a species to avoid predators and
further the most basic of all causes in the struggle for existence, that
of effective reproduction. Gould states that most species of bamboo
follow a synchronous pattern of reproduction with their respective
species. In addition, they spawn so much seed that predators cannot
possibly succeed in consuming it all and exterminating the race. One
final adaptation involves the length of the reproductive cycle consisting
of prime numbers that cannot be easily tracked by predators whose
life cycles are of a shorter duration and cannot regularly coincide with
the setting of the bamboo seeds. Gould states, “An effective strategy
of predator satiation involves two adaptations. First, the synchrony
of emergence or reproduction must be very precise, thus assuring that
the market is truly flooded, and only for a short time. Secondly, this
flooding cannot occur very often, lest predators simply adjust their own
life cycle to predictable times of superfluity” (101). The preservation
of predator satiation not only defends a species against the immediate
threat of predators, but also adjusts the reproductive cycle to prime
numbers to counter any useful variations being accumulated in the life
cycle of the predators. Natural selection, in this instance, functions in
one species to prevent natural selection from occurring in that species’
predator.
Natural selection by no means entails an irrevocable progress
toward perfection of those species somehow destined to be dominant.
Instead, it merely preserves random variations that prove useful in a
way that will aid a species sustaining its presence on the planet for the
present and subsequent generations. The variations eventually pile
upon one another until some significant change has occurred within a
species. Any progress that occurs is only that which allows for a species
to adapt to its present circumstances. As the examples given here
illustrate, natural selection may take on many forms and give a species
better defensive, offensive, or reproductive measures in the struggle
for existence, which, though it sounds dramatic and urgent, is nothing
more than being able to effectively cope with the external world and
reproduce.
Works Cited
Darwin, Charles. The Origin of Species. Cambridge, Massachusetts and
London, England: Harvard University Press, 1964.
Gould, Stephen J. “Of Bamboos, Cicadas, and the Economy of Adam
Smith.” Ever Since Darwin. New York, New York and London,
England: W.W. Norton & Company, 1977.
—. “Organic Wisdom, or Why Should a Fly Eat Its Mother from
Inside.” Ever Since Darwin. New York, New York and London,
England: W.W. Norton & Company, 1977. |