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Natural selection has no foresight, nor does it operate according
to some preconceived plan. Rather it is a purely natural process
resulting from the interacting properties of physicochemical and
biological entities. Natural selection is simply a consequence
of the differential multiplication of living beings. It has some
appearance of purposefulness because it is conditioned by the
environment: which organisms reproduce more effectively depends
on what variations they possess that are useful in the environment
where the organisms live. In a sense, natural selection is an
"opportunistic" process. The variables determining in
what direction it will go are the environment, the preexisting
constitution of the organisms, and the randomly arising mutations.
But natural selection does not anticipate the environments of
the future; drastic environmental changes may be insuperable to
organisms that were previously thriving.
Adaptation to a given environment may occur in a variety of
different ways. An example may be taken from the adaptations of
plant life to desert climate. The fundamental adaptation is to
the condition of dryness, which involves the danger of desiccation.
During a major part of the year, sometimes for several years in
succession, there is no rain. Plants have accomplished the urgent
necessity of saving water in different ways. Cacti have transformed
their leaves into spines, having made their stems into barrels
containing a reserve of water; photosynthesis is performed in
the surface of the stem instead of in the leaves. Other plants
have no leaves during the dry season, but after it rains they
burst into leaves and flowers and produce seeds. Ephemeral plants
germinate from seeds, grow, flower, and produce seeds—all
within the space of the few weeks while rainwater is available;
the rest of the year the seeds lie quiescent in the soil.
The opportunistic character of natural selection is also well-evidenced
by the phenomenon of adaptive radiation. The evolution of Drosophila
flies in Hawaii is a relatively recent adaptive radiation. There
are about 1,500 Drosophila species in the world. Approximately
500 of them have evolved in the Hawaiian archipelago, although
this has a small area, about one twenty-fifth the size of California.
Moreover, the morphological, ecological, and behavioral diversity
of Hawaiian Drosophila exceeds that of Drosophila in the rest
of the world.
Why should have such "explosive" evolution have occurred
in Hawaii? The overabundance of drosophila flies there contrasts
with the absence of many other insects. The ancestors of Hawaiian
drosophila reached the archipelago before other groups of insects
did, and thus they found a multitude of unexploited opportunities
for living. They responded by a rapid adaptive radiation; although
they are all probably derived from a single colonizing species,
they adapted to the diversity of opportunities available in diverse
places or at different times by developing appropriate adaptations,
which range broadly from one to another species.
The process of natural selection can explain the adaptive organization
of organisms; as well as their diversity and evolution as a consequence
of their adaptation to the multifarious and ever changing conditions
of life. The fossil record shows that life has evolved in a haphazard
fashion. The radiations, expansions, relays of one form by another,
occasional but irregular trends, and the ever present extinctions,
are best explained by natural selection of organisms subject to
the vagaries of genetic mutation and environmental challenge.
The scientific account of these events does not necessitate recourse
to a preordained plan, whether imprinted from without by an omniscient
and all-powerful designer, or resulting from some immanent force
driving the process towards definite outcomes. Biological evolution
differs from a painting or an artifact in that it is not the outcome
of a design preconceived by an artist or artisan.
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| Contributed by: Dr. Francisco Ayala
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