Rates of Speciation
How fast do new species form? Because their generation times
are short, new species of unicellular organisms may evolve in years, months,
or even days. For plants and animals Darwin theorized that new species
formed gradually over millions of years. Today scientists know of examples
in which species arose in only thousands rather than millions of years.
For example, archaeological evidence indicates that settlers from Polynesia
introduced banana trees to the Hawaiian islands about a thousand years
ago. Today several species of moths, unique to the Hawaiian islands, feed
on Bananas. These moth species are closely related to other plant eating
moths in Hawaii. Thus scientists suggest that the banana eating moths arose
from other plant eating moths, undergoing adaptive radiation in less than
the thousand years that banana trees have existed in Hawaii. Evidence from
the fossil record has led some scientists to propose that speciation need
not occur gradually but can occur in spurts. According to the theory of
punctuated equilibrium, all populations of a species may exist for a relatively
long time at or close to genetic equilibrium. Then the equilibrium may
be interrupted by a brief period of rapid genetic change in which speciation
occurs. Some scientists argue that if new species evolved gradually, the
fossil record should show many examples of transitional forms- species
with characteristics intermediate between those of ancestral species and
new species. However, for most organisms such transitional forms are absent
from the fossil record. Instead the fossil record shows that most species
remained the same for hundreds of thousands or millions of years. Then
new, related species suddenly appeared. Whether new species form gradually
or rapidly is still a point of debate among scientist. However, scientists
agree that natural selection, whether gradual or rapid, is the most important
factor in speciation.
Extinction
Just as new species form through natural selection, species
also die off - or become extinct. What causes extinction? For a species
to continue to exist, some members must have traits that allow them to
survive and pass their genes onto the next generation. If the environment
changes, for instance, the species will become extinct unless some members
have adaptations that allow them to survive and reproduce successfully
under the new environmental conditions. Changes in climate and competition
among species are examples of environmental changes to which species must
adapt in order to survive. Environmental changes caused by human beings
have led to the extinction of hundreds of organisms in the past few centuries.
Most of these changes involve the destruction of habitats. For example,
the conversion of the prairies of central North America into farmland and
grazing ranges caused a decline in the large population of prairie daogs
in the region. In turn the black footed ferret, which preys solely on the
prairie dog, has also greatly declined in numbers. This weasel like animal
may soon become extinct, because members of the species do not have variations
that result in reproductive success in this changing environment. The example
of the ferret shows how species depend on others for survival.
The decline
in population of one species has led to the near extinction of another.
Extinction is a natural process. However, the rapid rate at which species
are becoming extinct as a result of the destruction of habitats by human
beings may endanger the survival of many life forms. Over the billions
of years of evolution, species of organisms have evolved with unique adaptations
to problems presented by a diverse and changing environment. Once a species
has become extinct, the unique solution to life we call a species will be
lost forever.
