Encyclopedia of Evolution.pdf - Online Reading Center

vestigial characteristics Vestigial characteristics are traits
of organisms that no longer have a useful primary function
but are leftovers or vestiges of characteristics that had a useful
primary function in evolutionary ancestors. They usually
result from a process such as the following:
1. A characteristic has an important function in an organism.
For example, stamens in flowers produce pollen.
2. As circumstances change, the characteristic is no longer
important for the organism. natural selection no longer
eliminates individuals that lack the full development
of the characteristic. For example, flowers (at least, those
with male function) need stamens but may not need all of
the stamens that they have.
3. Mutations may cause incomplete development of the characteristic.
For example, many flowers produce staminodes,
which are sterile stamens—shafts that produce no pollen.
This structure is now considered vestigial.
4. If the vestigial structure represents no significant cost to
the organism, then natural selection will not eliminate it.
Also, in many cases, natural selection may be in the process
of eliminating a vestigial characteristic but has not
completed this process.
It is always risky to call a characteristic useless because
it is always possible to discover a use for the characteristic
later. Critics of evolutionary science (see creationism) like
to cite the example of the German anatomist Robert Wiedersheim
who, in 1895, listed about a hundred organs which he
considered vestigial in humans. Many of these organs turned
out to have important functions (for example, tonsils) even
though a person could in fact survive without them. The fact
that a person can survive with only one leg (or none) does not
prove that legs are useless. However, vestigial and useless are
not the same thing; and it is not possible to so easily dispose
of vestigial characteristics as evidence for evolution as the following
examples indicate.
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Vestigial characteristics can be found at all levels of
organism structure and function:
At the biochemical level. The DNA of eukaryotic organisms
contains a large, sometimes prodigious, amount of noncoding
DNA. In most eukaryotes, well over half of the DNA
is noncoding—that is, it does not result in the production of
a protein. While some of this noncoding DNA has important
functions, much of this DNA is vestigial. Pseudogenes, for
example, are very similar in structure to true genes, but they
do not have a promoter, therefore they are not used. They
are old genes that the cell no longer uses. The cell has deleted
the gene the same way a computer deletes a file—not by
eliminating it entirely, but by deleting the information about
how to find and use it (see DNA [raw material of evolution]).
Much human DNA consists of instructions for making
reverse transcriptase—an enzyme not used by humans but
which is the remnant of the past activity of retroviruses.
At the cellular level. Some cellular organelles, such as
chloroplasts and mitochondria, are the simplified descendants
of symbiotic bacteria (see symbiogenesis). Chloroplasts and
mitochondria still have some of their own DNA and genes,
and their own ribosomes, which allow them to produce proteins
from their own genes. As they no longer have all of
the genes that they need for survival, they cannot live independently
of the host cell. Many cell structures such as the
endoplasmic reticulum did not arise by symbiosis; they function
just fine without their own DNA or ribosomes, because
the nucleus of the cell has all the genes, and the cytoplasm
of the cell has all the ribosomes, necessary for the synthesis
and activity of the endoplasmic reticulum. Chloroplasts and
mitochondria are a transitional form—they have lost enough
genes that they cannot live on their own, but they have not
lost all of them. While their genes are, in fact, useful, these
genes do not need to be in the chloroplasts and mitochondria
themselves. Perhaps even more striking is the fact that the
chloroplasts of some protists (such as dinoflagellates) are the
evolutionary descendants not of bacteria but of eukaryotic