Encyclopedia of Evolution.pdf - Online Reading Center

0 mutualism
function of the protein. A mutation that causes the protein
to function poorly at normal temperatures but better at
hot temperatures may be detrimental, except in organisms
whose environments are getting hotter or that are migrating
into hotter environments.
• Some mutations involve more than a simple amino acid
substitution. In some cases, when a base is lost from
or added to the DNA, the entire frame of reference is
changed. For each codon “downstream” from the mutation,
the base that used to be number one now becomes
number two or number three. This frameshift mutation
would cause almost every amino acid downstream from
the mutation to be different, which could result in an
entirely different protein. Some mutations occur when an
entire chunk of DNA is eliminated, duplicated, or moved
to a new location. If the chunk of DNA in a new location
is ignored, it simply becomes one more of the many
chunks of noncoding DNA with which the chromosomes
are already crowded. But the translocated chunk may
cause a mutation in its new location.
While mutations may occur uniformly over time, the
mutation rate per generation is higher in species that have
longer generation times. Fruit flies (Drosophila melanogaster)
have 0.14 mutations per genome per generation, while
humans have 1.6 mutations per genome per generation. A
fruit fly sperm is the product of only about 25 cell divisions,
while a sperm from a 30-year-old man is the product of more
than 400 cell divisions.
Evolutionary scientists often assume that mutations
occur randomly. This, however, may not be entirely true.
Cells seem to be, in some cases, able to control the frequency
with which mutations occur, allowing an increased rate of
mutations during times of environmental instability. For
example, during times of stress, the mutation rate in bacteria
has been found to increase. Increased mutation rate during
times of stress has been observed in yeast and in maize as
well as bacteria. Bacteria have an S.O.S. response by which
they survive sudden increases in mutation. In addition to
this, during stress, bacteria tend to release and absorb small
circular chunks of DNA known as plasmids, some of which
have genes that allow the bacteria to handle the stress better.
Antibiotic resistance genes, for example, are often found
on plasmids; and bacteria swap plasmids more rapidly when
antibiotics are present than at normal times (see resistance,
evolution of). However, modern evolutionary theory
insists that mutations occur randomly with respect to the
needs of the organism. For example, an animal may need,
or greatly benefit from, a mutation that would cause an
enhanced ability to run away from predators. Such a mutation,
however, is no more or less likely to occur than is any
other mutation.
Mutations in developmental control genes can have large
effects on the structure of the resulting organism (see developmental
evolution). Most such mutations are lethal, but if the
organism with such a mutation survives and breeds, it might be
the founder of a new species, genus, or even higher group.
Evolutionary change tends to be conservative, largely
because most big mutations are bad. Evolutionary modification
occurs within existing patterns and structures, rather
than inventing entirely new innovations. Adaptations tend
to be, according to evolutionary biologist Stephen Jay
Gould, “jury-rigged contrivances,” rather than designs
from a master mind. Because most big mutations are bad,
there are some things evolution simply cannot do and has
not done.
Further Reading
Michel, Bénédicte. “After 30 years of study, the bacterial SOS
response still surprises us.” PLoS Biology 3 (2005): e255.
Available online. URL: http://biology.plosjournals.org/perlserv/
?request=get-document&doi=10.1371/journal.pbio.0030255.
Accessed July 10, 2006.
mutualism See coevolution.