Biology Reference
In-Depth Information
Unlike natural selection, which is a highly nonrandom process, mutation is a
chance process - random copying errors that may occur every time DNA is repli-
cated. The term “random” in this context means that the particular mutations that
occur are unrelated to their effects on evolutionary fitness. A better term would be
“accidental” or “undirected”, but the term “random” is commonly used. This differ-
ence between the properties of natural selection and mutation is another common
source of confusion in media debates about evolution.
So remember -
mutation is random, but natural selection is nonrandom.
There are several different ways in which mutation can occur - these are listed
on the left of Fig. 4.20. On the right, are some approximate rates of mutation.
RNA viruses have high rates of mutation because RNA, unlike DNA, is single-
stranded and RNA viruses lack proofreading mechanisms. All cellular organisms
contain proteins that are able to detect errors in base-pairing during the replication
of DNA and correct them, using the information in the strand of DNA being copied.
But RNA viruses lack such proofreading mechanisms, which is why the AIDS virus
evolves so quickly - I mentioned earlier that you can observe the AIDS virus evolv-
ing within a single human individual in a few weeks. The appearance of the MRSA
superbug in hospitals is another example of evolution in action - the selection here
is created by our use of antibiotics.
Rates of mutation are expressed in several ways. The rate per base per replica-
tion is very low in everything except viruses. For example, every time a human cell
divides about six new mutations arise on average, that is, six bases out of a total of
six billion bases are changed. This seems a very small change, but when you con-
sider the number of cell divisions required to make the gametes of an adult human,
it turns out that the average person will accumulate in their gametes during their
reproductive lifetime around 200 mutations. Most mutations turn out to be neutral
in their effect, either because they occur in regions of the DNA that do not code for
proteins or because they do not change the amino acid sequence. But a minority of
mutations are harmful. For example, about 1 in 25,000 people are born with a single
mutation in a gene that encodes a protein involved in the action of a hormone called
fibroblast growth factor. The result is the condition called achondroplasia, in which
the limb bones fail to elongate normally so that the affected person has short stature.
In most case the parents do not have this condition, so it is the result of a new muta-
tion. In about 98% of cases, the mutation is a single base change that results in the
replacement of one amino acid by a different amino acid in the protein that binds to
the growth hormone.
On the other hand, a very small fraction of mutations are positive in their effects
- they increase evolutionary fitness. An example of a positive mutation that has
been selected for recently in human history is a mutation that occurred in the gene
encoding the enzyme lactase. This enzyme is required so that babies can digest the
sugar lactose that they receive in their mother's milk. After weaning, the production
of this enzymes ceases, as babies prepare for an adult diet. Several thousand years
ago, a mutation occurred in a human that allowed him or her to continue to digest
lactose in dairy products into adulthood. From the ethnic distribution of this trait,
it is likely that this mutation occurred in a European who lived in an area where
