Travel Reference
In-Depth Information
for there would be no genetic variation on which natural selection could
act.
Mutations can be caused by many things, including radiation and the
invasion of our cells by viruses that can change our DNA. Most mutations,
however, arise from mistakes that occur when DNA is copied. Whenever
a cell divides into two daughter cells, all its DNA must be replicated. The
enzymes that do the copying are really good at their job, but they are not
quite perfect. This is not surprising—making a perfect copy of all your DNA
would be the equivalent of copying out
War and Peace
a thousand times with-
out a mistake.
There is now strong evidence that organisms living in a stressful environ-
ment have a higher rate of mutational mistakes than those living under more
relaxed conditions. To compensate, they also have better mechanisms for
repairing their DNA if it is damaged or if mistakes are made during the copy-
ing process. Korol has found that Drosophila on the sunny side of the canyon,
which are exposed to thermal stress and blasts of ultraviolet radiation, are
able to repair their DNA more readily than fl ies that live on the shadier side.
These genetic dif erences between the two fl y populations have also been
maintained in the face of gene fl ow between the two sides of the canyon.
Our current understanding of evolution has been much refi ned and
extended since Darwin's time. We now know that the composition of gene
pools changes because of natural selection, mutation, chance events, the fl ow
of genes among populations, and the powerful mixing process of genetic
recombination that accompanies sexual reproduction each generation. Tow-
ering over all, and driving these evolutionary changes, are the interactions
between organisms and the environment in which they live.
As Nevo roamed around the canyon, he pointed out mounds of fresh
earth that mark the burrows of some remarkable animals that have also been
shaped by these processes of evolution. In the canyon the burrows house
the mole rat (
Spalax ehrenbergi
), a blind naked mammal that lives all its life
underground. There are more than a dozen species of
Spalax
, four of which
are found in Israel.
Some species of mole rat that live in Africa have lifestyles that resemble
those of social insects such as termites. One female dominates each network
