The Inner Workings of Evolution - The Darwinian Tourist - page 54

The natural selection acting in the canyon must be strong, because it is
able to maintain the genetic dif erences between these adjacent populations.
These dif erences are maintained even though many of the animals can fl y,
crawl, or scuttle freely from one side of the valley to the other, and of course
the pollen and seeds released by the plants can blow everywhere.
Because of these movements, genes from both sides of the canyon mix
every generation. Even a limited amount of such gene fl ow should be enough
to wipe out any genetic dif erences on the two sides of the canyon. The mem-
bers of a species should look and behave the same on each side. Nonetheless,
the populations are dif erent. Selection preserves and reinforces the genetic
dif erences between the populations even though there is copious gene fl ow
between them.
Sometimes the ef ects of this selection can be measured directly. Two
common species of Drosophila fruit fl ies live in the canyon. In both of these
species, Nevo's colleague Abraham Korol has found a strong tendency for
the fl ies to choose partners from the same slope. 2 The fl ies have been selected
to detect dif erences between potential mates from the two sides of the val-
ley. This tendency to mate non-randomly preserves the genetic dif erences
between the gene pools on the north and south slopes, even though the fl ies
in theory are all perfectly capable of fl ying anywhere in the canyon and mat-
ing with whichever fl y they choose.
Korol's work illustrates an important aspect of natural selection that is
often overlooked when we talk about evolution. Natural selection is not just
about selecting new mutations. Natural selection can, and often does, act to
preserve the genetic dif erences between populations. It can also preserve the
genetic variability hidden within populations, which makes the variability
available for later evolutionary changes.
Korol and Nevo revealed another essential part of the network of evo-
lutionary forces operating in the canyon. Fruit fl ies that live on the hotter,
brighter side of the canyon are able to repair damage to their DNA more
readily than those that live on the cooler side. 3
Mutational changes in the DNA are the ultimate source of genetic vari-
ation. The many allelic forms of genes in gene pools were all originally pro-
duced by mutations. Without mutations evolution would grind to a halt,