Heres how:
http://www.christiananswers.net/q-aig/race-definition.html
(more genetic info)
http://creation.com/images/pdfs/cabook/chapter18.pdf
The dispersion at Babel broke up a large interbreeding group into
small inbreeding groups. This ensured that the resultant groups
would have different mixes of genes for various physical features. By
itself, this dispersion would ensure, in a short time, that there would
be certain fixed differences in some of these groups, commonly
called races. In addition, the selection pressure of the environment
would modify the existing combinations of genes so that the physical
characteristics of each group would tend to suit their environment.
There has been no simple-to-complex evolution of any genes, for
the genes were present already. The dominant features of the various
people groups result from different combinations of previously
234~Chapter 18 existing created genes, plus some minor degenerative changes, resulting from mutation (accidental changes which can be inherited). The originally created (genetic) information has been either reshuffled or has degenerated, but has not been added to.