http://www.sciencedaily.com/releases/2011/11/111103143237.htm "We find that families who are at the forefront of a range expansion into new territories had greater reproductive success. In other words, that they had more children, and more children who also had children," Labuda explained. "As a result, these families made a higher genetic contribution to the contemporary population than those who remained behind in what we call the range core, as opposed to the wave front." "We knew that the migration of species into new areas promoted the spread of rare mutations through a phenomenon known as 'gene surfing', but now we find that selection at the wave front could make this surfing much more efficient."