Highway Blocks Gene Flow

A four-lane highway has sent two groups of salamanders, living a mere 200 metres apart, down separate evolutionary paths.

Although the divided road has only been in place for 35 years, red-backed salamander populations living on either side now differ genetically.

The 104 metres of right-of-way that makes up interstate highway I-64 in Virginia separates the salamanders. This span is enough to block salamander travel across the road and thereby restrict breeding between the two populations.

Smaller paved roads, up to 47 metres wide, have not altered gene flow among neighbouring red-backed salamanders, even though some of these roads have been in place for over a century. Salamander populations from forests along both sides of these roads have the same genetic makeup.

Other studies have shown that narrow roads reduce dispersal of salamanders by up to 75 percent. When attempting to cross traffic, the slow-moving amphibians have a good chance of ending up as roadkill. Nevertheless, the smaller barriers still allow enough intermixing for this study to find that salamanders along either side of two-lane roads have essentially the same types of genes.

Even under ideal conditions, red-backed salamanders generally don't move far. Juveniles travel less than 100 metres from where they hatched, while adults stick to ranges no larger than 25 square metres.