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 m 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 m 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%. 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 m².