A new species of plant, which has overcome infertility to evolve, has been found on the bank of a stream in Scotland.

The discovery of the new "monkey flower" near Leadhills in South Lanarkshire was made by Stirling University's Dr Mario Vallejo-Marin.

He said it was a rare example of a species being found to have originated in the wild within the last 150 years. Only a handful of examples exists in recent history.

Dr Vallejo-Marin, a plant evolutionary biologist, said: "Our discovery will help enable scientists to understand how new species form.

"Finding examples of the process in action is rare, so this is an exciting opportunity to study evolution as it happens."

The new yellow flower is derived from the union of two American species, originally brought to the UK in the 1800s. Soon after their arrival, the parent plants escaped garden confines and began to grow in the wild along the banks of rivers and streams. Reproduction between these parents then produced hybrids which are now widespread in Britain.

Normally, genetic differences between two species render hybrid offspring infertile and unable to go beyond the first generation but, surprisingly, Dr Vallejo-Marin said he found wild hybrid plants that have overcome these genetic barriers to possess fully restored fertility.

The fertile hybrid therefore represents a completely new species, native to Scotland.

His research is published in the journal, PhytoKeys.

Meanwhile, a highly invasive plant called piri piri burr, originally from New Zealand, has been found on the sand dunes near the Forvie national nature reserve in Aberdeenshire.

Mike Smedley, Scottish Natural Heritage's operations officer, discovered several patches of piri piri burr growing near a path a few hundred metres from the Forvie reserve.

It has already been found around the River Tweed in the Borders and around the coast in East Lothian, but Forvie is the furthest north it has been seen yet.