Daily Mail, U.K. Genetic differences show advantages image:39452:2::0

The Red Squirrel which is the native squirrel to Britain is being overrun by a mutant Black Squirrel which first showed up in Britain about ninety years ago. The mutant Blacks squirrels are an offshoot from the Grey Squirrels which were formally introduced to Britain from America in 1876, although some sightings were reported in the 1820's.
The problem for both the Red squirrels which have been in Britain for ten thousand years and the Grey Squirrels is the food source issues.

Red Squirrels eat almost exclusively matured nuts, while the Grey Squirrels which is considered to have nearly wiped out the resident population of Red Squirrels since the Grey's introduction have a more varied diet.

The newly mutated form of the Grey Squirrel is a far less picky eater and will eat worms, nuts, or even insects which accounts for the Black Squirrels ever advancing movement against the range of the Grey Squirrels. The Black Squirrels also are reported to have higher testosterone levels and seem to be more attractive to female Grey Squirrels. (how does a scientist determine that? Do they eavesdrop at the local squirrel hangouts? How do they know it's not just higher pheromone production?)

The Red Squirrels are already a protected species in Britain.

In an interview with the London Telegraph

Lindsey Maguire, 50, a coordinator with the National Squirrel Rescue team, said black squirrels were an increasingly common sight and it may be that the greys will get their "just desserts.

"I wonder how long it will be before we see a 'save the grey' campaign," she said.

Dr Alison Thomas and Geneticist Helen McRobie conducted a year long study of the habits and ranges of the Black Squirrel as it makes inroads against the Greys territory.
One other quote worthy of note is from Dr. Thomas.

The rise of the black is the biggest change in squirrel demographics since the indigenous red squirrels almost disappeared 50 years ago from most of England.

Some have suggested Dr. Thomas has too much time on her hands if she can make a year long study of squirrel demographics, unless the squirrels have been given the vote.

Part of the problem for the Greys is the ability of the blacks to interbreed with the Greys, being part of the same breed, unlike the Red's who are a separate species.
The Black Squirrels also, having higher testosterone levels are more aggressive.

Daily Mail, U.K. Genetic differences show advantages image:39452:2::0

side note:
In scrolling through the Daily Mail vesion of the story I spotted an article I didn't read, but found the Intro absolutely grand regarding the recent divorce of John Cleese from his third wife. I just enjoy British newpapers a lot.

She's demanding £8million in homes and £71,000 a month to pay for clothes and holidays. No wonder John Cleese's brutal divorce battle has left him quivering with 'buttock-clenching' rage

In my not so humble opinion £71,000 per month is either a lot of clothes or a metric buttocks load of holidays.