One type eats small fish and lives in closely-related pods, the other eats marine mammals such as dolphins and is related to the Antarctic killer whale.
If the two continue on the same trajectory, they could evolve into two different species and conservation efforts will need to be focused on each separately, the researchers claim.
Dr Andy Foote, from the University of Aberdeen, undertook the study along with colleagues from universities and museums in Denmark and the UK.
“It’s exciting to think about two very different types of killer whale in the waters around Britain,” said Dr Foote.
“Killer whales aren’t really a species that we think of as being a regular visitor to Britain, but in fact we have two forms of these killer whales in our waters.”
Dr Foote and his team made their discovery by examining the teeth from remains of killer whales stranded over the past 200 years. They found that the teeth fell into two distinct camps: those that were well-worn and those featuring virtually no wear at all.
“We found that one form, which we call ‘type 1’ had severely worn teeth in all adult specimens,” Dr Foote said.
“The other form, ‘type 2’, had virtually no tooth wear even in the largest adults.”
The research team tested the teeth to find out about the mammals’ diets. They found that the killer whale with the worn teeth generally consumed fish such as herring and mackerel as well as seals, whereas the ‘type 2’ killer whale fed on smaller whales and dolphins.
Their differences in diet also affected their shape and size, along with colour, pattern and number of teeth.
Type 2 adult males were almost two metres larger than types 1 males. Whereas the type 1 whales are closely-related and found across the North East Atlantic and around Britain, the type 2 whales are regularly seen off the west coast of Scotland and Ireland and, according to Dr Foote, are more closely related to a group of Antarctic killer whales.
The discovery shows how the creatures have evolved to suit their environments, and according to Dr Foote, could have important implications for how they are monitored by conservationists.
“It’s similar to how Darwin’s finches have adapted to different ecological roles in the Galapagos, but on a larger scale,” he said.