By Matt Walker
Editor, Earth News
A tale of two orcas: the larger type B form
Orcas, commonly known as killer whales, are still evolving, and quickly.
Researchers have discovered that two distinct types of orca, a large and a pygmy form, are rapidly diverging, evolving away from each other.
The scientists' study reveals each type of orca carries a unique gene mutation that benefits its particular lifestyle.
The genetic change has occurred in the past 150,000 years, adding to evidence that the orcas are quickly evolving into two distinct species.
Details of the research are published in the journal Biology Letters by an international team of scientists led by orca expert Dr Andrew Foote of the Natural History Museum of Denmark based at the University of Copenhagen.
The orcas studied live in Antarctic waters, and are known as type B and type C orcas.
The dwarf type C orca
Type Bs are one of the largest forms of orca known and primarily feed on seals.
Type Cs, in contrast, are known as a dwarf-form of orca, and feed mainly on fish.
These differences in size and diet, as well as each type having distinct markings, has led Dr Foote and colleagues to previously propose that they could be two separate species.
Genetic evidence now backs that idea.
Dr Foote and his team analysed the mitochondrial genomes of 15 type B and 36 type C orcas, specifically looking at a gene known as cytochrome B, a gene that plays a significant role in the orca's mitochondria, the structures within cells that govern energy production and metabolism.
They found that type B and type C orcas have evolved different amino acids within this gene, that affect its performance.
All type B orcas have replaced one type of amino acid with another at a place on the gene known as site 279, and type Cs had replaced another amino acid at site 193.
"The mutation has spread throughout each type, so that all type B individuals we analysed the DNA for had the mutation and almost all of the type C individuals had the other mutation," says Dr Foote.
He explains how this mutation could be benefiting each type of orca.
"The gene under selection is important in producing energy for the body's cells, and so the mutations are probably linked to the metabolic requirements of these two types.
"Both types live in the Antarctic pack ice and therefore the low temperature of this habitat could be one selective pressure.
But the two mutations should have the opposite effect on metabolism to one another suggesting divergent evolution."
For example, type C is a dwarf killer whale morph reaching lengths of just over 6m, whereas type B is one of the largest killer whale morphs, being up to 50% larger than type C.
"So body size could also be the selective force on the gene linked to metabolism," Dr Foote told the BBC.
Neither mutation can be found in what is thought to be the mitochondrial genome of the orcas' recent ancestor.
That suggests the natural selection has fixed these mutations very quickly, and they appeared since type B and type C orcas diverged from their most recent common ancestor 150,000 years ago.