New proposal challenges definition of species

A fascinating new hypothesis promises to open another chapter in the long-running debate about whether species are real and how to define them.

A paper published in The Auk: Ornithological Advances puts an argument for a new species concept based on the relationship between two sets of genetic material found in every cell.

The most familiar concept of species is the one developed by Ernst Mayr, the famous German biologist and philosopher, in the 1940s.

This is the idea that distinct species form when subgroups of a parent species become unable to interbreed over time. Each group becomes “reproductively isolated’” and it is this that defines a species. This concept has problems, however, and can’t capture the complexity of real organisms in the wild.

However, Geoffrey Hill, Curator of Birds at Auburn University in Alabama, hypothesises that the relationship between the genes of a cell’s nucleus and those of its mitochondria might form a better defining characteristic.

Almost all multicellular organisms have mitochondria, small organelles inside the cells of animals, plants, and fungi. They function as the energy source for the cell and have their own DNA, different to that found in the nucleus. The two sets interact to enable the cell to efficiently produce energy.

However, mitochondrial DNA is prone to mutation and recent research indicates that the genes of the cell nucleus need to constantly evolve to keep up with these changes. If they do not ‘co-evolve’, then the cell, and the organism, will lose fitness as its energy production becomes increasingly inefficient.

Hill argues that this co-adaptation is the key to defining species, particularly in birds, and that being able to identify the interacting sets of genes in the nucleus and mitochondria is essential for establishing a species’ boundary.

It is the two sets of interacting co-adapted genes that define the unique essence of a species: what Hill calls the ‘mitonuclear compatibility species concept’.

For the moment, although promising, this concept is largely theoretical and awaits much empirical support. However, the concept of ‘species’ is a notoriously controversial one. It has been estimated by John Wilkins, a researcher in the philosophy of biology at the University of Melbourne, that there are 26 different species definitions in operation across the biological sciences.

Hill’s hypothesis makes that 27, so we can most likely expect his contribution to add to the debate, rather than provide a definitive conclusion to it. Nonetheless, the idea is intriguing and his paper makes several testable and significant predictions.

For those interested in the species debate it is a case of ‘watch this space’.