THE ONE THING THEY AGREE ON...:

In a paper, yet to be peer-reviewed, on the biological pre-print repository bioRxiv, Simpson has outlined a renewed case for species selection, using recent research and new insights, both scientific and philosophical. And this might be too much for the biological community to swallow.

The debate over levels of selection dates to Charles Darwin himself and concerns the question of what the 'unit of selection' is in evolutionary biology.

The default assumption is that the individual organism is the unit of selection. If individuals of a particular species possess a trait that gives them reproductive advantage over others, then these individuals will have more offspring.

If this trait is heritable, the offspring too will reproduce at a higher rate than other members of the species. With time, this leads to the advantageous trait becoming species-typical.

Here, selection is operating on individuals, and this percolates up to cause species-level characteristics.

While Darwin favoured this model, he recognised that certain biological phenomena, such as the sterility of workers in eusocial insects such as bees and ants, could best be explained if selection operated at a group level.

Since Darwin, scientists have posited different units of selection: genes, organelles, cells, colonies, groups and species among them.

Simpson's argument hinges on the kind of macroevolutionary phenomena common in palaeontology: speciation and extinction over deep-time. Species selection is real, he says, and is defined as, "a macroevolutionary analogue of natural selection, with species playing an analogous part akin to that played by organisms in microevolution".

Simpson takes issue with the argument that microevolutionary processes such as individual selection percolate up to cause macroevolutionary phenomena.

He presents evidence contradicting the idea, and concludes that the "macroevolutionary patterns we actually observe are not simply the accumulation of microevolutionary change... macroevolution occurs by changes within a population of species."

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