Macro or micro? Fight looms over evolution

Evolution over deep time: is it in the genes, or the species?
Credit: Roger Harris/Science Photo Library

A new paper threatens to pit palaeontologists against the rest of the biological community and promises to reignite the often-prickly debate over the question of the level at which selection operates.

Carl Simpson, a researcher in palaeobiology at the Smithsonian Institution National Museum of Natural History, has revived the controversial idea of ‘species selection’: that selective forces in nature operate on whole species at a macroevolutionary scale, rather than on individuals at the microevolutionary level.

Macroevolution, mostly concerned with extinct species, is the study of large-scale evolutionary phenomena across vast time spans. By contrast, microevolution focusses on evolution in individuals and species over shorter periods, and is the realm of biologists concerned with living organisms, sometimes called neontologists.

Neontologists, overall, maintain that all evolutionary phenomena can be explained in microevolutionary terms. Macroevolutionists often disagree.

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.”

How this paper will be received, only time will tell. A 2010 paper in Nature saw the famous evolutionary biologist E. O. Wilson recant decades of commitment to the gene as the unit of selection, hinting instead at group selection. The mere suggestion of this brought a sharp rebuke from 137 scientists.

Simpson’s claim is more radical again, so we can only wait for the controversy to deepen.

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