Wednesday, June 10, 2009

Where Domestic Selection and Natural Selection Collide

Discover reports that fishing is eliminating cod from shallow waters. The report draws on a study published earlier this year on the effects of human predation. The study notes that selective human predation ('harvest selection') has an effect on the rate at which phenotypes change and analyzes this effect in comparison with natural systems. The study conclude that human ‘perturbations’ – in particular, human targeting of select age- and size-classes – can cause more rapid phenotypic change than natural systems with strong directional pressure. While whether these phenotypic changes count as evolutionary or merely environmentally-plastic depends on which flavor of evolutionary theory one happens to favor, I suggest that such selective phenotypic changes are illuminating in light of Darwin's development of his theory of natural selection.

In their analysis, the researchers found that morphological traits declined in 94.9% of cases with an average decrease of 18.3%. Shifts in life history traits – reproductive age, etc. – occurred in 97.2% of cases with an average change of 24.4%. Comparing these results to a database on trait changes, they found that the rates of change in human cases far outstripped those in natural cases. The researchers concluded that the magnitude of change resulted from the direct nature of selective harvesting versus the indirect nature of other cases of selection, including other, non-targeted anthropogenic cases (eg. pollution). Although the study confines itself to a discussion of the science, the results obviously have broad economic implications, especially for those whose livelihoods depend on selective harvesting.

The study’s results seem trivial. Of course humans have an effect on biological systems which, in many cases far outstrips any natural cause. One need only look at dwindling fish stocks to come to this somber realization. However, the case of harvest selection raises interesting observations about Darwin’s evolutionary theory as he conceived of it. Darwin’s On the Origin of Species is clearly influenced by cases of targeted selection. In fact, as everyone should by now be aware, he uses domestic selection as a means of priming his audience for the possibility of some naturalistic mechanism for evolutionary change (ie. ‘natural selection’). However, in On the Origin of Species, Darwin used domestic selection as an analogy for natural selection and maintained a firm delineation between the two. His ‘laws’ of variation apply only to natural systems. In particular, Darwin sought to deprive natural selection of anything resembling a goal or telos. He remained dismissive of the ‘sports’ produced by selective breeding. Even when Darwin did venture into the realm of anthropogenic changes – for example, the famous case of the peppered moth – he confined himself to untargeted changes. I wonder whether Darwin’s failure to connect domestic selection and natural selection into a common theory was caused by his desire to contribute something profoundly new to science, rather than admitting that his views constituted a recombination of existing theories.

Ben Picozzi

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