[LINK] "Biology as a historical parameter"

[rfmcdonald]

Over at GNXP, Razib Khan has a rejoinder to the historians who don't believe that the mass depopulation of the New World by the diseases brought in the post-Columbian era was possible: the real, horrific, example of Tahiti, where populations fell to less than ten thousand in the 1840s (curiously, or not, about the same time that Tahiti became French).

One of the implicit aspects of Belich’s skepticism is that population crash models in the New World are rooted in inferences, not concrete censuses. But I recently stumbled onto a “test case” for contact between Europeans and indigenous people where we have some good data sets, Tahiti.

The initial data point of ~50,000 in 1767 is a low boundary estimate. But the subsequent data points are more concrete, from missionary surveys, or censuses. Even excluding the estimate the pre-contact population of Tahiti island seems to have dropped by one half in a two generation period. This is greater than the average decrease of the Black Death in Europe.

I think the reason for these massive population collapses when isolated groups meet more cosmopolitan ones is simple: they compress many generations of natural selection and immunity acquisition into just a few. In the historical record we know that the 2nd century A.D. witnessed the outbreak of plagues in the Roman Empire, and the subsequent decline and fall was concomitant with the endemic status of malaria in the Italian lowlands. The great Plague of Justinian in the late 6th century has been fingered as the causal factor behind the rise of Islam, the replacement of Celtic Britons by Anglo-Saxons, and the end of the Classical World more generally. Populations isolated from the grinding pathologies of Malthusian agricultural interlude just experienced it in all its glorious misery in a very short burst.

Islands can be quite vulnerable to epidemics, not only because of their isolation but because of the compactness that makes the rapid spread of a disease possible.