Tuesday, June 19, 2007

Winged insects and degree of civilization   posted by agnostic @ 6/19/2007 10:30:00 PM

A recent article in Nature, which we blogged about here, reviewed the consequences of agriculture on the nature and prevalence of pathogens that have plagued human beings. One key datum that Wolfe et al. (2007) discuss is the difference between the vectors (or transmitters) of infectious disease in the tropical vs. non-tropical regions, where agriculture has flourished the longest: almost all of the nasty infectious diseases in the tropics are spread by winged insects [1], whereas most of those in the more advanced areas are spread by human-to-human contact, polluted water, or parasites of small animals (such as the fleas that spread the Bubonic Plague). One consequence of this is that, as the authors note, infectious diseases in the tropics tend to be chronic rather than acute -- in crowded populations that characterize agricultural societies, it won't take long for you to pass your germs to someone nearby, after which point you've served your purpose and can be left alone for the time being (if you aren't shortly killed). If there aren't many people nearby to infect, you're going to have to serve as the host for much longer.

The authors do not note, however, an important evolutionary reason for why the geography of tropical regions causes them to be more plagued by insect-transmitted disease. This shortcoming is odd considering that one of the authors, Jared Diamond, has written best-selling books on human evolution (The Third Chimpanzee) and geography and civilization (Guns, Germs, and Steel). I haven't read either of these in full, so to be sure he didn't cover this issue in GGS, I searched it at Amazon and found no discussion of the prevalence of wingedness among insects. In any case, the key pattern is that the proportion of insects that are winged increases as both latitude and altitude decrease. At a more fine-grained level, wingedness is more common in habitats that are in some sense temporary or unstable, while flightlessness is more common in more permanent, stable habitats [2].

The basic insight comes from life history theory: in unstable habitats, an individual may be born into awful conditions due to temporal and/or spatial hetereogeneity. Here, it will pay to have a means of migrating to a more hospitable area, while in less volatile habitats an individual probably won't get caught with their pants down, and so flightlessness would increase. Just think of the energy a bug would save by not growing and maintaining their wings if it didn't need them. Most tropical areas have all three features: low in elevation, close to the equator, and more unstable habitat-wise [3]. It's no surprise, then, that such areas are more wracked by insect-borne infectious diseases. There are simply far more winged bugs that can travel far distances transmitting pathogens to humans.

One consequence of all this chronic disease must surely be increased difficulty in founding, let alone maintaining, a great human civilization. Chronic diseases which begin to strike early on in life are likely one reason that mean sub-Saharan African IQ is about 70, while mean African-American IQ is about 85, a full standard-deviation above. Possible mechanisms are not difficult to think of: the parasite that causes Sleeping Sickness get into your brain and slowly destroys it, your body may divert resources to disease defense and repair rather than on "luxury" items like higher IQ, and so on.

Even controlling for IQ, being afflicted with chronic disease must sap one's ability to doggedly pursue long-term projects, whether artistic or scientific, that foster civilization. Probably the best shot sub-Saharan Africa has is south of the Zambezi River, which doesn't suffer from a tropical hellhole climate. At best they could reach the level of African-Americans, who don't dominate Silicon Valley, but who have contributed scores more to the world's culture than Africans in sub-Saharan Africa. [4] Even in the US, most high African-American culture has largely sprung from cities outside of the dreadful "humid subtropical" climate of the Southeastern states (for example, New York and Chicago).

That pattern is also evident among American Whites, by the way: at the most northern fringe of the Southeastern US there are first-rate research universities (Duke and UNC - Chapel Hill, both in North Carolina), but the region is largely bereft of civilization-propelling institutions. In fact, blogger Inductivist has shown, using General Social Survey data on Whites, that it is a larger source of and magnet for duller Whites, compared to other regions (see here and here). Now, clearly I'm not proposing that epidemic Sleeping Sickness, malaria, etc. are causing the problem in the US. But whatever the more numerous bugs in the Southeast are transmitting to humans, it could partially account for the discrepancy between its level of culture and that of the Northeast. Indeed, from Inductivist's reckoning, it appears that most intelligent people with any sense from that region decide to haul ass to the Mid-Atlantic and New England states.

[1] From p. 280 of Wolfe et al:

A higher proportion of the diseases is transmitted by insect vectors in the tropics (8/10) than in the temperate zones (2/15) (P less than 0.005, chi-square test, degrees of freedom, d.f. = 51). This difference may be partly related to the seasonal cessations or declines of temperate insect activity.

[2] For a brief overview, see pp. 349-56 of Roff (2002). For extensive literature reviews, simulations, and so on, see Roff (1990) and Roff (1994).

[3] As for the non-obvious claim of greater temporal variation as you move toward tropical areas, see Roff (1990: 405):

I tested the hypothesis that habitat persistence varies with latitude with data on the rates of succession on abandoned farmland. In the northerly states of the United States (Wisconsin, New Jersey, Illinois, and New York) shrubs appear only 10-20 yr after abandonment, and even after 40 yr succession does not proceed beyond a very open woodland/parkland condition (Thomson 1943, Bard 1952, Bazzaz 1968,1975, Mellinger and McNaughton 1975, Pickett 1982), while in the more southerly states of North Carolina, Tennessee, and Georgia a closed canopy is formed within 15-30 yr (Billings 1938, Oosting 1942, Quarterman 1957, Nicholson and Monk 1974, 1975, Lindsay and Bratton 1980). In the Mexican tropics invasion by trees occurs within the first 2 yr, and these may reach a height of 10 m within 5 yr (Purata 1986): in the upper Rio Negro region of the Amazon Basin a loose canopy of Cecropia spp. 5 m high was formed within 22 mo (Uhl et al. 1981).

Sidebar: Detroit is fortunate to be situated as far north as it is, or else the reclamation of the city by the wild would have wholly swallowed up most of the area long ago (see here too).

[4] Alternatively, they could follow the lead of the elite strata of South Asia, who have managed to build a civilization despite vying with tropical Africa for status as the world's chamberpot of infectious disease. There, though, the elites have striven for centuries to isolate themselves genetically from those in lower castes, as well as to minimize their physical contact with the even more bug-bitten lower classes.


Roff, D. (1990). The evolution of flightlessness in insects. Ecological Monographs, 60(4), 389-421.

-------- (1994). Habitat persistence and the evolution of wing dimorphism in insects. The American Naturalist, 144(5), 772-98.

-------- (2002). Life History Evolution. Sinauer Associates: Sunderland, MA.

Wolfe, N., C. Dunavan, & J. Diamond (2007). Origins of major human infectious diseases. Nature, 447, 279-83.

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