What do parasites and mountains have in common? They both keep populations apart and drive evolution, say researchers.

In the absence of geographical barriers such as mountains and oceans, parasite “wedges” keep populations of the same species apart, say Corey Fincher and Randy Thornhill of the University of New Mexico in the US. They claim this can provide the opportunity for populations and even new languages to evolve separately.

Fincher and Thornhill say their hypothesis explains the longstanding ecological debate about why it is that biological diversity decreases as you move away from the equator and towards the poles.

“Individuals must balance the benefits with the costs of contacting other members of the same species,” says Fincher. For humans interacting with each other, for example, benefits include the opportunity to mate and trade, but these come at a cost: the risk of contracting a parasite or disease.


“These costly interactions especially come from interacting with people who do not belong to your society or group, whose immune systems are adapted for a separate set of parasites than your own,” adds Fincher.

Socialising risk

The notion was first suggested in the 1970s by researchers who noticed that baboon populations living in the African savannah typically carried similar populations of bacterial fauna and would frequently interact.

In the rainforests, however, where each population tended to carry its own set of bacteria, primates typically interacted far less.

In the parasite-rich forest populations, interacting with others came with a high chance of contracting a lethal illness, making parasites an evolutionary driving force.

According to Fincher and Thornhill, similar situations can be found in human populations. Taking languages as their measure of diversity, the pair looked at the concentration of different languages within an area and the number of parasites in that same area.

Lacking detail?

They collected data on parasites and languages around the world and found that the diversity of indigenous human languages is correlated with the diversity of human disease-carrying parasites. The correlation was true on every continent and independently of historical contexts such as colonialism.

Mark Pagel, an evolutionary biologist at the University of Reading, UK, says the correlation is interesting but insufficient.

Fincher and Thornhill did not divide the world by linguistic region, but into six large continents. This means they do not have enough detail to link differences in parasite infections with different languages, Pagel says.

However, he adds that linguistic diversity does increase as you move from north to south in North America and the pattern is the same worldwide, mirroring the increase in diversity biodiversity as you move to the tropics.

‘Great dark secret’

But rather than parasites driving linguistic diversity, Pagel believes the explanation lies in an intrinsic human tendency to wage war. “I believe humans will separate and split whenever they can,” Pagel told New Scientist.

“You’ve got all these people wandering around the Amazon all doing more or less the same thing – hunting and gathering – so why do they all speak different languages?

“There must be some ecological force driving them to live in separate groups. We are intensely competitive and when the environment will support a small group living separately I believe humans will do that,” he says.

In ecologically poor environment, humans and other animals need to range further in order to find food and inevitably bump into each other and mix. But in ecologically rich areas, such as Papua New Guinea, villages that are just a few kilometres apart speak different languages.

To Pagel, this proves that populations that do not need to interact to survive won’t. “I think this is a great dark secret about humans,” he adds.

Journal reference: Oikos (DOI: 10.111/j.2008.0030-1229.16684.x)

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