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Sharing diseases with relatives and neighbors

Not enough people voted on the Reader’s Choice, so this week’s paper is “Phylogeny and geography predict pathogen community similarity in wild primates and humans? by Jonathan Davies and Amy Pedersen, published in Proceedings of the Royal Society.

Many humans diseases, from flu to AIDS, come from other species. Similarly, diseases from dogs are an increasing threat to lions, while cat diseases kill sea otters. Are there general rules that predict how likely two species are to share diseases?

To find out, the authors analyzed several large data sets on diseases of humans and 117 other species of primate (apes, monkeys, etc.). They hypothesized that species are more likely to share diseases if they live near each other and/or if they are more closely related, that is if they share a more recent common ancestor. This is similar to how we define relatedness in humans: brothers and sisters have more recent common ancestors (parents) than cousins do (grandparents). Fortunately, the family tree for primates is relatively uncontroversial, at least among scientists.

Overall patterns were consistent with both of their hypotheses. For example, pairs of species whose last common ancestor lived 20 million years ago were more than twice as likely to share diseases, relative to less-related pairs of species, whose last common ancestor lived 40 million years ago. Within each relatedness class, pairs of species were about three times more likely to share diseases if they live near each other.

How can species that don’t live near each other share the same disease-causing microbes? Maybe they lived near each other some time in the past, or maybe the microbes were spread by species that travel widely: migrating birds, say, or ecotourists. Wash the soil off your boots before you travel to the Galapagos, OK?

For protozoa (microbes bigger and more complex than bacteria), host relatedness was most important. But for viruses, host relatedness was less important. Because viruses evolve faster than protozoa, they can more easily adapt to new hosts.

Guns, Germs, and Steel suggested that the European diseases that killed so many Americans after 1492 originally came from livestock. Consistent with the results in this paper, these diseases were mostly caused by viruses, which could more easily adapt to new hosts.

Cows and sheep are much less related to humans and chimps than humans and chimps are to each other, but I've never gotten a clear answer from creationists as to where they think the boundaries of "created kinds" lie. Since they like to caricature evolution as a frog "turning into" a cow -- an even larger change that would take millions of generations -- maybe apes, or even mammals, are all the same "created kind"?

Also this week:

Environment-contingent sexual selection in a colour polymorphic fish

Encoding choosiness: female attraction requires prior physical contact with individual male scents in mice

Subtle cues of predation risk: starlings respond to a predator's direction of eye-gaze

Local resource competition and local resource enhancement shape primate birth sex ratios

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