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Flying Snails and the Strength of Weak Ties

A population of horn snails dwelling in intertidal habitats split into two about 3 million years ago when a land bridge form near Panama. These two populations the proceeded to evolve into the two species known today as the Atlantic horn snail and Pacific horn snail. Being separated, it would appear that the two populations of snails could not interact with each other. However, ecologist Mark Torchin and his colleagues found that the snails shared genes at least twice after the land bridge formed, a distance of over 200 kilometers. How could this have happened? Going off the work of George Simpson, Torchin suspects that birds such as herons may have consumed the snails live – shell and all – and ferried them across to the other ocean.

Events such as these can be considered a weak tie between two populations of snails. However, like in Granovetter’s interview subjects, this weak tie may have much larger effects than a strong tie between two nearby snail populations. For example, the ferried snail may bring over completely different new genes that help snails fight off a particular disease. In contrast, two closely related populations of snails would already have shared any meaningful genes, and be prone to an attack. This particular weak tie is a bridge, as interactions between snails on the Atlantic and snails on the Pacific are quite rare.

Biology has a wealth of relationships such as these. Bacteria have developed their own method of sharing genes, in the form of a circular piece of DNA called a plasmid. Two populations of bacteria will commonly trade plasmids with each other, giving each other advantages such as antibiotic resistance or the capability to metabolize some sugar. While two bacterium from within a population may have a strong tie, the weak tie from a third bacterium from a neighboring population may offer the antibiotic resistance needed to survive another day.



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