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If the association between two species is very close, they may speciate in parallel. This is called cospeciation. It is especially likely to happen between parasites and their hosts.
To see how it works, imagine a species of louse living on a species of gopher. When the gophers get together to mate, the lice get an opportunity to switch gophers and perhaps mate with lice on another gopher. Gopher-switching allows genes to flow through the louse species.
Consider what happens to the lice if the gopher lineage splits into lineages A and B:
Evolutionary biologists can often tell when lineages have cospeciated because the parasite phylogeny will mirror the host phylogeny.
This example is somewhat idealizedrarely do scientists find hosts and parasites with exactly matching phylogenies. However, sometimes the phylogenies indicate that cospeciation did happen along with some host-switching.
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Spanish translation of Understanding Evolution For Teachers from the Spanish Society of Evolutionary Biology.