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The big issues

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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.

When gophers mate, their lice have an opportunity to mate as well

Consider what happens to the lice if the gopher lineage splits into lineages A and B:

  1. Lice have few opportunities for gopher-switching, and lice on gopher lineage A don't mate with lice living on gopher lineage B.
  2. This "geographic" isolation of the louse lineages may cause them to become reproductively isolated as well, and hence, separate species.

When gophers become isolated, their lice become isolated as well

Evolutionary biologists can often tell when lineages have cospeciated because the parasite phylogeny will "mirror" the host phylogeny.

Observing parallel host and parasite phylogenies is evidence of cospeciation
Observing parallel host and parasite phylogenies is evidence of cospeciation. Download the graphics on this page from the Image library.

This example is somewhat idealized — rarely 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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