How birds are like islands (for lice)

Researchers document a new example of convergent evolution in bird lice.

In terms of evolution, islands are special.  The Galapagos Islands played a central role in the development of Darwin’s ideas on natural selection.  Alfred Wallace found the same inspiration while traveling among the islands of the Malay Archipelago.  With hindsight, it’s no surprise.  Island chains are great places to see how isolation and natural selection lead to the array of unique and exotic species that are found there.  Islands isolate groups of individuals from breeding with their counterparts on other islands and over time these isolated groups may diverge from each other, eventually leading to the emergence of new species.

Isolation doesn’t just occur on little bits of land in the middle of an ocean.  Lakes are like islands for fish.  The tops of mountains are like islands for alpine plants.

And birds can be islands, too — at least to lice — according to the work of Kevin Johnson and colleagues in a recently published paper in BMC Biology.

Think about it:  Since lice are tiny parasites that must stay put while feeding on their host, they will usually have an opportunity to mate only with other individuals that are hitching a ride on the same bird or when their host bird meets another bird carrying another potential lousy mate.  As a result, these isolated lice populations have a good opportunity to diverge and evolve into different species.  There are at least 2700 documented species of feather parasites that have evolved on these bird-islands.

But the really interesting part of their study is that they were able to determine, using molecular techniques, how the species of feather lice are related to each other.  In the process, they have added another example of convergent evolution to the textbooks.

In a nutshell, here’s how their story goes …

There are four general groups of lice: head, body, wing, and generalist.  Individuals within each of these groups tend to look alike.  Here’s a figure from their paper to demonstrate:

The particular form (morphological type) of each group is related to the “microhabitat” in which they live and the ways that the bird tries to get rid of the lice when they preen.  Wing lice will need to try and keep their ground as a bird’s beak is trying to pick them off, while head lice may have to avoid the same bird’s feet scratching to pry them off.

One reasonable hypothesis about how the many different species are related is that all the species that exist in the same microhabitat (and share the same morphological type) come from a common ancestor.  If true, then all the head lice would have arisen from one species of ancestral louse that was really good at living on the head of a bird.  If a group of head lice on a crow made their way onto a robin, a new species of head lice on robins might emerge.  Eventually, all the subsequent head lice (and all the head lice species we see today) would be related to each other and not as closely related to wing lice.

However, the genetics of lice do not follow this pattern.  The study documents many good examples where different species of lice on the same type of bird are more closely related to each other than you would expect.  In parrots, for example, the head, body, and wing lice are more closely related to each other than to the same kind of lice on other birds.

This appears to be another case to add to the long list of examples of convergent evolution, which means that different members of the same group (wing lice, for example) look alike because they experience a similar problem (preening by birds) not because they are closely related.

Check out the video on YouTube — it does a really nice job summarizing their findings and is narrated by the first author of the paper. (I wish more researchers would do the same!)

Thanks to Bug Girl’s Blog for tipping me off to this story.  Great for my introductory biology class.

Johnson, K.P., Shreve, S.M. & Smith, V.S. (2012). Repeated adaptive divergence of microhabitat specialization in avian feather lice, BMC Biology, 10 (1) DOI: 10.1186/1741-7007-10-52