Tag: Anatomy

islands diverging
beaks for seeds and bugs and blood
letters rearranged

Darwin’s finches are a group of 18 species of passerine birds found across the Galápagos Islands (hence their other name of Galápagos finches). The group are a poster child for Darwin’s theory of evolution by means of natural selection. During his voyage on the HMS Beagle he collected specimens from what later turned out to be 12 of the 18 species, although Darwin himself didn’t realise the significance at the time, not realising they were all types of finch (ornithologist John Gould corrected him about the species) and not recording which islands they came from (he was later able to correctly assign them based on the notes of others on the voyage).

As the finches colonised the islands and began to adapt to the varied habitats and food resources available, the different groups diverged from each other, resulting in the separate species we see today. The finches themselves have a huge variety of forms (likely leading to Darwin’s confusion), most notably in their beak shapes and sizes. Their beaks are highly specialised to the food sources available on the different islands, with different species feeding on nuts, seeds, flowers, nectar, leaves, cacti, and invertebrates (including insects, parasites, larvae and spiders).

Perhaps the strangest of all is the Vampire Ground Finch (Geospiza septentrionalis) which feeds on the blood of other birds such as blue-footed boobies and Nazca boobies. It’s theorised that this behaviour evolved from mutualistic behaviours where the finch would clean parasites from the plumage of larger birds. These days their sharp beaks are used to peck their victim’s skin until it starts bleeding and the finches feed on the blood. Their unpleasant behaviours don’t stop there, however, as they steal eggs and roll them into rocks to break the shells, and they’ll also eat guano – excrement from seabirds. Since fresh water is scarce on their home islands (Wolf Island and Darwin Island), they also feed on nectar from Galápagos prickly pear flowers.

Molecular studies of Darwin’s finches suggests that the timing and spatial expression of at least four genes are responsible for the differences in beak structure, alphabetic changes that led to anatomical changes: BMP4 (which encodes Bone morphogenetic protein 4), CaM (which encodes Calmodulin), ALX1 (which encodes ALX homeobox protein 1), and HMGA2 (which regulates the expression of other genes).

A note about the sciku: this sciku has been written using a scale and focussing structure – narrowing in from the vast islands to the beaks to the individual letters of DNA. Have you ever tried writing sciku with a focussing structure? If so, how did you get on? Let us know in the comments below!

Interested instances of evolution in action? Check out this sciku by Prof Matthew J. James on the classic example of evolution, the Peppered Moth: Dark Moths.