Urbanization may affect many aspects of marine ecosystems, but as ecologists we often ignore the potential evolutionary consequences of urban development. Several years ago now, there was a great study on just this by Andrew Hendry and colleagues in Proceedings of the Royal Society. They studied the most famous of model species for evolution: Darwin’s finches (the ground finch, genus Geospiza).
Specifically, Hendry et al. were studying two morphs (right) of the Medium Ground Finch (Geospiza fortis) on the island of Santa Cruz in Galapagos. The two morphs had different beaks sizes (used for eating different foods), different songs, and tended to mate preferentially within their morph (ie – with other birds that looked like them).
By documenting the differences between these two morphs, Hendry and colleagues expected they were in the process of observing a speciation event. But then something strange happened: in areas on the island where there were lots of humans, the morphological differences between the two morphs diminished. In particular, the “bimodal” (distinct large and small) beak sizes of the two morphs fused and beak size became widely variable in more urban locations. Why would this happen? Human activities had made a wider variety of food sources available to the finches (see photo of finch on cereal box). The little birds indulged in the anthropogenic cornucopia urbanization offered, and human food of all shapes and sizes was sure to satiate regardless of one’s beak morphology. Selection against intermediate beak size was thus eliminated, allowing birds of all beak sizes to thrive in urban areas. As Hendry describes it, humans were causing “reverse speciation”.
This is old news really, as it was published almost a decade ago. Two things have led me to post about it now: (1) I recently came across this blog post by Hendry on eco-evo, which I highly suggest, and (2) Hendry’s findings were featured in Episode 3 of Galapagos, which was made available on YouTube earlier this month. (The episode in its entirety is worth making an evening of! But skip forward to 36:08 if you just want to hear about urban finches).
To my knowledge, there have yet to be any comparable examples documented in the marine environment. Other human activities are known to influence evolutionary traits in marine organisms. For instance, fishing can lead to an earlier age or smaller size at which marine fish become reproductive. But evolutionary consequences from marine urbanization specifically are poorly understood.
With time and further study, we hope to know more!