Cormorants of the world. Convergent or divergent evolution?

There are 37 known species of cormorants on the planet.  Although some look vastly different from the ones I photographed above, many of them look eerily similar.  For them to all be cormorants, they would have all evolved from an ancestral organism, a single species of fish-eating bird.  The success of this ancestor, its ability to fly great distances and eat fish wherever it found itself, no doubt led to its global presence.

Ecological pressures, such as food parameters, availability, nesting site, predation, and competition from other fish-eating species play a role in gene selection.  Then, there are the rare but beneficial mutations that occur and the process of random assortment that all play a role in speciation.  The question I am posing is, "Do these factors cause the birds to be more different or more alike?"  I believe the answer to be, "Both."

Convergent evolution happens when the driving forces of selection cause unrelated species to become more alike.  Polar animals have short ears, not that polar bears, arctic foxes, and artic hares have much in common other than being mammals.  In the same way, I believe that many of the parameters that have made cormorants successful remain very selectable traits and so the birds' shape, colour, feather specifics, eating habits, and remain consistent.  

Divergent evolution is when mutation, random assortment, and natural selection all power change in a species.  Darwin's study on the finches and tortoises of the Galapagos is a prime example.  Another great read on the subject is the speciation of Cichlids in Lake Victoria, Africa.  The idea here is that change is inevitable, even if what worked 50 million years ago still works today (thus the crocodile and the coelacanth).  Small changes in bill colour, changes in average size, and even changes in the choice of nesting sites and mating behaviour are likely to occur.  You can get an idea of how the ancestral organism has given way to the existing species by clicking here.

Should divergent forces cause a change, convergent forces may well bring an organism back toward the average.  There is quite a wide variety of cormorant characteristics besides the ones I photographed here.  Of the 37 cormorant species, some are white with black and some mostly black with white.  The bill has some mild differences and so on.  

I believe evolution to be an ongoing force that shapes a species' gene pool.  The problem is about our impact on that process.  We have created conditions whereby no amount of selection or adaptation will allow a species to survive.  Since the year 1500 it is estimated that some 900 species have gone extinct.  I believe man's influence on the environment has had a major part in that.  How many more will go extinct in the next 100 years?  Your guess is as good as mine, but I would eager a lot.

Thanks for reading.

Eric Svendsen     www.ericspix.com