Over the past half-century, we have witnessed some remarkable paradigm shifts in science. Perhaps the best known is the acceptance in geology of plate tectonics, the idea first offered by Alfred Wegener that the surface of the earth is divided into large regions that shift over time. For example, eons ago Africa and South America fit together like jigsaw puzzle pieces.
Another striking change has been in the basis for classification of all living things on earth, the so-called tree of life. You may recall your school biology book illustrating the various life forms by this tree diagram with larger branches representing divisions like plants and animals, offshoots standing for groups like families and genera, while the outermost twigs are finally species like scarlet tanager, common buttercup, monarch butterfly, gray fox, sea cucumber, E. coli and, yes, human being.
Until about 30 years ago, this diagram was not only considerably less complicated, but it was determined solely on the basis of similar physical structures. Lineages among animals with skulls could be determined partly by differences in their jaws.
Classification today is instead largely based on molecular biology: DNA differences separate those tree of life branches. While the genetic analysis largely confirms the earlier physical divisions, many differences have been discovered. If, for example, you looked at a modern bird field guide, you would find the order unlike that of earlier guides. Those earlier versions had the loon on the first page; now it follows all the waterfowl and even the game birds.
I had the great good fortune of enjoying the friendship of one of the first ornithologists who worked on this reclassification. I defended a widely criticized position taken by Charles Sibley and he wrote to thank me. Based on this, we developed a friendship and corresponded as he continued his work at Cornell and other universities until his death in 1998. According to his Wikipedia entry: “He had an immense influence on the scientific classification of birds, and the work that Sibley initiated has substantially altered our understanding of the evolutionary history of modern birds.”
I feel that I was especially fortunate because Sibley remained a controversial figure with an acid tongue. According to his friend Richard Schodde, “lesser mortals were not tolerated easily and, as has been said by others, collegiate friends were few.” I saw instead the gracious side of this famous scientist and I was deeply saddened by his loss.
I thought of Sibley when I read an interesting technical paper in the Condor by Amy Weibel and William Moore about downy and hairy woodpeckers. I am convinced that Sibley would have been delighted to read it. (I cannot recommend it to regular readers, however, because it is rife with technical language including words like synapomorphies, intron, homoplasies, clade and dimorphism.)
If you have a bird feeding station, you surely know these two woodpecker species. They are not at all easy to tell apart. The hairy woodpecker is slightly larger than the downy and has a larger bill; except for a feather or two, that’s it. (Males of both species have a red rear crown dot.) The only species I know that look more alike are the alder and willow flycatchers that even banders cannot differentiate.
It turns out, however, that these two woodpeckers belong to distinct groups, and tracing back along their branches, you pass through three divisions for the downy and two for the hairy before you reach a common branch-ancestor. Although this is a bit like tracing your own genealogy back to your forebears, these ornithological branches each stand for many generations.
Thus these two look-alikes represent convergent evolution: two different branches leading to twigs with similar characteristics. Other examples of convergence are monarch and viceroy butterflies (also appearance), bats and birds (flight), porcupines and hedgehogs (quills) and koalas and humans (fingerprints).
Weibel and Moore trace how the woodpeckers’ physical characteristics changed in parallel with their molecular changes, some turned on and then off over time, but finally leading to this remarkable similarity.
What caused this final convergence? The authors offer several possibilities related to flocking, aggression and territoriality, but these remain untested hypotheses. Molecular analysis provides no help there.