Flightless Birds Run Down Evolution
Newly Discovered Example of Convergence Challenges Biological Evolution
Lately, my wife has had trouble hanging onto cell phones. Within the span of two weeks she lost not one cell phone, but two.
But my wife is not the only one who has lost the same thing over and over again. Recently, evolutionary biologists have discovered that birds lost the ability to fly on several separate occasions.
According to new research each time birds lost the ability to fly, the outcome was virtually identical. This remarkable result makes little sense within an evolutionary framework, raising significant questions about the validity of the naturalistic explanation for life’s history and diversity.
Evolutionary biologists have long had interest in understanding the origin of a group of flightless birds known as ratites. This group consists of birds like ostriches, emus, kiwis, rheas, cassowaries, and the extinct moas and elephant birds.
According to the standard evolutionary fare the ratites had a single origin, with all of them descending from a common flightless ancestor. The basis for this view stems from the fact that they all share common behavioral traits and anatomical features like the absence of keels in breast bones, smaller, simpler, and fewer wing bones, bigger leg bones, and non-aerodynamic feathers.
One nagging problem with this evolutionary model is the biogeographical distribution of the flightless birds. They are found all over the world, with ostriches in Africa, rheas in South America, emus and cassowaries in Australia and New Guinea, kiwis and moas in New Zealand, and elephant birds in Madagascar. One way that evolutionary biologists account for this pattern is by appealing to continental drift. Accordingly, the common ancestor of the flightless birds supposedly emerged before continental breakup. Then as the different lineages formed they became separated and distributed around the world as landmasses drifted apart.
As appealing as this explanation appears to be, it doesn’t quite mesh with other evolutionary studies that estimate the emergence of the various lineages of flightless birds to have occurred at times that don’t correspond to the breakup of the continental landmasses.
A new study helps resolve this problem, but in turn creates other more significant issues for the evolutionary explanation of flightless birds and the evolutionary paradigm in general.
As part of this study, scientists used 20 different regions of nuclear DNA taken from 18 different bird taxa to build an evolutionary tree for ratites. It turns out that they identified not one but three distinct lineages for the ratites. The lineages include: one for ostriches; one for rheas; and one for kiwis, emus, and cassowaries. In other words, flight was lost on three separate occasions in ratites, not once.
This result nicely accounts for the biogeographical distribution of ratites. Loss of flight occurring at separate times is not that hard to envision. What is difficult to fathom is how independent instances of loss of flight would yield practically the same behavior and anatomical adaptations. This makes little sense within the evolutionary framework because evolution shouldn’t repeat.
As the late evolutionary biologist Stephen Jay Gould highlighted in his book Wonderful Life, if one were to push the rewind button, erase life’s history, and let the tape run again, the results would be completely different each time.
The very essence of the evolutionary process renders evolutionary outcomes non-repeatable. According to the concept of historical contingency, chance governs biological and biochemical evolution at its most fundamental level. Evolutionary pathways consist of a historical sequence of chance genetic changes operated on by natural selection, which also consists of chance components. As a consequence, if evolutionary events could be repeated, the outcome would be dramatically different every time. The inability of evolutionary processes to retrace the same path makes it highly unlikely that the same biological and biochemical designs should appear repeatedly throughout nature among unrelated organisms.
Contrary to what’s expected, evolutionary biologists note that “biological convergence” is widespread. This term refers to the widespread pattern in nature in which unrelated organisms possess nearly identical anatomical, physiological, behavioral, and biochemical characteristics. (Go here, here, and here for some recent articles I wrote on biological convergence.) The loss of flight in the ratites, on three separate occasions, adds one more example of biological convergence to an already lengthy list.