Freaks of Nature: What Anomalies Tell Us About Development and Evolution
Mark S. Blumberg
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In most respects, Abigail and Brittany Hensel are normal American twins. Born and raised in a small town, they enjoy a close relationship, though each has her own tastes and personality. But the Hensels also share a body. Their two heads sit side-by-side on a single torso, with two arms and two legs. They have not only survived, but have developed into athletic, graceful young women. And that, writes Mark S. Blumberg, opens an extraordinary window onto human development and evolution.
In Freaks of Nature, Blumberg turns a scientist's eye on the oddities of nature, showing how a subject once relegated to the sideshow can help explain some of the deepest complexities of biology. Why, for example, does a two-headed human so resemble a two-headed minnow? What we need to understand, Blumberg argues, is that anomalies are the natural products of development, and it is through developmental mechanisms that evolution works. Freaks of Nature induces a kind of intellectual vertigo as it upends our intuitive understanding of biology. What really is an anomaly? Why is a limbless human a "freak," but a limbless reptile-a snake-a successful variation?
What we see as deformities, Blumberg writes, are merely alternative paths for development, which challenge both the creature itself and our ability to fit it into our familiar categories. Rather than mere dead-ends, many anomalies prove surprisingly survivable--as in the case of the goat without forelimbs that learned to walk upright. Blumberg explains how such variations occur, and points to the success of the Hensel sisters and the goat as examples of the extraordinary flexibility inherent in individual development. In taking seriously a subject that has often been shunned as discomfiting and embarrassing, Mark Blumberg sheds new light on how individuals--and entire species--develop, survive, and evolve.
without any constraints imposed from within the animal. Thus, the driving force of evolutionary change—natural selection—has to be an external one. But Bateson sensed that not all variation is equally possible—that developmental factors internal to the organism constrain or bias the range of forms that nature produces.35 Rather than view natural selection as the engine of evolution, Bateson placed his bet on variation. Indeed, in his Materials for the Study of Variation, Treated with Especial
cases of cyclopia, especially those that 82 S FREAKS OF NATURE run in families.43 Moreover, we should not forget that even environmental factors can produce their effects by modifying the activity of genes or the action of their products. In other words, both Wilder and Stockard were right and wrong, their disagreement reﬂecting an either–or, dichotomous mentality concerning the developmental roles of genes and environment. This mentality continues to confuse many people to this day. But
pup is on its way to upright walking, but it is not there yet. Day 1: Short hindlegs and tail Day 20: Lengthy hindlegs Day 41: Quadrupedal walk Day 50: Horzontal posture of the trunk Typical postures of the jerboa from birth through ﬁfty days of age (left). The skeleton of an adult jerboa (right). It is at this stage, at about 40 days of age, that a curious thing happens. With hind legs roughly three times the length of its forelegs, the pup is at an awkward stage. As it tries to walk on
locomotion and that the brain overrides the spinal cord to produce hopping. In a nutshell: The spinal cord “walks” and the brain “hops.” To explain this arrangement, we might wonder if the hop arrived late on the evolutionary scene, by which time the predominant locomotor pattern was ﬁrmly ingrained within the neural circuitry of the spinal cord. Such a scenario might explain the “walking” spinal cord and the need for the brain to take control to produce hopping. At its heart, this style of
in a random collection of male beetles. From this simple analysis we quickly ﬁnd that each species has a threshold body size that separates, so to speak, the men from the boys. Below that threshold, males are both small and hornless. Above that threshold, as bodies get bigger horns get longer. So it appears that a developmental switch generates two sub-populations of male beetles: The horned and the hornless. The haves and the have-nots. When we see such distinct groups of individuals, we may be