Other challenges and anomalies
In my view the various examples of stasis, some discussed in Chapter 9, form the most serious set of difficulties facing evolutionary theory today. In this chapter I try to continue in the spirit of Darwin's Chapters 6 and 7, and my own Chapters 8 and 9, and discuss various other theoretical challenges. Other investigators can undoubtedly identify major anomalies that I have overlooked. To these might be added an enormous list of single investigations that yielded results somewhat different from what had been expected. Some of these might well be of enormous importance, given the proper interpretation. The finding of mitotic irregularities and unexpected pigment patterns in maize leaves and kernels might have been shrugged off by many geneticists as a minor technical annoyance. Happily McClintock ( 1965, 1987) did more than shrug. She took the trouble to keep detailed records of the annoyances through successive generations and thereby discovered the previously unsuspected transposable elements.
Twenty years ago this was a much debated matter. Discussions began with J. B. S. Haldane's ( 1937, 1957) calling attention to the seemingly great demographic cost of natural selection, gained greater attention with the growing recognition of high levels of genetic variability in nature (history reviewed by Ayala ( 1982)), reached a high prominence with a decisive demonstration of this phenomenon ( Lewontin and Hubby 1966), and climaxed in Bruce Wallace's ( 1970) book Genetic Load (updated by Wallace ( 1987, 1989) and Reeve et al. 1988). A formally rather similar problem is currently worrying investigators of the evolution of developmental mechanisms ( Kauffman 1987; Wimsatt and Schank 1988).
In my opinion the problem was never solved, by Wallace or anyone else. It merely faded away, because people got interested in other things.