A New Theory
At one time, before I had become a geneticist and an expert on hybrids, it would never have occurred to me that hybridization might play a direct and frequent role in the production of new forms of life. In fact, until I read the Origin of Species, I had never given hybrids much thought at all. I knew hybridization was a technique used in plant breeding and that a mule is a hybrid of a horse and an ass. But beyond these facts, my knowledge of hybrids was minimal. And yet I still made certain general assumptions about hybrids. For example, I had always believed organisms could interbreed only if they closely resembled each other and that some rule allowed biologists to predict with certainty whether a given cross was possible. I had also assumed hybrids were always sterile.
I was surprised, then, when I first encountered Darwin's observation that "No one has been able to point out what kind, or what amount, of difference in any recognizable character is sufficient to prevent two species crossing" and that "the facility of making a first cross between any two species is not always governed by their systematic affinity [that is, by how closely they are related] or degree of resemblance to one another." It was difficult for me to believe biologists lacked a firm predictive rule. I also balked when Darwin pointed out that hybrids are not always sterile:
Finally, looking to all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal.
Darwin's comments made me wonder: How often are hybrids fertile? Do they occur naturally? What is their evolutionary significance? I decided to investigate reported accounts of hybrids. As I read, it became clear that thousands of different crosses produce fertile hybrids. Eventually, in authoring a book on bird hybrids (Handbook of Avian Hybrids of the World, 2006), I found that the great majority of avian crosses, at least the majority of those for which data on fertility is available, actually do produce hybrids that are themselves capable of having offspring. Moreover, about half (i.e., about 1,800) of the crosses listed in my book occur in a natural setting. Many of these crosses occur on an ongoing basis and have produced permanent hybrid populations. I am currently working on a survey of hybridization among mammals. There, too, I have found that many crosses produce fertile hybrids and that many such crosses occur in a natural setting. In other categories of organisms, other workers have reported similar findings. Among fish and plants, such hybrids are even more numerous than among mammals or birds. Hybrids among invertebrates seem innumerable. After looking at so much data, I began to wonder: what becomes of these myriad hybrids that, from the standpoint of geological time, are being produced in such vast numbers? If many kinds of natural hybrids can produce offspring, which is clearly the case, what is the role of hybridization in the evolutionary process?
Over the years, I have accumulated information bearing on these issues and have made a particular study of genetic mechanisms known to produce new types of organisms, many of which involve hybridization. In the process, I have come to believe that certain important flaws in standard evolutionary theory have passed unnoticed, primarily because most people are unfamiliar with the actual facts of hybridization as revealed by observation. Indeed, in comparing modern attitudes toward hybrids with the statements of naturalists of bygone eras, I have come to the realization that we have retained certain potent presuppositions derived from ancient systems of thought without change and without substantiation. Some of these unfounded claims are axiomatic in the modern scientific account of the natural world. In particular, a stereotypic conception of hybrids as sterile evolutionary nonentities has remained largely in force. Even many biologists think this way. In the discussion offered on this website I have tried to replace this stereotype with a new image that better corresponds with available data. I also offer a different theory of evolution that, I am convinced, is more consistent with observation than is Darwin's. I hope the reader will be convinced as well. Of course, I expect no one to accept such a radical assertion on faith. The validity of any theory can be assessed only by sifting through the evidence, point by point, fact by fact. I have therefore done my best to construct my arguments with what James Boswell once described as "that diligence which alone can collect those scattered facts that genius, however acute, penetrating and luminous, cannot discover by its own force."
I want to thank the many friends and colleagues who in some way helped with this project. They are too numerous to name individually, but I would like to express my special appreciation to Stuart Katz and my wife Rebecca. Their endless enthusiasm and support for this project kept me going when I would otherwise surely have set it aside. NEXT PAGE >>
1. Book I, line 1. Translated by E. M. McCarthy. Original Latin: "In nova fert animus mutatas dicere formas corpora."
2. Darwin (1859: 170).
3. Lovejoy (1936: 7).
4. Even Buffon (1749-1804: XIX, 20) was aware hybrids capable of producing offspring are usually not as fertile as their pure parents.