Natural Hybridization: The successful matings in nature between individuals of two populations, distinguishable by heritable characteristics.

Michael Arnold & Hybridization
"The role of hybridization in evolution has been one of the most controversial topics in the whole field of evolutionary study"  (Stebbins 1963)
So starts Michael L. Arnold's (1997) book "Natural Hybridisation and Evolution."  In the 200-odd pages of text the associate professor of the department of Genetics at the University of Georgia attempts to resurrect the idea that hybridization may have played a role in evolution.

The sleeve summarises the book this way...

The occurrence of natural hybridization has far reaching implications in the evolution of some plant and animal species. This paradigm is not new. However, it seems to have fallen on hard times during the past several decades. Contrary to the popularly held view of natural hybridization as maladaptive or even a violation of divergent evolution, this book presents evidence that it plays a significant role in furthering diversification in organisms.
Natural Hybridization and Evolution examines the role of crosses between genetically divergent individuals as a creative force in organismal evolution. Instead of concentrating on natural hybridization as a tool for defining barriers to gene exchange, this text encourages the examination of these barriers to facilitate genotype predictions. The array of hybrid genotypes, after all, represents the material for evolution.

Arnold's introduction (p10) reminds us that in the world of botany the concept of hybridisation is taken for granted everywhere. He says "two viewpoints concerning the evolutionary importance of natural hybridisation crystallized during the period 1930-50. On the one hand, botanists emphasised the evolutionary potential of hybrid genotypes to occupy novel habitats   and thus act as the progenitors of new clades. In contrast, zoologists championed the view that hybridisation was maladaptive because individuals involved produced fewer and/or less-fertile progeny.

The 'zoological' viewpoint developed into the major paradigm for process-oriented studies of natural hybridisation, and the phylogenetic perspective was largely adopted by botanists."

The Cladistic Revolution
It would appear to be the case that today, most evolutionary theorists ignore hybridization as an important consideration. A whole methodology - cladistics - has arisen based upon the assumption that phylogenetic trees always split and never merge.
Even such eminent lights in the field of evolution such as Richard Dawkins seem to ignore the possibility that natural hybridisation may have played a role in the ancestry of the millions of living organisms on the planet. His books, such as "River Out of Eden" emphasise the splitting nature of gene flow and say nothing about the possibility that gene pools might come together. For example he describes speciation in the following way... "A new species comes into existence when an existing species divides into two. The river of genes forks in time. From a gene's point of view, speciation, the origin of new species is "the long goodbye." After a brief period of partial separation the two rivers go their separate ways forever, or until one or the other dries extinct into the sand." That's a pretty explicit denial, albeit only one in a popular science book, that speciation could ever result as a merging of two gene pools -  a strong endorsement of the cladistic model to explain all life on Earth.

My experience at UCL reinforced my view that cladistics was de rigueur and that everything in evolution had to make sense in terms of phylogenetic splitting where two or more groups radiate from one ancestral group and never the reverse.

Cladistics seemed to be so revered at UCL around  the time that one masters student even received a distinction for his thesis about the cladistics of Persian Rugs in 2000. The assumption was that cultural transfer, as embodied in the rug design, would be, predominantly from parent to child with occasional 'cladistic splittings'  accounting for the diversity seen today. The possibility that a rug designer might have seen two or more other designs and assimilated them into a new one was not considered.

When Kenyanthropus platyops was announced at UCL in 2001, Fred Spoor described in detail the difficulty they had had in placing this fossil safely in an existing paleospecies. It had traits that seemed to be derived from two separate lineages but rather than consider that this individual might have

been the result of a hybridization it was decided to place it in a brand new paleospecies of its own. Not just a new species, in fact, but a whole new genera. This is just one example of a fossil find with contradictory traits that has had to be placed into a brand new genus rather than accommodate those changes through some hybridisation model.

Hominoid Karyotypes
Everyone these days is aware of how close humans are to chimpanzees genetically. The figure 98.5% is bandied around as common knowledge as the amount of genic (genetic information) data that we share with Pan.
Not so well known is the fact that we alone have a very big difference in the way that genetic information is packaged. We have 46 chromosomes, unlike chimpanzees, gorillas and orang-utans which have 48.
This karyotypic difference is very significant and, as Max King describes in detail in his book 'Species Evolution', is likely to have caused a massive barrier to gene flow between populations with different karyotypes.
Surprisingly, considering the potential significance of this fact, chromosome number differences in Hominoidae does not seem to have been studied a great deal and the models that have been put forward to explain how our chromosome number changed do not seem (to me at least) to have very much explanatory power.

It occurred to me that these problems might be linked. Perhaps this is another piece in solving the puzzle of human evolution that has simply been overlooked. In the same way that the role of water in human evolution appears to have been overlooked perhaps hybridisation as a mechanism of speciation has been overlooked in zoological evolution theory generally.

Links: Hominoid Karyotypes and Human Evolution.

Evidence of animal hybridisation:
Hybridisation in fish