Science in Christian Perspective
From: JASA 7 (March
The Role of Recombination in Speciation: The three previous articles have discussed the parts played by point mutation, chromosome rearrangement and polyploidy in the formation of new species. Examples were given whenever available. The fourth method, it seems to us, is that of crossing, hybridization or recombination. The term hybrid has had several meanings in the past but in this article the term is restricted to crosses between species or higher categories.
Hybridization was the most obvious method of species formation prior to the discovery of the first three methods mentioned above. Lotsy, in 1916, was the great proponent of crossing. Conway Zirkle's book "The Beginning's of Plant Hybridization" traces the history of the subject and is well worth reading. He mentions, among other interesting things, that Galen, 130-200 A.D, recognized hybrids as being interInediate in appearance between the parents. The test for a hybrid character now in the twentieth century has become quite involved and employs the techniques of a number of disciplines including morphology, cytology, breeding, genetics and biometry. The best test involves the resynthesis of the hybrid from its putative parents. Secondarily, chromosome lagging in the meiotic divisions and more than 5017o bad pollen are good clues.
A great deal has been made of the sterility of hybrids by those believing in the fixity of species. The mule is almost always given as the prime proof of the futility of crossing in promoting new species. The writer is now compiling a list of all known hybrids as found in the literature (list will be finished about 1980) and it is quite surprising to note the number of hybrids that are listed as fertile. I fully expect to have several hundred fertile hybrids listed by the time the study is finished. A few examples may be useful -Platanus acerifolia (P. arientalis x P. occidentalis) has normal meiosis and is highly fertile-Salvia mellifera and S. apia,na form fertile hybrids and Paecilosis pomona~ria x P. isabellae cross to produce fertile offspring. The first two examples are plants and the third one is a fish cross. There are dozens more that could be cited.
It is well-known fact that the more similar the chromosomes and genes in two species, the more easily they can cross. We therefore expect to find fewer successful crosses between genera and higher categories than between species. It cannot be denied that the majority of hybrids are sterile but there are enough fertile ones to completely discredit any belief in the fixity of species.
In fairly recent years it has been discovered that sterile hybrids can become fertile by chromosome doubling or allopolyploidy. This doubling provides mates for otherwise mate-less chromosomes and pairing in meiosis can go on more or less normally. It has been said that a large proportion of genera and even higher categories of angiospermous plants may be polyploid and of presumably hybrid origin. Goodspeed and Bradley (Bot. Rev. 8 (5) : 271-316, 1942) listed over one hundred plants which combined hybridity with chromosome doubling in their fori-nation. Julian Huxley in "Evolution, the Modern Synthesis" says that allopoloyploidy has undoubtedly played an important role in the evolution of many plant genera. Stebbins says in "Variation and Evolution in Plants" that allopolyploids have been synthesized in forty or more instances and that they have regular pairing of the chromosomes..
Another interesting facet of the matter is the possibility of confusing mutations with hybridization. Dr. Ezra Brainerd worked with the genus Viola in the early part of this century. He found that Viola affinis produced black seeds instead of yellow ones, that V. cucMllata produced dark purple capsules instead of clear green ones and that V. nephrophylla had buff seeds instead of black ones. These abberations were termed mutations by many scientists but Brainerd showed that the plants in question were hybrids and he was able to reproduce them to show their exact parentage.
Probably the most stalwart supporters of hybridization in this country is Dr. Edgar Anderson of the Missouri Botanic Garden. He suspects that there are more hybrids abroad than is commonly supposed, even going so far as to doubt the genetic purity of the common organisms used in genetic research. His catch-words are "introgressive hybridization" and "hybridization of the habitat". In regard to the first term we might say that hybridization under natural conditions results in repeated backcrossing to one or both of the parents. With each backcross, the hybrid nature becomes less apparent. Hence it is suspected that many supposedly normal species are, in reality, hybrids. The unraveling of the derivation of these plants is a major operation but Anderson has succeeded in enough cases to prove his point and to open up an entirely new and enlarged vista in respect to evolution.
Through the course of time, species have become stabilized and have their own ecological niches to which they are adjusted. If the habitat is distributed or hybridized, then species not normally near one another, may be brought together and frequent hybridization may take place. This phenomenon (frequency of hybrids in disturbed habitats) can be seen in nature by the careful student where ditches have been dug, where timber has been cut and where roads have been built. Tornadoes, hurricanes, floods, and fires are natural hybridizers of the habitat. The fossil record is peculiar in many ways but one thing that stands out is the frequency of bursts of evolution that seem to occur, such as the proliferation of amphibians after the mid-paleozoic, or reptiles and ammonites after the late paleozoic earth movements and of mammalian forms in tertiary times. Natural forces which hybridized the habitat in those times may have been indirectly responsible for the bursts of evolution. Anderson believes that introgressive hybridization is more important than all the other forces put together in providing raw materials for natural selection to work on.
It will be quite interesting to note the course of development of our ideas regarding the forces causing speciation as time goes on. As indicated earlier hybridization was in great favor at one time; then, as other factors became known, it receeded from popular favor. Lately the idea has again come to the fore. Of course, many workers still think of it as a minor cause of speciation but until we have as many examples of species arising by mutation, chromosome rearrangement and polyploidy as we have by hybridization, we shall have to accord it a prominent place in the evolutionary scheme.