Introgressive Hybridization (1949)
Edgar Anderson

Chapter 5

Introgression and Evolution

It is premature to attempt any generalizations as to the importance of introgressive hybridization in evolution. There is some evidence, mostly inferential, that it did indeed play a role. There are as yet no critical data to indicate whether that role was a major or minor one. Though it is certainly true that one cannot state with assurance that introgression was a major factor in evolution, it is quite as true that we cannot yet be certain that it was not a major factor. The chief purpose of this book is to indicate the kind of critical data that are needed before such questions as this can be discussed intelligently.

One problem that cannot be settled satisfactorily without further information is the extent to which the term introgression can be validly used. In the original instance it described introgression of one species into another. In many ways the flow of genes from one subspecies into another, or from one variety into another, or from one genus into another presents the same phenomenon. In other ways there are distinct peculiarities at each of these levels. We shall need to be much more fully informed before we can intelligently set exact limits to the use of the term. Throughout this book an attempt has been made to discuss the phenomenon on so fundamental a level that the term introgression would apply with equal validity whether the entities involved were subspecies, or genera.

If introgression proves to be a primary factor in evolution it will be because it so greatly enriches variation in the participating species. As raw material for evolution, the bizarre hybrid swarms described in Chapter I are not so important as the the Asclepias introgression described by Woodson (1947), which was barely noticeable in any one locality and extended as a trend through a long intermediate zone. By the time of the third backcross of the original hybrid to one of the parental species, there would be little or no external indication of hybridity in the mongrel progeny. Yet in terms of gene frequencies, the effects of introgression in such mongrels would far outweigh the immediate effects of gene mutation.

Such otherwise excellent studies of hybridization under natural conditions as those of Epling (1947) on Salvia, and those of Valentine (1948) on Primula, fall short of their greatest possible usefulness because they present neither precise data nor even rough estimates on this important point. Having in each case demonstrated that hybridization occurs frequently in nature, that the hybrids are partially fertile, and that some backcrossing does occur, they rest their case. Impressed by the evident fact that hybridization is not occurring on a scale large enough to have taxonomic consequences, they do not inquire into the more biologically significant problem whether it is having genetic consequences. A trickle of genes so slight as to be without any practical taxonomic result might still be many times more important than mutation in keeping up the basic variability of the parental species. The critical question, on which we have as yet almost no data, but which it should eventually be possible to answer exactly, is "How much of the variation in the supposedly pure parental populations is due to introgression?" There are some circumstantial data suggesting that introgression may be one of the main sources of that variability which provides the raw material for evolution. Woodson's detailed studies of Asclepias tuberosa and Turrill's and Marsden-Jones' work on Silene (see Marsden-Jones and Turrill, 1946) are examples of the kind of data we shall need before we can even discuss such a problem.

Nearly all the published data on introgression demonstrate its importance in areas where man has upset natural forces. We might logically expect that introgression would be equally effective when nature herself does the upsetting. Floods, fires, tornadoes, and hurricanes must certainly have operated upon natural vegetation long before the advent of man. Like man himself all these phenomena alter conditions catastrophically, break down barriers between species, and provide unusual new habitats in which hybrid derivatives may for a time find a foothold, thus serving as a bridge by which groups of genes from one species can invade the germplasm of another.

Not until one has lived in close proximity to a large midcontinental river does he realize what a restless neighbor such a waterway can be. It is forever changing its course and altering the habitats of plants that grow near it. Trees are undermined and swept away; sand to the depth of several feet is deposited on top of heavy clays or silt, thus changing the soil type and the ground-water level; plants are transported bodily; and not only do water levels change from day to day and week to week, but also the average level of the previous decade may be drastically altered by a whim of the river. In such a variable environment species that (through introgression) are able to achieve a great increase in genic variability should be at a selective advantage. It is apparently true that river-valley plants are more generally adaptable than those from other habitats. It would seem likely that introgression may be one of the natural forces that have brought about this greater adaptability. Exact data bearing on this point should not be difficult to obtain.

A demonstration of the evolutionary importance of "natural" introgression on a much wider scale is emerging from a series of studies by various workers which are already well under way but for the most part have not yet been formally published. All suggest the probable importance of introgression at particular times and places when diverse floras were brought together in a changing environment. Mason and his collaborators (1942; see also Cain, 1944), working with living and fossil populations of the closed cone pines, are finding it possible to demonstrate these phenomena in a surprisingly exact fashion. Areas that were once a series of islands off the California coast have been united to the mainland by natural causes. In these areas species of pines that were previously isolated have been brought together in a newly emerged area in which somewhat diverse floras were in the process of settling down into a new, and supposedly more stable, equilibrium. Hybridization and introgression under such conditions might be able to play a much greater role than in a stabilized community of which all the members have long been selected for their ability to interlock effectively.

Woodson (1947) has presented data on the introgression between three well-differentiated geographical races of Asclepias tuberosa (butterfly weed). One of these is centered upon peninsular Florida, a region that was an island, or series of islands, in Tertiary times and was later connected with the mainland. Through introgression, the fusion of these two varieties has now become a gradual process, extending over an intermediate zone hundreds of miles in depth. The infiltration of the two varieties is so gradual as to be imperceptible to anything less acute than refined statistical methods. From what is generally known about the flora of northern Florida and the Gulf and Atlantic coastal plains it seems probable that the introgression of these two varieties of Asclepias is rather typical of that area. For genus after genus in the flora of the eastern states, there are well-differentiated species or varieties in southern and central Florida and equally well-differentiated entities on the Coastal Plain. In northern Florida there is centered an intermediate zone in which various transitions between the typical coast-plain type and the typical peninsular type make up the bulk of the populations. It would seem as if, when "Orange Island" was united to the mainland for the last time, two rather differentiated floras may have met in this intermediate zone. Under these unusual conditions, not only would there have been special opportunities for hybridization, but also, with two sets of plants readjusting themselves into new communities, some of the backcrosses would have been at a selective advantage. Thus introgression would have been encouraged in much the same ways as when man upsets the ordinary balance of nature.

It is probable that the same kind of phenomenon took place in the eastern United States after the last glaciation. Whenever the retreat of the continental ice was rapid, large areas must have been open for colonization, and sometimes at least they must have presented the invaders with new sets of soil types and habitats different from those previously known. When the ice front advanced again it may very likely have left isolated pockets of vegetation well behind the readvancing front. If these areas were small, the "Sewall Wright effect" would have produced local differentiation within the pocket so that at the next time of retreat there would be opportunities for these new highly localized varieties to introgress into the main body of the species. The distribution and differentiation of the northern blue flags (Iris versicolor and Iris virginica) suggest that a considerable area in the interior of the lower peninsula of Michigan may have been isolated for quite a time in this fashion. W. H. Camp has already given an informal report (1943) on his studies of hybridization in North American beeches (Fagus) which demonstrate the effect of the various retreats and advances of the ice front on introgression in that genus. With a series of studies on different genera we should be able to approach the subject experimentally rather than dogmatically.

It seems probable that a somewhat similar maw introgression may have taken place in the northern and eastern Ozarks in post-glacial times. During the xerothermic period when the prairie grasslands extended much farther east than they do now, many of our common woodland species of eastern North America must have existed in the Ozarks in small, isolated refuges. Today, in much the same way, small patches of isolated woodland are to be found in sheltered canyons in western Oklahoma. When the climate was distinctly hotter and drier than it is now, the central Ozarks in southern Missouri must have had a climate more like that of western Oklahoma today. With its increasingly severe climate and with small populations, opportunities for differentiation would have been great. As the hot, dry period came to a close and the mesophytic forests moved westward again, these remnants probably first spread out locally and then hybridized with their remote cousins as they came back into the territory. Desmarais (1947) has made an intensive study of the sugar maples which demonstrates something of what took place in that genus. More than one observant naturalist has noted slight regional differences in the Ozark representatives of many other wide-ranging species, which would indicate that the phenomenon may have been a very general one.

In his studies of introgression in Cistus (1941) Dansereau presented circumstantial evidence that the North African variety of C. ladaniferus originated through introgression of C. laurifolius into the typical variety (which is now limited to the Iberian peninsula and southern France). Although he presented no cytological or genetical evidence in support of this hypothesis, he did possess a detailed understanding of the genus Cistus from having monographed it and from having, as a trained ecologist, studied the problem in the field. Furthermore, he made detailed population samples that were analyzed by some of the methods discussed in Chapter 6. His explanation seems to be well established as a working hypothesis. If confirmed, it would be a further demonstration of the role of introgression in differentiating geographical varieties.

INTROGRESSION AND EVOLUTION UNDER DOMESTICATION

Such disturbances of the habitat as those previously described certainly must have occurred in prehuman times. It is just as certain that the appearance of man greatly accelerated such processes. On the one hand, by moving himself and his domesticated animals from place to place he removed geographical barriers between previously isolated species. On the other, he created new ecological niches in which hybrid segregates might find a foothold. Some of these niches were of definite types, and he created them everywhere he went. Of these one of the most important was his trash and dung heaps. He made these everywhere he halted, and, as he unconsciously bred the quick-growing weeds capable of utilizing soils high in nitrogen, he also unconsciously carried them about from place to place and gave them previously unparalleled opportunities to cross with others of their kind and thus build up into superweeds. From these weeds some of his crops were bred. There is good evidence that hemp started in that way, and from what was originally a weed plant there were at length evolved hemp as a fiber plant, hemp as a source of oil (from the seeds), and hemp as a narcotic drug (Vavilov, 1926; Parodi, 1935). The primitive chenopodiums and amaranths which are so widely grown as cereals by primitive peoples, in both the old world and new, show every indication of having originated in this fashion. Many of the cucurbits probably originated in the same way. Most, if not all, of the wild cucurbits are bitter or insipid. Introgression produced weed types that became camp followers. These were probably used first as dishes or rattles. Increasing variation produced some whose seeds were edible, and, still later, varieties with edible flesh were selected.

Evolution under domestication has been so complete that it is difficult to get exact data on the subject. In only a few instances can we point to the exact wild species from which a cultivated plant or a weed was derived. For some of the cultivated plants we know closely related wild species, though we have little or no evidence of the exact relation between them and the cultivated plant. In many other cases we can point to a group of weeds that are related to a cultivated plant. This is no solution to the problem. We now know that weeds may be bred from cultivated plants, as well as vice versa. Since weeds as we know them are largely man-made and inhabit ecological niches that are either directly or indirectly the results of man's interference, our "explanation" of the origin of such a crop is merely the posing of a much larger problem. Where and how were the cultivated plant and its related weeds bred out of the prehuman elements in the genus? Most of our cultivated plants, therefore, merely tell us that evolution has proceeded apace under domestication. Few of them are the kind of research material from which we can get a precise answer as to how the changes that occurred under domestication were brought about.

Accordingly, we shall first present (in simplified, pictorialized form) a hypothetical, generalized diagram of the way in which domestication of weeds and cultivated plants most probably took place. With that for reference, there will then be presented detailed evidence from various genera supporting the hypothesis. Plate 3, therefore, is a diagram of the way in which cultivated plants and weeds have been consciously and unconsciously developed from their wild progenitors. It is greatly simplified as compared with the actual history of most cultivated plants and weeds. For one thing, the special and complicating effects of polyploidy and apomixis are not included. With the occurrence of apomixis or of ploidy either before or after domestication, further complications would be added to the existing complexities of relationship.


Plate 3. Introgression under the influence of man. Diagram showing the role of introgression in building up cultivated plants and weeds in the hypothetical genus Planta. The ranges of the various species and varieties are represented upon an area of supposedly continental size. The plate shows the ranges of the species and varieties in prehuman times (at the top), then the successive steps by which the present condition (bottom of the plate) has been brought about. Further discussion in the text.
Turning to Plate 3, the diagram at the top of the plate concerns the five original entities in our mythical genus Planta and their fate under the influence of man. The diagram represents an area of continental size with one highly localized species, "P. endemica," in the east, and another species, "P. occidentale," in the far west. In the center of the continent are three entities, "P. laxa" and the two entities that we have grouped under "P. mixta," the variety "cruciformis" and the variety "punctata." Planta cruciformis and P. punctata are fairly well differentiated and for the most part occupy different areas, but in the zone where they approach each other (even in prehuman times) there was some hybridization and consequent introgression of genes from each into the germplasm of the other.

The second part of the diagram shows the unconscious effect of man upon this assemblage. When he occupies the territory, even though at first he takes no particular interest in the genus Planta, he removes barriers between the species and creates new ecological niches in which some of the hybrid segregates might survive. Consequently there is greatly increased introgression of P. cruciformis into P. punctata (we visualize cruciformis as being a weedy, rank, quick-growing, many-seeded plant even under natural conditions and likely, therefore, to contribute genes that would be at a selective advantage after the appearance of man). In addition, the barriers between P. laxa and P. mixta are broken down enough so that we get introgression of laxa into P. mixta var. punctata. Since laxa and punctata are highly differentiated species, the introduction of a relatively few genes will produce an increase in overall variability.

As this reciprocal introgression continues it produces certain new recombinations that are outstandingly useful to man, and at length some of these are gradually brought into cultivation. A new crop plant has come into being which we shall call P. utilis. Similarly, the addition of cruciformis genes to this same complex produces a more aggressive plant that grows of its own accord in the fields where utilis is being cultivated. Eventually, under the combined effects of natural selection, conscious human selection, and unconscious human selection, there are produced an aggressive weed, P. sativa, and an important world crop, P. utilis, both of which are spread more and more widely as they become increasingly adapted to their new roles. After many years P. utilis is cultivated within the narrow area to which P. endemica has been so long restricted Eventually an occasional hybrid is produced which backcrosses into the original P. endemica. The introduction of a very few genes from P. utilis greatly increases the variability and adaptability of P. endemica. As a result, though only slightly changed morphologically, it is now able to colonize a much larger territory than that to which it had previously been restricted, and it does, in fact, become almost "weedy" in its habits.

Meanwhile, by other routes, man has unwittingly carried his new weed P. sativa into the area of P. occidentalis. There the two hybridize and the hybrids backcross to P. sativa, increasing its variability still more. From the resulting intermixture there is bred a new and particularly aggressive form of this weed which spreads around the world and eventually becomes recognized as P. sativa var. peregrine.

So much for a part of the history of domestication in the hypothetical genus Planta. Let us now consider the difficulties of unraveling this history had Planta been an actual genus. We would have had little or no evidence about it as it occurred in prehuman or even in early human times. From the bewildering array of specimens in our herbaria, collected by different people and in a more or less haphazard fashion, from notes by agronomists who had cultivated P. utilis, and from our own powers of observation we should have had to put the story together. This would have been difficult. Someone interested in P. sativa might never have been able to make field studies in the original region where introgression took place so actively in P. mixta. Only occasionally would careful local field studies reveal to the scientific world such interesting phenomena as the effect of P. utilis on P.  endemica. Were the work to be done by purely conventional taxonomic methods, based upon the critical study and comparison of single specimens, a first-rate taxonomist might separate the genus into the following categories: (1) endemica, (2) mixta, (3) utilis-sativa, and (4) occidentale. From collections of single individuals it would not be possible to distinguish between the original endemica and its variety robusta. One could not in every instance separate some variants of sativa from some of those of utilis. Planta sativa peregrina could not be differentiated from sativa, and the intergrades between punctata and cruciformis would be confused with sativa and with utilis. Had population samples of these entities been examined, however, it would have been possible to define these entities exactly and to distinguish between them. Furthermore, by such methods as those outlined in Chapter 6 one could have considered dynamics of the whole group. He could have demonstrated that sativa peregrina differed from sativa by genes acquired from P. occidentalis and shown how a slight introgression from utilis had produced P.  endemica var. robusta.

For the great bulk of our cultivated plants it will be difficult, or impossible, to bring together the data on wild populations, weed populations, and geographical distribution which will permit us to demonstrate step by step them complicated process of domestication. The major areas of domestication (Asia Minor, Southwestern Asia) are difficult of access to most students. However, there am a few cultivated plants and weeds whose histories are more accessible, and for a few of them data on introgression are already beginning to appear. Of these the common cultivated sunflower, Helianthus annuus is in a class by itself in the degree to which we may some day hope to demonstrate in detail the steps by which it became a cultivated plant and a weed. It was domesticated in precolumbian times within the boundaries of the present United States. A considerable amount of prehistoric remains from archaeological sites are already available in museums. Its wild progenitors are still to be found in the United States in the west, south, and southwest. Heiser has already (1947a, 1947b, 1949) made a promising beginning at unraveling the story of its domestication. Though, in comparison with the great world crops such as rice, wheat, and maize, the history of the sunflower is a relatively simple one, it is so complicated that a decade or so of intensive work will be needed to establish the main points. As the story takes shape with such data as are now available, it is about as follows:

If we use the expression Helianthus annuus in its widest sense, there can at present be recognized the following different entities:

  1. Cultivated large-headed varieties (chiefly monocephalic), grown for their large, oily seeds.
  2. Large-headed and small-headed varieties grown for ornament.
  3. Weeds of the Great Plains and adjacent prairies, oftentimes growing in corn fields, gardens, etc.
  4. A second set of weeds, distinct from the preceding, limited to trash heaps, railroad yards, and the like, typical "camp followers."
  5. A third set of weeds in the irrigated valleys of the far west.

It is already known from careful experimental work that the large-headed condition is due to a single recessive gene, whose exact expression is conditioned by a few modifying factors. It suppresses the production of axillary buds and therefore forces the maximum amount of growth into the single head, which consequently bears much larger seeds. We do not yet know from archaeological evidence just where this mutation was picked up. We do know that it occurred very early, possibly before the Christian era. Sauer (1936) has suggested that the sunflower was domesticated before maize reached North America. Certainly, by early Basket-Maker times in the southwest, the large-flowered sunflower was being grown; we have not only the large seeds as evidence but also some prehistoric collections of the heads themselves.

The large-headed sunflowers, both in prehistoric times and at the present day, were a diverse lot, including purple-seeded varieties with long, narrow seeds (still grown by the Hopi and in northern Mexico) and white- and gray-seeded varieties with shorter, flatter seeds. Morphologically all these varieties are closer to Weed D than they are to Weed C, suggesting either that the weed originated after the cultivated variety had been differentiated or that in some way or other the weed arose out of the same complex. Both A and D (the cultivated varieties and the camp-follower weed) show morphological relationships to more than one of the wild-growing species of category C. Heiser has already been able to demonstrate the introgression that is going on between the C variety of H. annuus and the very different H. petiolaris of the Great Plains. It seems very probable that A and C originated in early prehistoric times when the natural introgression between the various original entities in this group was accelerated by the presence of man Out of the ensuing mixture came the cultivated plant and the camp-follower weed, the development of the former being very greatly accelerated by the appearance of the mutation of a large single head. Being recessive, single-headedness bred true as soon as its importance was realized, producing a superior crop that was more and more widely dispersed. In many areas to which it spread, it could by introgression contribute genes to the wild and weed sunflowers of the new area. Occasionally it might, through backcrossing, pick up a few useful genes from the wild sunflowers of that area. Ordinarily, however, the recessive nature of its most useful character (large-headedness) would have kept it from acquiring as many genes in this manner as it might otherwise have done.

Heiser's most complete evidence is for one of the later steps in this process. He has been able to demonstrate in detail the way in which one of the B categories has originated and is continuing to evolve. Helianthus Bolanderi was originally a distinctive, highly localized sunflower restricted to serpentine areas in northern California. Since the introduction of Helianthus annuus into that region, hybrids have occurred between the two species. Though they are very different from each other and the hybrids are partially sterile, enough introgression of annuus into Bolanderi has occurred to produce a vigorous weedy variant of the original serpentine sunflower. This more aggressive type is now spreading with increased rapidity in irrigated areas, continuing to cross occasionally with H. annuus, and is indeed a weed in the making. The main morphological facts are summarized in Table 3. Heiser analyzed the situation by field methods similar to those described in the next chapter and produced the above explanation as a working hypothesis. He then repeated the suspected cross between Bolanderi and annuus, grew progenies from suspected hybrids, and worked out the cytology of both species and their hybrids, both natural and artificial. His experimental data confirm and extend his original hypothesis, and the case has been proved beyond a reasonable doubt.

A similar demonstration of introgression between a cultivated plant and its weedy relative has been made by Marion Ownbey (unpublished). In the vicinity of Pullman, Washington, a variety of garden lettuce (Lactuca sativa) with dark red leaves is widely grown. This color difference is dominant in crosses with weed lettuce (Lactuca serriola), and one can therefore recognize naturally occurring hybrids between the two lettuces. Ordinarily, because so many of the characteristics of cultivated lettuce are recessives accumulated under domestication, the hybrid looks so unlike garden lettuce that it escapes critical notice. Using the red-leaved character as a marker, Ownbey has been able to demonstrate the extensive introgression that is continually going on from garden lettuces into weed lettuces, previously largely unsuspected because the hybrids and hybrid derivative mongrels were superficially so similar to wild lettuce and so unlike garden lettuce.

TABLE 3*
Comparison of Morphological Features of Helianthus annum, H. Bolanderi, and Their Hybrid
1
H. Bolanderi
(Serpentine, Foothill Race)
2
H. Bolanderi
(Valley Weed Race)
3
H. annuus x
H. Bolanderi
4
H. annuus (Western)
Height 3-10 dm. 6-13 dm. 6-15 dm. 8-18 dm.
Leaf Shape Linear-lanceolate to ovate lanceolate, cuneate at base Ovate-lanceolate to ovate, cuneate, rarely truncate at base Ovate-lanceolate to ovate, cuneate to truncate at base Ovate-lanceolate to ovate; truncate to cordate at base
Involucral Bracts 3.0-4.0 mm. broad; oblong to anceolate, gradually attenuate 3.5-4.5 mm. broad; otherwise much as in 1 5.0-7.0 mm. broad; lanceolate to ovate; more abruptly attenuate than 1 or 2, less so than in 4 5.0-7.0 mm. broad; lanceolate-ovate to ovate, abruptly attenuate
Pubescence Hirsute or hirsute-villous Hirsute, rarely somewhat hispid Hirsute to hispid Hispid
Ray Number 10-13 12-17 14-20 17-24
Diameter of Disk 1.5-2.0 cm. 2.0-2.5 cm. 2.0-3.0 cm. 2.5-3.5 cm.

* Adapted from Heiser (1947b).

An effective demonstration of the role of introgression in building up weed complexes is afforded by two species of fleabane, Erigeron annuus and Erigeron strigosus (=E. ramosus). These two native American plants were originally quite distinct from one another and had very different ecological requirements. Erigeron annuus prefers rich, moist situations; E. strigosus is a plant of dry, barren areas. In the eastern United States they have introgressed so extensively into each other that somewhat intermediate types are found exclusively over wide areas. Apomictical forms of both annuus and strigosus have occurred, some of which seem to have been very widespread. Weed strains of both species have spread far outside their original habitats, and have been carried to other continents.

In parts of their present ranges the two species have been so extensively blurred that it is difficult to conceive of what they may have been like before the advent of man. In other areas, however, they are well differentiated, though introgression is still continuing. Their relationships are quite clear in the northern Ozarks. There Erigeron strigosus forms large and only slightly variable populations in dry, rocky areas while Erigeron annuus, in essentially pure condition, is limited to rich and fairly moist locations, such as barnyards and fertile vegetable gardens. Intermediate populations are common throughout the area, the degree of intermediacy being proportional to the dryness and sterility of the habitat. Yet this intermediacy is something inherent, since cultures raised in the experimental garden retain the characteristics of the populations from which they were derived.

With many cultivated plants the nature and degree of introgression have probably changed as man has found new uses for each cultivated plant. The probable histories of cucurbits and of hemp have already been alluded to. Seibert (1947, 1948) has discussed the role of introgression in the domestication of Para rubber (Hevea). The wild-growing species of Hevea are native mostly to alluvial soils, and Seibert thinks that there may have been some introgression in these areas before the advent of man. Apparently the species was first cultivated for its edible nuts (Baldwin, 1947; Baldwin and Schultes, 1947). Either accidentally or with intent, seedlings from wild trees came up in clearings where they were being used for food. These areas were often outside the natural range of that species or variety and sometimes within pollination distance of other species. Consequently these isolated trees tended to be cross-pollinated. Under the primitive agriculture of these areas, clearings were occupied for a time and then deserted. As the disturbed land gradually reverted to jungle there were many opportunities for the hybrid seedlings of the isolated nut trees to germinate and survive. They crossed back to the native species of that vicinity, and thus the process of introgression might have started in hundreds of little clearings in the jungle. The more or less casual use of Hevea for its edible nuts increased the natural introgression between some of the species. When man gradually learned that the latex of Hevea also had its applications, he already had at hand variable, introgressed, semidomesticated populations, in which trees superior in latex were more likely to be found.

The extent and frequency of introgression must certainly vary greatly with the type of agriculture that is being practiced. Under the jungle-clearing pattern, like that just described for Hevea, it must have been at a maximum. Today it can be seen to vary widely between areas of pastoral agriculture and those devoted exclusively to field crops. In the latter, in the so-called cotton belts, wheat belts, and corn belts, the native vegetation is completely removed over wide areas. Alien crop plants are introduced. There are few opportunities for hybridization and almost no niches in which the hybrid segregates may survive when they do occur. A pastured area is very different. The native vegetation is removed only in part, though natural ecological conditions are drastically changed. The plants introduced in pastures and hayfields are of many kinds. There are new opportunities for hybridization between various components of the native vegetation previously isolated, or between them and their close relatives among the introduced plants and weeds. When hybrids do occur there are various new niches in which some of them may possibly succeed. It is significant that most of the studies of introgression up to the present time have been made in pastures or in heavily pastured areas. Riley's studies of Iris were made in pastured swamplands. Anderson and Hubricht worked in overpastured areas in the Ozarks. It would seem to be significant that New Zealand— where the frequency of hybridization has been the subject of several special investigations (Allan, 1937) — is very largely given over to pastoral agriculture. Such genera as Crataegus, in which thousands of new species have been described in the last century, are nearly all plants of pastures. For Crataegus, Marie Victorin has outlined the main steps in the production of the swarms of these new forms in the pastures of French Canada. The great majority of the species described by the late Charles S. Sargent came from such pastured areas in which opportunities for hybridization and consequent introgression were very high. Crataegus (a genus in which both polyploidy and apomixis are frequent) produced a complicated introgression pattern, which has led to great taxonomic confusion. Without these two complications there would have been a less ruffled gene flow between the original hybridizing entities.

The demonstration that cultivated plants and weeds are very largely the products of introgression is particularly important for plant genetics. It is almost exclusively upon such plants that the theory of plant genetics has been based. From Mendel's original peas to Blakeslee's Daturas, we have worked chiefly with introgressed germplasms. Some of our marker genes are certainly introgressive segments from another germplasm. That does not vitiate their use as marker genes but it does mean that our estimates of the role of the gene in evolution may need a correction factor, because nearly all our evidence comes from plants that are somewhat exceptional.

Bibliography