by Göte Turesson (1922)
Thus, while the belief that the Linnean species of the present genetically represent complicated products of recombined Mendelian factors, or genotype compounds, has been strengthened nobody would maintain that the problems connected with the formation of the Linnean species are exhausted by this demonstration. Most of these species are, as every earnest inquirer will find, in their natural habitats rather stable products, which do not live in any extensive connubium with congeners of other species. The bridgeless gaps found between species of the same genus, the final moulding of the Linnean species, remain then to be explained. The Darwinian idea of selective processes at present offers to most minds a plausible explanation of the differentiation of Linnean species. Although very little is known with regard to the actual play of these selective processes, certain facts likely to demonstrate the complex nature of selection [[p. 101]] have been brought to light. KÖLREUTER (1761) showed that a species when pollinated simultaneously with its own pollen and pollen from another species breeds true to type in spite of the fact that it otherwise gives hybrids when crossed with that species. That the native pollen is favoured as compared with foreign has been shown by HERIBERT-NILSSON (1920) in the case of Oenothera Lamarckiana. He found that pollen tubes of O. gigas grew more slowly in the styles of O. Lamarckiana than the O. Lamarckiana's own pollen tubes. The terms elimination, certation, prohibition and substitution discussed by HERIBERT-NILSSON refer to phenomena which give rise to aberrant types of segregation. The importance of such gametic and zygotic complications has been discussed more recently by NILSSON-EHLE (1921). They are all particularly well calculated to throw light upon selective processes of great weight. However this "prenatal selection" may limit the output of new organisms, hybrids between already existing species would no doubt be more numerous and more widely distributed in nature were it not for the controlling effect of living and non-living factors of the outer world. Various disturbances involving different organs are frequently seen in hybrids and in "artificial" species, and this fact does not support the idea that such organisms are able to hold their own with nature. We are thus forced to the conclusion that the present-day species represent the only possible and necessary outcome of the complex processes of selection in this epoch of the earth's history (cp. HERIBERT-NILSSON, 1918). As a natural consequence we are led to the inference that a change in the inorganic world must bring about a corresponding change in the organic, inducing a recombination of Mendelian factors now distributed in living organisms and resulting in the formation of new genotype compounds or species (=evolution).
The species problem is thus seen to be in a large measure an ecological problem. As such it has hitherto remained almost unattacked from an experimental point of view. While at present the purely genetical side of the problem is fairly well understood, we have nothing like a reliable picture of the significance of the ecological factors in the differentiation process of the organisms. Our knowledge with regard to the species problem may be shortly expressed by stating that while our understanding of the genospecies1 is [[p. 102]] reasonably satisfactory, such a statement is not warranted with regard to the ecospecies.
In the efforts made by the writer to arrive at an understanding of the Linnean species from an ecological point of view--of the ecospecies, as I prefer to say in the following--studies have been made of a number of plant species. Usually species with extensive distribution, each occurring in as many different localities as possible, have been selected. The conditions in the natural habitats have been noted as far as possible, and seeds and pieces of turf have been collected and brought home from various habitats in order that the types might be followed in permanent cultures laid out in the experimental fields. A review at this stage of some of the results of these studies is intended to show the necessity and importance of a comparative study of organisms in their natural habitats and in the experimental field when the question of the differentiation of species and varieties is to be discussed.
It is much to the purpose to start with certain species belonging to the genus Atriplex, which have been in my cultures since 1916. Several species of this genus grow abundantly along the Swedish coast. One of these species characterized by thick, fleshy, rhombic-lanceolate leaves, and flowering earlier than any other of our species grows all along the coast in the zone nearest to the water. Sets of seeds and small plants of this species have been collected from some thirty different localities and cultivated. A remarkable hereditary variation has been found to occur. The distribution of these varieties in nature offers, however, a most interesting study. It is found that the eastern coast strip is inhabited by a certain group of types distinctly different from the population inhabiting the rest of the coast (fig. 1). This group of types has been described under the specific name Atriplex praecox by HÜLPHERS (in LINDMAN, 1918). If the species is examined on the west coast, we find that the coast farthest to the south, the marshy coast strip along the Sound, is inhabited by types which in the fruiting stage are easily seen to be quite different from other types of the species but which during the flowering period closely resemble the eastern representative. Types of this southern group were described as long ago as 1838 by DREJER (1838) under the [[p. 103]] specific name Atriplex longipes (fig. 2). The name has disappeared from modern handbooks, or else it has been discarded with the remark that it refers to abnormal plants of other species. The "abnormal" characteristic meant is the long-stalked, leafy, fruiting bractlets, a characteristic found by me to be a hereditary feature peculiar to these southern types. Going up along the west coast, north of the Sound, we meet with a third set of types, which by cultivation have been found to be genotypically different from the eastern and southern representatives. Forms belonging to this variety are now included under the specific name Atriplex hastifolium, SALISB. in systematic handbooks (fig. 3). They have certain characteristics in common with the southern and eastern groups, but are easily distinguished from these. The flowering period, although coming earlier than in other Atriplex species, falls later than in A. praecox and A. longipes.
There can be no question that these three "species" belong to the same ecospecies. They form as such a natural group with an almost uninterrupted distribution all along the coast, occupying the lowest zone of the shore nearest to the water, where no other species of the genus thrive well. A cross made between A. longipes and A. [[p. 104]] [[figures]] [[p. 105]] praecox has been found to give a progeny which does not show any decrease in fertility2. In the further discussion of this complex other well defined species of the genus must be taken into consideration. Hybrids between the above-discussed "species" and these latter ones are occasionally found on the east coast and along the Sound, although they are usually very difficult to trace. On the west coast, north of the Sound, these hybrids however become so frequent that it is sometimes difficult to find representatives of the above-mentioned western type except in certain places, where other species of the genus, such as Atriplex Babingtoni and A. latifolium, grow but sparsely. Here, then, the typicalness of the type depends on the number of other species present on the same spot. When these latter are present in sufficient number the "swamping" of the variety in question is very likely to occur. As A. Babingtoni and A. latifolium are more abundant on the west coast than on the east coast there are fewer chances of hybridization between A. praecox and these species, but the distinctness of the latter variety cannot be due to this circumstance alone, since places are known where all the species occur indiscriminately without inducing any swamping of A. praecox. It has already been stated that the types under consideration flower very early compared with other species of the genus, and that this is particularly true of A. praecox and A. longipes, while the flowering of the western variety falls somewhat later. That this circumstance accounts for the differences in the degree of hybridization between the eastern and western varieties is more than likely. The western variety flowers late enough to find particularly early individuals of other species flowering at the same time. Hybrids are thus formed yearly, and back-crossing might ensure the persistence of hybridogeneous forms even if the primary bastards are less vital than the parents.
A natural circumscription of the ecospecies is thus seen to exist to the east and south, while it is often swamped on the west coast. Similar peculiarities in the behaviour of a great number of plants and animals are well known. The species may be well defined within a certain area and the systematist has no difficulty in tracing it; then suddenly a nearly related species (or several of them) is found to intrude upon the area at certain points, causing great variation in type and unsurmountable difficulties to the systematist. These [[p. 106]] intermediate areas harbouring mixed types would seem to be best suited for the study of the differentiation of the species from an ecological point of view, as are also the zones of contact between varieties belonging to the same ecospecies to be discussed in the following. The case so often seen where a species at its boundary-line hybridizes and gets swamped by other species in spite of the fact that these same species may occur side by side within the "natural" area of that species without any swamping, is most likely due to the different genotypical constitution of the species near the border line. This has been shown to be the case in the above-mentioned western Atriplex variety, and a certain peculiarity, which distinguishes this variety from all other known types of the ecospecies, viz. the late flowering, has been pointed out as a reasonable cause of the mixing with other species on that coast. Other cases of hybridization in the zone of contact between different species will probably also be found to be due to changes in the genotypical construction of the species at their limits of distribution.
The factors likely to bring about changes in the genotypical construction of species at different points of their geographical distribution have now to be considered. In order to evade the complications arising when parts of a plant population become geographically isolated from the bulk of the population, only such plants as combine an extensive and uninterrupted range with an extraordinary frequency have been investigated. Certain Atriplex species, as for instance the one discussed in the above and known under the different names of A. praecox, A. longipes and A. hastifolium, comply with this demand fairly well. The Atriplex ecospecies just discussed occurs and has been followed along the coast from Stockholm round southern Sweden up to the middle part of Bohuslän, on the west coast. Cultivations have shown that the hereditary variation within each of the three above-mentioned varieties is by no means slight, and different forms belonging to these varieties have been described and named by systematists. When well acquainted with the material one has no great difficulty in stating from which of the three coast strips a certain form has been collected. For in spite of the considerable variation within each of the three varieties there are certain characteristics which are common to all the forms of the particular variety. When therefore it is found that the ecospecies is split into three well-defined varieties, each occupying different but distinct stretches of a continuous strip of coast, the question becomes pressing whether or not climatic and [[p. 107]] edaphic factors are responsible for the splitting up of the ecospecies in these distinct varieties. The climatic differences would seem to be sufficiently great to produce a dissimilar development and differentiation of the population in the three localities. That the flora of the much exposed west coast of Sweden is very different in character from the flora of the more sheltered, less extreme eastern coast is well known. I have pointed out in a previous publication (TURESSON, 1919) that numerous prostrate plant forms are found on the exposed west coast. This is especially true of the species belonging to the genus Atriplex, and therefore it is not surprising to find that the west coast variety of the Atriplex ecospecies under discussion differs from the eastern representative precisely in regard to stature. Forms with hereditary erect branches are rare in the west coast variety. That prostrate forms of the species are "favoured" on the west coast, while erect ones tend to become eliminated, seems then to be a very reasonable assumption.
The experience gained from other species in the course of these studies considerably strengthens the assumption of the importance of climatic and edaphic factors in the differentiation process of varieties. Some examples are here given. The coast form of the common scentless camomile, Matricaria inodora var. maritima, occurs both on the west coast and on the east coast. The coast form differs from the inland forms in many characteristics but especially in having thick, fleshy leaves and bushy growth, both these features being hereditary and constant in cultures. When sets of individuals of this coast form collected in various places (where hybridization with the inland form is excluded) along the east and west coasts are studied comparatively in cultures, important differences between the eastern and western sets will at once be seen. The west coast sets always contain more numerous prostrate types than the east coast sets, and the fleshiness of the leaves is much more pronounced throughout in the former than in the latter. Collections of other plant species show similar peculiarities. Mention may be made of Solanum dulcamara, a plant with an extensive distribution in Sweden. Sets of this plant from different points on the east and west coasts have been cultivated and studied comparatively. It is found that the eastern sets contain all kinds of types as to characters of leaf, while sets collected on the exposed western coast show a remarkable conformity in type being all referable to the hairy, thick-leaved form called S. dulcamara f. marinum, BAB.
[[p. 108]] The features dealt with belong strictly to the category of characters usually called adaptations. As far as this term is employed to mean a Lamarckian mode of origin of the characteristics in question no support can be given it; if it is employed to express the idea that certain hereditary forms of an ecospecies are favoured in a certain locality, while others are expelled in some way or other, much is to be said in support of it. The study of the coast plants just mentioned has clearly brought out the fact that the genotypical constitution of the ecospecies under discussion varies with the locality, and that the west coast forms of the plants represent throughout more extreme varieties than the east coast forms (when compared with the inland forms). When such a parallelism is found to exist between the extremeness of type and the extremeness of locality there can be little doubt as to the profound importance of climatic and edaphic factors in the differentiation process of varieties and so-called adaptive forms.
Before discussing any further the significance of such forms, mention should be made of the behaviour of an ecospecies distributed within a continuous area, where localities different in nature alternate with one another. The species which has been particularly studied from this point of view is Hieracium umbellatum, a typically cross-fertilizing plant (not apogamous) with an extensive range in Sweden. Material collected in different [[p. 109]] habitats and now in culture tends to bring out the fact that at least three groups of types of this plant are differentiated in southern and middle Sweden. The ordinary variety (fig. 4) grows in open, herbiforous woods, the second grows on the dunes along the coast (fig. 5); a third variety occurs in arenacious fields in certain parts of the south (fig. 6). When sets of turfs of these groups are cultivated side by side, their respective and distinctive characters are at once seen. The material from the woods represents erect, rather broad-leaved forms, while the plants from the dunes show more narrow-leaved, less erect, and strongly shoot-regenerating forms. The third variety, which is perhaps to be called H. umbellatum var. dunense, REYN, is a prostrate plant with much hairiness at the base of the stems and is unable to regenerate during autumn. There is great variation within the groups in regard to the shape of the leaf margin, the inflorescence etc., but the groups as such are quite distinct in the majority of cases.
The close relation of these groups, or varieties, to the habitat presents a most interesting study. The varieties are sometimes found growing within a short distance from each other, as for instance in the south-east corner of Skåne (southernmost Sweden), where woods, dunes, and sandfields alternate. The prostrate variety is found growing abundantly on the sandfields north of Kivik. This variety is succeeded by the variety typical of the woods [[p. 110]] at a point just south of that village, where the woods of the Stenshuvud region begin. The point where the two forms meet has been found, and the cultivated material from this place shows all kinds of intermediate types. The geographical point to the south where the variety typical of the woodland region ceases has not yet been located, but the prostrate variety inhabiting the sandfields north of Kivik reappears in its most typical form on the sandfield north of Simrishamn. It then dominates the coast at least down to Skillinge, a small fishing village, but disappears again a little farther to the south, where the sand-dune region of Sandhammar begins. The typical sand-dune variety now flourishes unrestricted. The successions of varieties here described take place within an area found to be about 35 kilometres in extent. The point northwest of Kivik, where the prostrate variety of the arenacious fields meets the inland variety of the woods, has been located. The prostrate variety follows the sandfields and is found quite typically about 15 kilometres from the sea. The sandfields are now replaced by woods. Woodlands situated about 20 kilometres from the sea have been found to harbour intermediate types. The intermediate zone is probably quite narrow, as cultivated material collected at random at a distance of 2 kilometres to the west of the latter locality contains the typical woodland variety only.
The evidence furnished by the distribution of these varieties of Hieracium umbellatum within a limited and continuous area shows that the different varieties are restricted to distinct localities within the area, and that the reappearance of a distinct locality occasions the reappearance of the variety typical of that locality. These facts, as well as those discussed above with regard to coast plant varieties, are inconsistent with the view often held and defended that the kind [[p. 111]] of variety present in a certain locality is mainly determined by chance. The intimate relation of the above discussed varieties to the environment is, indeed, the only possible explanation. It is for several reasons inappropriate, however, to employ the terms adaptation or adaptive forms for these kinds of varieties. It introduces and propagates the teleological idea that need controls the origin of characters. The term likewise brings with it the erroneous idea that the origin of so-called adaptive characters represents a phenomenon sui generis, viz. the origin of these characters through the direct (Lamarckian) effect of environmental influences, in contrast to the origin of so-called "Organisationsmerkmale." The concept of adaptation, further, has led biologists to believe that the primary cause enabling a certain plant to live in an extreme locality is to be sought for in the habitus of that plant and in morphological details. However, it is evidently the genotypical constitution of the individual plant which is the point of primary importance. The morphological details and the habitus of a given plant represent only the result produced by the reaction of the genotype to definite external conditions, it represents the reactiontype3. We further know that similarity in reactiontype does not mean similarity in genotype. The prostrate character in certain Atriplex forms, for instance, may be in the one case a modification, in the other case a hereditary variation, for even some of the usually erect branched forms may become prostrate if the intensity of light becomes strong enough, as I have shown some years ago (TURESSON, 1919). That the process of selection, however, intervenes and directs the genotypical differentiation of the population in a given locality becomes apparent when it is found that the majority of the individuals on the exposed west coast consist of hereditary prostrate forms, i.e. forms reacting very readily to light and becoming prostrate even in ordinary light.
The differing phenotypical reactions, or the different reactiontypes, of a given plant, when exposed to differing environmental factors, are thus factors of great moment. However, the differing genotypical reactions of a plant population, when distributed over a continuous area comprising different types of localities, is a fact of still greater importance from an ecological point of view. From what has been said in regard to the occurrence of distinct varieties of an [[p. 112]] ecospecies distributed in that way it appears reasonably safe to conclude that the varieties in question each represent the genotypical response of the ecospecies to the respective localities. When the development of a distinct variety in a distinct locality is thus expressed in terms of reaction even such concepts as "selection" and "survival of the fittest" also become rather superfluous.
The reaction products of the above discussed ecospecies have been called varieties. That this term gives an inadequate conception of the result of the genotypical reaction of an ecospecies to a specific habitat is obvious. It is rather arbitrarily used to denominate different kinds of hereditary deviations from a valid or supposed type and cannot therefore be advantageously employed in any ecological sense. The term ecotype is proposed here as ecological unit to cover the product arising as a result of the genotypical response of an ecospecies to a particular habitat. The ecotypes are then the ecological subunits of the ecospecies, while the genotypes are purely Mendelian subunits of the genospecies. A knowledge of the ecology of an ecospecies presupposes a knowledge of its most important ecotypes, and the knowledge of the ecology of the latter involves primarily a study of the variation and the distribution in nature of each of these ecotypes.
The study of the species along these lines furnishes a necessary complement to the Mendelian study of the species problem. The importance of this line of research for the understanding of biogeographical and biosociological questions is also evident.
February 5th, 1922.
1. DREJER, S.T.N. 1838. Flora excursoria Hafniensis. -- Hafniae.
2. HERIBERT-NILSSON, N. 1918. Experimentelle Studien über Variabilität, Spaltung, Artbildung and Evolution in der Gattung Salix. -- Lunds Universitets Årsskrift, N.F. Avd. 2, Bd 14, Nr. 28.
3. _____ 1920. Zuwachsgeschwindigkeit der Pollenschläuche and gestörte Mendelzahlen bei Oenothera Lamarckiana. -- Hereditas I, 41-67.
4. JOHANNSEN, W. 1913. Elemente der exakten Erblichkeitslehre. -- 2. Aufl. Jena.
5. KÖLREUTER, D.J.G. 1761. Vorläufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden Versuchen and Beobachtungen. Leipzig.
6. LINDMAN, C.A.M. 1918. Svensk fanerogamflora. Stockholm.
[[p. 113]] 7. LOTSY, J.P. 1916. Evolution by means of hybridization. --The Hague.
8. NILSSON-EHLE, H. 1921. Über mutmassliche partielle Heterogamie bei den Speltoidmutationen des Weizens. -- Hereditas II, 25-75.
9. RAUNKIAER, C. 1918. Über den Begriff der Elementarart im Lichte der modernen Erblichkeitsforschung. -- Zeitschr. f. ind. Abst. u. Vererbungsl., 29, 225-240.
10. TURESSON, G. 1919. The cause of plagiotropy in maritime shore plants. -- Lunds Universitets Årsskrift, N.F. Avd. 2, Bd 16, Nr. 2.
1. The term genospecies is here meant to embody the known facts about the genotypical construction of the Linnean species; it has nothing to do with the genospecies of RAUNKIAER (1918), who employs the term in the sense of Lotsyan [[p. 102]] species, i.e. homozygotic biotypes. Since there is already in existence an older and well-established term, which covers this conception, viz. JOHANNSEN'S pure line, no valid objection can be raised to the use of the term genospecies in the sense given to it in the above. [[on pp. 101-102]]