Popular Science Monthly/Volume 82/April 1913/Natural Selection
| NATURAL SELECTION |
By Professor T. D. A. COCKERELL
UNIVERSITY OF COLORADO
THE lowest forms of life show the greatest stability in their specific characters. Any one who will examine a little pond water under the microscope will see numerous minute Protozoa belonging to different families, genera and species. Close study brings out the fact that although we regard these as very low types, they are complicated little animals, with remarkable characters. When we look at lists of these creatures, based on collections from different parts of the world, we are astonished to find that most of the species are the same, no matter how remote the localities. When identifications were based merely on comparisons of descriptions and figures, we suspected that the alleged wide distribution of some of these fresh-water Protozoa might be due to mistakes. In recent years, however, protozoologists have frequently traveled, and Dr. Penard, the greatest authority on rhizopods, has been able to determine by study on the spot the identity of Rocky Mountain forms with those of Switzerland. Even more remarkable are the results of Dr. Edmondson, who has visited Tahiti, high altitudes in the Rocky Mountains, and various places in the western central states, with the result of entirely confirming the opinion that most species of fresh-water Protozoa are spread over the world, almost without regard to climate or altitude.[1]
These studies and reports, however, have been based on the species as generally understood. These species are not entirely uniform, but consist of groups of minor races, which also appear to have constant characters and to be of general distribution. Dr. Penard told me that he could greatly increase the list of "species" of rhizopods were he to describe as distinct all the apparently constant forms which he had learned to recognize, and which, so far as he knew, did not conjugate one with the other. He did not describe and name them because their separation required such critical comparisons and familiarity with the subject that very few naturalists would be willing to consider them. Professor Jennings, in his studies of Paramæcium, has isolated a number of races or varieties which possess constant characters by which they can be recognized, and which are believed to be common throughout the country.
Are these Protozoa, then, indifferent to their surroundings? By no means. The experiments of Jennings show that uniformity of environment is not possible even in a watch glass, and that the animals respond readily in many ways to the conditions under which they exist. This fact has tended to obscure the genetic independence of different races, the characters of which overlap, but "pure line" cultures have made it possible to separate them. It has been shown that distinct races may differ only in average characters, a large proportion of the individuals, under ordinary conditions, being indistinguishable by inspection. Such a pair of races would only show absolute differences if subjected to conditions ensuring for every individual the maximum or the minimum growth and efficiency. Such conditions are practically unattainable, and only "pure line" breeding and statistical study will separate the races.
Consequently, in the Protozoa, we have three recognized grades:
1. The species of ordinary taxonomic writings.
2. The minor types recognizable by inspection when the investigator is very expert.
3. The races or strains separable only by breeding combined with statistical study.
Do the third originate frequently without evident cause? Do they then turn into the second, and the second into the first? Jennings did not find it so, but his experiments necessarily occupied a limited time and were concerned with an infinitesimal fraction of the unthinkable myriads of Paramæcia in the world. We have, however, the results of nature's large-scale experiment with Paramæcium. The genus, notwithstanding its universal distribution and the very diverse conditions under which it must exist, is very poor in species. Either the imagined process does not go on, or it fails before reaching the stage of species-formation, as species are understood by the taxonomists.
In the ease of bacteria, and even trypanosomes, it is commonly alleged that environment will change the type. This is constantly asserted by the highest medical authorities, and in a certain pragmatic sense it is of course true. It is found, however, that if the environmental factor is carried too far, or continued too long, the process can not be reversed. It seems nearly certain that the observed phenomena are due to nothing more than a selective process operating on a mixture of races, isolating the one least able to endure. Thus, suppose in a given case we have a culture consisting of one million pathogenic bacteria and ten of an allied non-pathogenic race (presumably there will usually be several grades or races, as with the Paramæcia). Apply some treatment favorable to the ten and destructive to the million, and presently the ten are a million and the million reduced to ten or none. The appearance is that of changing the type by environmental means, but nothing more than selection has been at work.[2]
From all this we are led to conclude that natural selection is continually operative on the lower types of life, the unicellular animals and plants, everywhere affecting the proportions or existence of the races and species. These creatures are not adapted to life under all conditions; they are, on the contrary, sensitive to relatively slight changes, many of them probably too slight for us to appreciate. The history of a single culture in the laboratory indicates this. Why, then, are the species so widely distributed, and, on the whole, so constant? Why are they not infinite in number? Why are they not exterminated in great numbers, instead of being of tremendous antiquity, as their wide distribution and the paleontological records show? Where the environment is highly specialized, as in the case of groups parasitic on the higher vertebrata, there is considerable evidence that evolution has to a certain extent kept pace with that of the hosts; yet always tending to lag behind, as Kellogg showed even in the case of the bird-lice, which are of far higher organization than the types now under discussion. In the case of such animals as the fresh-water Protozoa, however, the selective processes have always acted piecemeal, rarely if ever sufficiently widely to destroy a species which had once gained a good footing. They have no doubt destroyed many incipient species, but any tolerably successful type, once widespread, may defy the ordinary processes of nature. In a wide country there is nothing which renders every puddle uninhabitable, or every part of each pond and river, and survival in a number of places permits the reappearance of the creature in millions when good conditions for reproduction occur. All that is necessary for permanence is an inherent stability of type, which will prevent automatic modification independent of conditions. This stability surely exists in an amazing degree, and may itself be regarded as a product of selection acting through the ages; for automatic instability, manifested too much or too often, would lead to series of changes eventually fatal to existence. A certain looseness of adjustment to surroundings is advantageous, but even slight variations, piled one upon the other, would before long throw the organism out of gear.
Perhaps we may picture the condition of affairs somewhat as follows: There are, let us say, 500 common "situations" in the fresh waters of the world, differing in the temperature and chemical content of the water, in the presence or absence of particular enemies, in the quantity and quality of available food, and I know not what else. These are not constant for any one body of water for any length of time, owing to seasonal and other changes. An organism adapted equally to any one of the 500, if that were physically possible, could not be closely adapted to any, and would be perhaps more or less inefficient under all. An organism adapted exclusively to one or two of the 500 could not in practise confine itself to them, and would be in danger of extermination. There would accordingly arise an optimum condition of adaptability, according to which any given organism would exist under at least 300 of the 500 postulated conditions, would do well under 100, and would flourish exceeedingly well under perhaps 10. Hence the species would be very widespread, would often be common, and would occasionally occur in excessive numbers; which is approximately what we find.
All of this would require in the animals much stability of type. If they varied freely and indiscriminately, the variations being inherited, they would not only tend to lose their standards of efficiency, but the selective processes might make playthings of them, changing them temporarily to meet this or that condition, but rarely able to reverse quickly enough as conditions altered.
The rhizopodous genus Difflugia contains a great number of species, differing in the size and shape of the little shells they make. It is not necessary to suppose that each species is specially adapted to some particular set of conditions, though some of them may be more or less so. It is only necessary to suppose that the difflugian type reached in these animals so many "positions of organic stability," which persisted and survived simply for this reason. There is a "survival of the fittest" in inorganic chemical compounds, following analogous lines.
There is the greatest contrast between these fresh-water protozoans and some of the marine groups, particularly the Radiolaria. Haeckel's great Challenger report on the radiolarians only partially indicates the enormous diversity of skeletal structure in these animals. They remind us more of snow crystals than anything else and it is useful to remember here that snow crystals, with all their wonderful diversity, are simply . It is impossible to believe that all this radiolarian diversity can be adaptive in more than the most general way; we would rather believe that it is possible because of the relative simplicity and uniformity of the conditions of life, which permit infinite diversity in the details of skeletal structure without injury. There is perhaps a high degree of stability in the protoplasmic structure of the radiolaria, and the modifications in the skeletons or shells may not indicate much fundamental diversity. To what extent the described species are permanent and constant is not known.
In multicellular animals, conditions are in many ways different; yet even here we notice a remarkable limitation in the types of cellular structure produced. What multitudes of animals are made out of essentially the same kinds of tissues! How limited in number those kinds are! The plants show the same lack of cellular diversity. Evolution has proceeded by means of new arrangements rather than new materials. This cellular stability, well fitting the needs of organisms, must have been fostered by selection. The nerve cell, the striated muscle cell, are astonishingly modified from ameboid ancestors, but the power that could do so much has left us only a few masterpieces. Is this the result of orthogenesis? Did development proceed along these lines, looking neither to the right nor to the left; or did selection oppose impassable barriers? Perhaps both, since orthogenetic trends may themselves be favored by selection.
Passing from tissues to organs and characters, we seem to find much greater, almost infinite, diversity. Recent research has, however, indicated the presence of determiners in the germ-plasm, factors which may be combined in endless ways in inheritance, but are themselves remarkably stable. It seems nearly certain that, so far from constantly presenting heritable variations, they rarely do so. This conclusion is based not merely on the Mendelian phenomena observed by experimenters, but also on the paleontological evidence. There are many groups of species, such as the oysters and the oaks, which have existed since Mesozoic times, producing innumerable species, but so far as we can see, practically all by the shuffling of characters present within the genus all along. Among the Unionidæ, the fresh-water mussels, Ortmann has recently commented on the occurrence of practically identical shell characters in different genera; while land snails afford a number of similar instances. In insects, these phenomena are constantly observed; types of color and marking are nearly the same in Lepidoptera of diverse structure; and in some of the Hymenoptera peculiar characters, such as spines on the cheeks, appear here and there as if at random. In one genus of bees the sexes differ in the character of the tongue, one having that organ pointed, the other having it obtuse; differences hitherto considered to mark families.
We are, therefore, led to see a certain stability amidst all the instability of the multicellular animals; a stability of types of tissue on the one hand, a stability of determiners on the other. Change in the stuff of which living things are made is not a common phenomenon; indeed, we know little or nothing about it. The experiments of Tower and MacDougal, in which heritable variations were apparently produced, can be explained rather on the supposition that certain determiners were destroyed than on the idea that they were altered.
Natural selection, it has often been said, creates nothing. It merely makes use of the variations already present. In Darwin's time, it was not appreciated that so many of the observable variations are due to the direct effects of the environment, and are not inherited. To-day, we must throw these out and consider heritable variations only. Now we find that these heritable variations mainly (at least) represent no more than a shuffling of the stock-in-trade of the organism, and if any of them involve absolutely new determiners, we do not know it. The matter is complicated by the frequent appearance of new characters, which experimental evidence shows to result from new combinations of the old ones. Thus the pink-flowered and yellow-flowered stocks (Matthiola) give white and red-cream flowers in the third (second filial) generation, no matter if the two original strains had been bred true and had remained constant from the beginning of time. Here we seem to see something entirely new, but analysis shows that we have no more than new combinations of certain of the grandparental characters.
What will natural selection do with such materials? It can do no more than favor certain characters or combinations of characters and eliminate others. It can not even eliminate the recessives. The result will be the production of a number of distinct types, without necessarily any forward evolution—anything more than a shuffling and sorting of determiners. So far as we can see, this is exactly what has happened in the case of the oak leaves and many mollusc shells.
The modern school of Mendelian experimenters, who have from necessity confined themselves to determining the inheritance of relatively simple characters, have come to think little of natural selection. They have seen how various combinations can arise, greatly altering the appearance of animals and plants, without selection having anything to do in the matter. They have also seen how certain of these modified types, or others like them, may multiply and spread, without being obviously helped or hindered by selective agencies. Where the characters came from, they do not know; but neither do the selectionists. Let it suffice that we have here an apparently mechanical arrangement, which if left to itself will people the earth with diverse animals and plants, a large proportion of which will get along well enough to survive. Possibly this description is unjust in its application to modern experimenters generally, but it at least represents the attitude of some of the more influential and at the same time marks the recognition of a number of real and important facts. I think that while we shall gladly incorporate the facts into our system, we shall in time come to wonder at the limited view of nature implied by the attitude described. It is all too simple and too easy, it does not take into account the real complexities of life or of organization. It reminds us a little of the school of zoogeographers who would bridge the oceans whenever it seems necessary for some animal to cross, or to have crossed. The experimental work itself is revealing this, as day by day new complications arise.
Darwin and Wallace have talked of the accumulation of small variations, of the effects of natural selection on the perpetual minute variations which all species exhibit. Antiselectionists answer that most of these are non-heritable "fluctuating variations," and as for the rest, they are usually not small, nor is the variation "continuous." Moreover, selection soon gets to the end of its rope. So it seems when we are looking at a single pair, or two or three pairs of Mendelian allelomorphs, all active independently of one another. This, however, is not a true picture of living animals, which are bundles of uncounted "factors," acting together in various ways. Any single factor depends for its appearance and form of manifestation on all the others, as Wilson has urged. It is not a thing by itself; it is the result of a complex equation. Sometimes we are getting along well enough with our experiments, when suddenly things go wrong; not because of error in our theory, but because some new factor, which we were not watching, has come in and disturbed the results. Thus, in breeding red sunflowers, we predicted, and got, a dark red homozygous flower. We also got, but did not predict, a homozygous red in which only the basal half of each ray was of that color. The fact is that many of the wild sunflowers carry a factor for marking, which can be seen with difficulty on close inspection, but in the red it comes out strongly. For reasons of this sort we have not only the complications due to the multitude of factors or determiners, but also those caused by their interactions. Inasmuch as they may influence each other strongly or slightly, and in all sorts of different ways, the net result is that in the more complex types of life we have almost infinite possibilities of variation, quite aside from any question of the alteration of the determiners themselves. Add to this the complications due (it appears) to accidents in the maturation process of the germ cells—such an "accident" probably gave rise to the red sunflower—and we have in most cases as much material for natural selection to operate upon as Darwin or Wallace ever postulated. Enough, indeed, to account for all the wonderful adaptations in the tropical fauna and flora, when we consider the time available for their production.
It has recently been announced, as the result of the museum work of Dr. K. Jordan, combined with the field and breeding observations of Dr. G. D. H. Carpenter,[3] that an African butterfly of the genus Pseudacræa occurs in a variety of forms, which imitate species of Planema flying with it. The extraordinary thing is that one phase of this Pseudacræa is sexually dimorphic, imitating a dimorphic Planema, while in the same forests it also occurs in two monomorphic forms, resembling two other monomorphic species of Planema. Dr. Carpenter succeeded in breeding one of the monomorphic Pseudacræas from an egg laid by the other. In a case like this, we have the result of a Mendelian experiment performed by nature. The different phases are represented by interchangeable units, and interbreeding normally occurs. Hence, an extreme case of polymorphism, in which all the alternative forms which have been preserved are at present favored in the struggle for existence. Of those which, in the ages past, have disappeared, we have now no trace, but theoretically we should expect some non-mimetic recessive combinations to occur as occasional aberrations. This, I believe, accords with the facts.
Those who examine remarkably adapted forms are always impressed by their striking characters, and find it hard to believe that they have arisen by gradual steps from the more ordinary types. They do not appreciate the ages during which these forms have been evolving, and the multitudes that have perished. Among insects, however, the number of surviving species is usually much greater than in any other group of animals, so that it is possible in a certain sense to compare a specialized type with its ancestors, or at least with contemporaneous species having many of the marks of its ancestors. For this reason insects are exceptionally valuable for the study of evolution; though hardly equal to mammals, which have changed so rapidly within comparatively recent times, and have left us such admirable fossil remains. It would be a useful contribution to the theory of evolution to take up a number of the cases of mimetic or otherwise peculiar insects and show how they are connected by many steps with the more ordinary forms. This has, indeed, been done in part, but it has been difficult, requiring immense and carefully worked out collections. In the Lepidoptera, where these studies are most interesting, the work is being immensely facilitated by the publication of Dr. Seitz's magnificent volumes on the Macrolepidoptera of the world, which place descriptions and good colored figures of all the principal larger Lepidoptera at the service of any one who can afford the very moderate price charged.
We may consider, for example, the "Aristolochia Papilios." These splendid butterflies feed in the larva state on Aristolochia, rarely on allied plants. They occur on both sides of the world, and are doubtless, as a group, of great antiquity. They are strong-smelling and apparently distasteful to most predatory animals; the other two groups of Papilio, not thus protected, frequently produce species which closely imitate them, so much so that "until quite recently models and mimics have often been regarded as closely allied." The great Indo-Australian series of Aristolochia Papilios shows the largest size and extraordinary sexual dimorphism in the Ornithoptera series, usually treated as a distinct genus. The great diversity of the sexes, both in form and color, is extremely impressive in view of what we now know about sex-inheritance. The bright colors are most commonly orange, often green, while the male of P. urvilliana, a Solomon Islands species, is marked with blue. The absence of red is noteworthy,[4] although it is not complete, several of the species having a little red on the anterior edge of the thorax or back of the head, or on the under side of the thorax. In the allied tailed series (including such forms as P. hector and coon) light red spots are frequently developed on the hind wings. The American (neotropical) Aristolochia Papilios, which are much smaller on the average than the oriental, have the markings and form for the most part much like the orange and black oriental group (P. darsius of Ceylon, etc.), but where there is orange on the hind wings of the darsius group, it is usually bright red in the neotropical series, though occasionally orange, or orange shaded with red. Most of the American species have well-defined patches on the anterior wings also, but these are green, yellow, white or rarely blue, never red. American Papilios of the lysithous group resemble the Aristolochia Papilios of the same region in the most amazing way, and these mimetic butterflies are said to usually imitate the sluggish flight of their models. When we have figures of all these insects and their allies before us, we can see how some of the most peculiar types are connected with quite ordinary ones by intermediates, and how each group works on a certain series of available colors and patterns to reach its results.
- ↑ University of Colorado Studies, IX., pp. 65-74; Science, September 9, 1910.
- ↑ There is some evidence, the precise value of which can not at present be determined, pointing to a selective destruction of certain germinal elements under special conditions. This, if adequately confirmed, may equally explain some of the results obtained with trypanosomes, and the often-quoted work of Tower and MacDougal in inducing heritable variations.
- ↑ Entomologists' Record, XXIV. (1912), p. 233.
- ↑ P. hypolitus Cr. (male) is figured as having red on the abdomen. This is probably a mistake, as the description says dark yellow.