Showing papers in "Evolution in 1949"

Thermoregulation in reptiles; a factor in evolution.

Abstract: Vertebrates are commonly divided into two groups, the "cold-blooded" or poikilothermic, and the "warm-blooded" or homoiothermic. Unfortunately both the vernacular and technical terms carrv erroneous connotations and moreover imply a dichotomy that does not exist. It is properly assumed that the body temperature of the poikilotherm varies directly with that of the environment. Even though ecologists have long recognized the fact (see Chapman and coauthors, 1926, for example), it is not so generally understood by others that the environment includes not only the air and the substratum, but solar radiation as well, and that animals avail themselves of the great variations in temperature to be found in time and space to avoid extremes and to exercise a measure of control over the thermal level of the body. When confined in the laboratory cage a reptile cannot control its temperature, which may indeed approximate "that of the surrounding atmosphere" as stated in many texts. When active under natural conditions it often maintains the body at a thermal level that is higher than that of man and many other mammals. With their elaborate mechanisms to produce heat internally, the birds and mammals are appropriately characterized as endothermic, unlike the reptiles that derive

Pollination systems as isolating mechanisms in angiosperms

Abstract: 82 The opinion, once held rather widely by biologists, that related species are generally, if not universally, separated by barriers to reproduction involving incompatibility and hybrid sterility has undergone some qualifications in recent years. The modification of the time-honored view has been in part a consequence of the discovery that species of higher plants, which occupy the same range in nature without evident hybridization, may nevertheless yield fertile progeny when crossed in the breeding plot (Anderson, 1948). Two sets of processes are now known whereby intercompatible and interfertile species of flowering plants can occur sympatrically and still maintain their distinctness. The first of these is the non-establishment of the hybrid seedlings, due to the lack of an available ecological niche; the second is the non-production of hybrids, due to the failure of interspecific pollination. The, effect of ecological factors on the isolation of interfertile, sympatric species has been dealt with in several recent publications (Epling, 1947a, 1947b; Stebbins, Matzke, and Epling, 1947; Anderson, op, cit.). The purpose of the present paper is to investigate the occurrence and distribution in angiosperms of isolating mechanisms operating at the critical stage of pollination.

Suggestions as to quantitative measurement of rates of evolution.

Abstract: The best known recent work on rates of evolution is probably that of Simpson (1944), though Small's (1945, 1946) work on diatoms is more extensive. Small is concerned with the origin of species, which in this group seems to be a sudden or almost sudden process. Simpson compares evolutionary rates in different groups mainly by means of data on the origin and extinction of genera. Now among living animals and plants the distinction between genera is much less objective than that between species. On the other hand the paleontologist often finds it hard to be sure of differences less than generic. Moreover the criteria of generic distinction are certainly different in different groups. For example good systematists from Linnaeus onward have assigned the polecat (Mustela putorius) and the ferret (Martes furo) to different genera, while in fact they give quite fertile hybrids and are probably to be regarded as at most subspecifically different. On the other hand I know of no viable, let alone fertile, hybrids between animals assigned to different genera of Diptera or Amphibia. This sugests that animals are rather more readily placed in different genera among mammals than in some other classes. And in view of the incomplete character of the best fossil remains, the systematics of extinct animals are certainly less uniform than that of living ones. On the other hand the dimensions of a solid organ, such as a tooth, a shell, or a bone, are often measureable with great precision; and when we have a series of fossil populations, believed to form a lineage, we can calculate the rate of change of the mean value of any measure. In speci-C-ne o _ t lxr .."a l *".. r+ I:rnilc for% +111 time and for the character. From data on radioactivity we have a fairly precise measure of the duration of the whole tertiary period, and less accurate dates within it; and estimates of ten million years during this period are not likely to be out by much more than 25%o. The precisely dated strata from earlier rocks are not so common. The error is likely to be a good deal greater in paleozoic or mesozoic marine deposits where we have continuous records over some millions of years, as in the English chalk or Lias. Here estimates of time may very well be out by a factor of 2, but are hardly likely to be so by a factor of 10. As biologists we should like to be able to measure time in generations. In a fair number of insects the generation is exactly a year, in others there are two generations per year, and so on. Where generations overlap, as in most vertebrates, we can theoretically define a mean generation length exactly by definite integrals if we have life and fertility tables. In fact we can never do so for fossil forms. But we can be fairly sure that for a small rodent the mean generation length was less than a year, for most ungulates more so. For mammals not larger than a cow we may be fairly sure that it was under ten years. The longest mean initerval between sexual generations is probably to be found in clonally propagated invertebrates such as corals, where it may possibly extend over centuries. If evolutionary rates depend on mutation rates (which is doubtful), the year is as natural a unit as the generation. For it seems possible that the intensity of natural radiation produces a minimum mutation rate per unit of time which is often exceeded (as in Drosophila) but below

Phyletic size increase, an important trend illustrated by fossil invertebrates.

Abstract: One of the crowning achievements of paleontology, and of surpassing importance in the development of evolutionary theory, has been the discovery of innumerable graded morphological series of fossils showing progressive change as we ascend the geological scale of time. Many of the evolutionary modifications follow simple patterns, or trends, which recur again and again in related, or even unrelated stocks. One of the most prevalent, and perhaps the most important of all the trends among invertebrates-increase in mean size during evolution-is the subject. of the present discussion. Long ago Cope showed that there is a persistent and widespread tendency for body size in animals to increase during their phylogeny (Cope, 1885, 1896). Although the principle was illustrated mainly by examples from mammals and dinosaurs, Cope (op. cit.), Deperet (1907), and Rensch (1943, 1947, 1948), among others, have shown that there is' also a tendency for increase in size during the evolution of invertebrates. As a matter of fact Haldane andHuxley (1927, pp. 276-280) have concluded that there has been an increase in mean size accompanied by increased structural complexity in large sections of the animal kingdom. This trend was known to Deperet (op. cit.) as the "law of phyletic increase in size" and Rensch (1948) terms it "Cope's law." For the purpose of this discussion the expression "phyletic growth" or "phyletic size increase" will be used. In reviewing the history of any group of animals one is impressed by the fact that the largest known representatives are usually geologically younger than their smaller relatives. Furthermore, individual size commonly averages smaller when a group first appears than at any subsequent stage in its history. Among living forms are found the largest known representatives of the vertebrates, crustacea, echinoderms, pelecypods, gastropods, cephalopods, coelenterates, and annelids, and these are appreciably larger than the largest known fossil relatives. However, size increase is more characteristic of the evolution of some groups than of others, and is not, by any means, universal. In some groups of Foraminifera (e.g. fusulines, nummulites) maximum size was attained at an intermediate period in their history and was followed by decreasein mean body size, Likewise the largest known representatives of the brachiopods (Mississippian), eurypterids (Silurian),

Selection of an unfavourable gene-complex.

Abstract: It has been the general basis of the theory of natural selection that genes or gene-combinations improving the "fitness" of a species will tend to increase in frequency within the species, while those which are harmful will tend to decrease, as a result of selection. But Fisher (1941) has shown theoretically that a gene affecting the breeding system but not species fitness can be strongly selected; that is, intense selective activity need not be accompanied by an increase in the survival value of the species. This paper describes a breeding System in which there is selection of a genecomplex which affects that system and which at the same time reduces species fitness, and evidence is presented which shows that such a system probably exists in two recently discovered groups of populations of Prinula vulgaris. Selection of unfavourable genes must be clearly distinguished from the purely fortuitous increase in frequency of such genes which Wright (1940) has shown to be possible in small populations.

Geographic variation of adaptive characters in Rana pipiens Schreber.

Abstract: As currently understood the origin of a new species in terrestrial vertebrates appears to depend on three processes. The first consists of the appearance of a variety of genotypes through recombination and to a much lesser extent through mutation. The second process, which usually occurs in a limited portion of the species' range, involves the increase in frequency of genotypes that better adapt the individuals possessing them to the conditions of the local environment. This increase in frequency is due to natural selection. The third process, which may be a special case of the second, consists of the development of mechanisms that can prevent gene exchange between the regionally adapted population and the remainder of the species. The first process results in an increased variability within the population. The second process, which involves the regional differentiation of the population, may lead to the division of the species into subspecies. The third process, leading to the development of isolating mechanisms, is the final step in the formation of new species. This outline is obviously a simplification of the extremely complex phenomenon of· speciation and represents but one of the many methods by which a new species may originate.

The willistoni group of sibling species of Drosophila.

Abstract: Species in sexual cross-fertilizing organisms are reproductively isolated populations. Such populations may or may not be distinguishable in morphological characteristics. Mayr (10942) has proposed the designation "sibling species" for species that are morphologically similar or identical. Camp and Gilly (1943) have called such species phenons, and other authors have referred to them as physiological species (Lancefield, 1929), cryptic species, etc. The theoretical interest of sibling species lies in that their existence shows that reproductive isolation may arise without divergence in morphological traits, and that physiological differences are not necessarily accompanied by morphological ones. As disclosed especially by the work of J. T. Patterson and his school, the genus Drosophila contains several groups of closely related species with small morphological differences between them. The present article reports the results of a study of a cluster of four sibling species native to tropical America, that previously have been confused under the name of Drosophila zvillistoni Sturtevant.

Some problems in the evolution of a group of ectoparasites.

Abstract: The Mallophaga are a group of ectoparasitic insects found on birds and mammals. Their complete life-cycle from egg to egg is spent on the same host form, away from which, under natural conditions, they cannot feed nor live for more than a short time. This group is of especial interest in that a large number of species may be found on one host. Most bird groups have five or six species of Mallophaga and some many more. Twelve species of Mallophaga belonging to eight genera and three families have been recorded from one species of Tinamidae (Tinamous), Crypt'urellus obsoletus punensis, and fifteen species belonging to twelve genera and three families from another, Tinamus major. In this paper the various factors which may have been responsible for speciation in this group of parasites are discussed and comparison made with the process of speciation in free-living animals.

Seasonal variation in the morphology of Drosophila robusta Sturtevant.

Abstract: In previously published studies of genetically controlled morphological variation in Drosophila robusta, Stalker and Carson (1947, '48) have shown that both north-south geographical clines and altitudinal clines exist. It has been demonstrated that in their broader aspects these two types of clines are parallel, with the morphological peculiarities of northern strains also found in strains from high altitudes, while southern strains resemble those from low altitudes. On the basis of this correspondence it seems probable that certain morphological characteristics may be adaptively associated with cool climates, while different characteristics would be adaptively associated with warm climates. If this were the case it would be possible that a population in one locality might adapt itself to the cyclic climatic changes associated with season, and undergo morphological change by a rapid type of natural selection. This change would presumably be toward a southern morphology during hot weather and toward a northern morphology in cold. Dobzhansky (1943, '47, '48a) has repeatedly demonstrated such seasonal fluctuations in Drosophila pseudoobscura, some of them undoubtedly due to natural selection, although in that species he followed changes in the frequencies of chromosome inversions, rather than changes in the morphology. Timofeeff-Ressovsky (1940) has shown that in the beetle Adalia bipunctata the dark-colored form increases in frequency during the warm weather, while during the winter, when a large part of the overwintering population is killed, the lightcolored individuals survive in the largest numbers. The proportion of lightto darkcolored individuals thus shows a cyclical change, this change apparently being caused by a rapid and vigorous natural selection. In the present study we have attempted to i answer three questions regarding seasonal morphological variation. 1. Is there genetically controlled seasonal morphological change within a population, and if so, what are its characteristics ? 2. What is the probable direct (non-genetic) effect of the changing environment on the morphology of the wild population? 3. What part of the genetically controlled morphological variability is associated with specific gene arrangements found in the population?

Genetic analysis of the polymorphism of color pattern in Drosophila polymorpha.

Abstract: Natural populations of most species of Drosophila are very uniform in external morphology. This uniformity is a serious drawback of Drosophila as material for studies on population genetics. Since genetic analysis is predicated upon existence of genotypic variability among materials to be studied, traits other than externally visible structures had to be used in genetic population studies on Drosophila. Variations in the gene arrangement in chromosomes, and recessive genes carried in populations concealed in heterozygous condition, have been investigated by many authors. These investigations have led to great advances in our understanding of the evolutionary mechanisms that operate in Drosophila as well as of general problems of population genetics. The labor involved in detection and study of chromosomal variants and of concealed recessive genes is, however, considerable; a species of Drosophila with genetically determined discontinuous variability in external traits in natural populations would have a clear advantage over species hitherto used. Two such species are known, Drosophila polymorpha Dobzhansky and Pavan (1943) and D. montium de Meijere (see Duda 1924); both show heritable variations in the color pattern of the abdomen. In the first of these two species the variation is caused almost entirely by a single gene, the heterozygotes and the two homozygotes being phenotypically recognizable. The present article reports the results of a study of populations of D. polymorpha which inhabit the states

Cytological evidence on the phylogeny and classification of the Diptera.

Abstract: For several reasons the Two-winged Flies (Diptera) occupy a unique position in the field of evolutionary studies. In the first place this order of insects includes the genus Drosophila, whose genetic system has been investigated so intensively, in the early period (1914-30) from the standpoint of formal genetics and more recently from physiological and evolutionary viewpoints. Drosophila, however, is not the only genus of the Diptera upon which such studies have been carried out; several species of Fungus-gnats belonging to the genus Sciara have been investigated by Metz and his collaborators (general review in Metz, 1938), and preliminary work by Gilchrist and Haldane (1947) on Culex and Lerche (1941) and Bauer (1946) on Phryne fenestralis (see also Bauer and Lerche, 1943) suggests that other species of Diptera are highly suitable for genetic studies. In the second place, it is only in the Diptera that one finds the giant \"polytene\" salivary gland chromosomes, which render possible a type of cytogenetic analysis that cannot be carried out in any other group of organisms. It is true that not all the Diptera possess chromosomes of this type, and that even among those that do the salivary gland chromosomes are seldom as favorable for a detailed analysis as they are in the genera Drosophila, Sciara and Chironomus. But even so, the fact that such an analysis can be carried out in many species of Diptera lends a special interest to all evolutionary studies on the group. Lastly, as will be shown in detail below, the Diptera present several entirely different.dypes of genetic systems whose re\"l~i~ns~~ to one another constitutes in

The role of sexual selection as an isolating mechanism in three species of poeciliid fishes.

Abstract: The role of sexual isolating mechanisms in maintaining the int-egrity of ecoIogically coincident yet specifically distinct populations of sympatric species does not seem to have been very fully investigated. In part, this may be due to the actual rarity of such complete ecological coincidence among species mixtures under natural conditions. It is well known that, among a very considerable proportion of sympatric terrestrial species, ecological differentiation, if only in response to minorfeatures of the common environment, is sufficiently marked so that its importance as an isolating mechanism in speciation cannot be neglected. In part, the paucity of observations of this kind is undoubtedly due to the relatively small number of species mixtures of terrestrial organisms which are sufficiently well understood to make possible any certain conclusion as to the presence or absence of such ecological isolating mechanisms, or any estimate of their relative importance. Observations upon mixtures of sympatric species the members of which. were known to be essentially identical in food habits and environmental preferences would obviously be of great value in a study of the role of physiological and sexual isolating mechanisms in the absence of ecological factors. On the whole, populations of aquatic organisms seem to present more promising material for investigations of this sort than do terrestrial ones. Such truly mixed populations are perhaps most evident among freshwater organisms which inhabit streams and restricted lake environments. It seems likely, for instance, that certain members of the so-called "species swarms" of Cichlidae found in some African lakes, such as those studied by Bertram, Borley, and Trewavas (1942) and others, may properly fall within this category. In other cases, although the ecological coincidence is less perfect, there is still a large and important environmental area in which intimate species mixtures occur without any loss of distinctness on the part of either population. The geographical overlap of the viviparous Poeciliid fishes Xiphophorus hellerii and Platypoecilus maculatus in Mexican streams is well known and has been extensively studied by Gordon and his coworkers (1947). The present report is concerned with a single aspect of a study now in progress of the ecological relations within a mixture of three sympatric species of PoeciIiid fishes which show an extraordinary degree of habitat coincidence in certain of the fresh waters of Trinidad. These three species are Lebistes reticulatus Peters, Micropoecilia parae Eigenmann, and Poecilia vivipara Bloch and Schneider. All three are closely related taxonomically, being included by Hubbs (1926) in a single tribe, the Poeciliini of the subfamily Poeciliinae. Mixed schools of these three species of fish are found in many Trinidad waters, and they exhibit almost identical feeding habits and prefer closely similar stream environments. Such populations appear to offer unusually good opportunities for a study of the operation of sexual and physiological isolating mechanisms in speciation quite apart from the influence of ecological isolation.

A new measure of sexual isolation.

Abstract: In recent years, a good deal of experimental work has been done on the detection and measurement of sexual isolation. One of the most common types of experiment, largely used for Drosophila, is to place a number of males of strain 1 with n1i, females of strain 1 (homogamic females) and nl,2 females of strain 2 (heterogamic females). After a period of time the females are dissected, and xi,,, the number of homogamic females inseminated, and Xl, 2, the number of heterogamic females inseminated, are observed. The measure of isolation usually used is the isolation index, devised by Donald Charles and introduced by Stalker (1942). If we let p1l1=xll/nl,l and Pl, 2 = Xl, 2/nl,2 be the relative frequencies of insemination, then the isolation index is

Analysis of data on sexual isolation.

Abstract: Following the recognition by Dobzhansky of the importance of sexual isolation in speciation, especially in the genus Drosophila, a number of experiments have been made on mating discrimination between races of various Drosophila species: willistoni (Dobzhansky and Mayr, 1944), sturtevanti (Dobzhansky, 1944) and prosaltans (Dobzhansky and Streisinger, 1944) and between the species pseudoobscura and persimilis and their hybrid (Mayr, 1946a). Tan (1946) has made a similar study of the effects of mutants in pseudoobscura. A brief account of the technique is necessary here. Equal numbers of two strains of females are enclosed with males of one of these strains. After a lapse of time sufficient to produce about 50% insemination the distribution of inseminated females between the two strains is observed. The percentage of intra-strain matings minus the percentage of inter-strain matings, divided by their sum, is the isolation index. If all matings are intra-strain the index is 1. If all matings are inter-strain the index is -1. If both strains are equally inseminated the index is zero. Since one strain of males is enclosed with two strains of females it is tempting to assume that the results are determined by discrimination on the part of the males. Both the direct (Mayr, 1946b) and indirect (Bateman, 1948) evidence is quite contrary to this assumption. Virtually conclusive was Streisinger's experiment (1948). Etherization of females, by inhibiting any discrimination on their part, enabled interspecific matings which were otherwise impossible to take place freely. It is justifiable to suppose that the determining factor in "choice" matings is the degree to which the two strains of female repel the same males. It is unfortunate, in view of the role of the females in determining sexual isolation, that in experiments of the "choice" type it is always the males rather than the females which have been offered the choice. The technical difficulties confronting the female "choice" matings are great. If they could be overcome, however, they might be more sensitive than the matings usually used. The repulsion exercised by the females will be the result of two sets of factors. The first, which is strictly concerned with sexual isolation, will depend on the ability of females to perceive whether the male is of their own or another strain and the degree to which they are capable of repelling males which they "recognize" as foreign. The second is concerned with the non-specific mating propensity of the females (what might be called their "heat"). Mayr (1946 a and b) has proposed that the more active the female the more likely she is to be courted. This does seem to apply when one compares females of the species D. pseudoobscura and D. persimilis. On the other hand, if a female is trying to avoid mating, the more active she is the more successful she will be. The situation is probably a complex one. The relative mating propensity of females of different strains introduces a complication to the study of sexual isolation which it may mask or-exaggerate, sometimes even producing negative isolation indices, which are inexplicable by the theory of sexual isolation. It is possible, however, to separate the effects of sexual isolation from those of mating propensity by the use of the isolation indices from complementary matings. "Complementary matings" is here used of the pairs of matings of the types (A + B) X A ee and (A + B)

Mammalian evolution in the Quaternary of southern and eastern Asia.

Abstract: Paleontological contributions to the subject of evolution mostly refer to what is now customarily called macroevolution. The paleontologist usually has insufficient material from a particular horizon and locality to establish the variation limits of the population from which his sample was taken. Study of infraspecific variation hence is left more or less to geneticists and taxonomists. However, phylogenetic work on recent material alone has the disadvantage that the time factor cannot be used in the establishment of phylogenetic units. Paleontology introduces the time dimension, but the paleontologist is forced by the incompleteness of his material to use characters for discrimination other than those by which neozoologists distinguish between their species and subspecies. Consequently, the taxonomic literature on recent animals is of little help to him, and he has to determine whether species or subspecies of mammals that are readily distinguishable by the characters of their skin and, skull can also be separated on the basis of a single tooth or (part of a) bone. This requires conscientious study of the odontology and osteology of recent material, a work which he is not always in a position to do. The student of the Pleistocene and prehistoric fauna deals with the immediate forerunners of the animals constituting the recent fauna. Great evolutionary changes are not to be expected, and at best the student is able to trace certain trends in the evolution of the dentition or of the skeleton rarely extending beyond the limits of a species. Previous to his researches on the Pleistocene fauna of Java, which culminated in the discovery of Pithecanthropus erectus (Dubois), Eug. Dubois (1891) spent some years in the Padang Highlands in central Sumatra, exploring caves yielding a mass of isolated teeth of mammals, including man. In the absence of extinct species in the Sumatran cave fauna, Dubois referred the cave deposits to the prehistoric portion of the Holocene. In Java he collected truly Pleistocene fossils as well as prehistoric teeth and bones, e.g., from the Wadjak cave and from Goea Djimbe, res. Kediri, E. Java. As a matter of fact the Dubois collections contain many species that are still living, either on Java or on the neighboring Greater Sunda Islands. A comparison of the Pleistocene and prehistoric material with that of the recent forms reveals a certain amount of differentiation which the various forms have undergone in the course of time. In the following I shall give examples from five families, viz., the Rhinocerotidae, the Tapiridae, the Hystricidae, the Felidae, and the Pongidae.

Seasonal variation in gene arrangement frequencies over a 3 year period in Drosophila robusta Sturtevant.

Abstract: Previous studies of the gene arrangements of Drosophila robtsta have dealt with their broad geographical distribution in the eastern United States (Carson and Stalker, 1947) and their distribution over an 18-mile altitudinal transect in the mountains of Tennessee (Stalker and Carson, 1948). In both instances, geographical gradients, or clines, in the frequencies of many of the chromosomal types were found. Over the broad geographical range of the species, these clines are gradual, and in a number of cases show distinct north-south relationships; on the mountainside, some, at least, are very steep. Whereas the preceding studies have dealt with the distribution of the gene arrangements in space, the present paper deals largely with their distribution in time. The data presented herein were obtained from an intensive study of the frequencies of the various gene arrangements in a single local population near St. Louis, Missouri, over a three-year period.

Successional speciation in paleontology; the case of the oysters of the sellaeformis stock.

Abstract: The successional mode of speciation of Julian Huxley (1942) may be defined as gradual transformation of one species into a successor species, or several of them, during the course of geologic time without primarily involving geographic, ecologic, or adaptive segregation. The purpose of this paper is to present a particular case of such a mode of speciation and to discuss problems involved therein. The example chosen is the stock of Cubitostrea sellaeformis (Conrad) [Mollusca, Pelecypoda], which contained 4 separate successional species of oysters and lived in Middle Eocene time in the ancient Gulf of Mexico.

Mating between two strains of Drosophila melanogaster.

Abstract: Considerable study has been made of mating in the genus Drosophila, but many aspects of the process are not yet well understood. Much of the recent work (e.g. Dobzhansky, Mayr, etc.) has been designed to investigate the nature and origin of sexual isolation. Many sympatric but distinct species are not separated by sterility barriers, yet fail to cross because of sexual isolation. In such cases, if present theories of species' origin are accepted, sexual isolation must have developed between similar groups such as geographic strains or races. Otherwise, when the two lines once again inhabited the same territory, any differences between them would be lost by interbreeding. For this reason, different strains or races of various species have been tested by investigators for indications of incipient sexual isolation. The present work with Lausanne Special and Oregon R strains of D. melanoqaster (Bridges and Brehme, 1944) was undertaken to test the possibility of sexual isolation between strains and to study the mating behavior of the flies. The results showed no indication of sexual isolation between the strains, unlike the unpublished results of Mather and Harrison, who found isolation between geographic races and also between selection lines of D. melonoqaster, However, the data are of interest because of their bearing on the approach to the problem of sexual behavior and sexual isolation in Drosophila. Each strain of flies had at one time been inbred (brother x sister) for many generations, but this close inbreeding had been discontinued. However, there must have been a fairly high degree of genetic homogeneity within each strain. The flies were cultured, and the experiments conducted in a constantly lighted room at 20.0 ± 0.5 0 C. Virgin males and females were aged separately prior to the mating tests for periods ranging from 48 hours to 14 days, the usual period being about a week. After aging, one male and one female were introduced into an empty glass vial without etherization. Four types of matings were possible, each of which was made up in triplicate. Twelve vials were thus under observation at once, and all types of matings were observed simultaneously in order to minimize unknown or chance environmental fluctuations. All flies used in any set of twelve vials were of the same age. Three readings were taken: the time the flies were introduced into the vial, the time copulation began, and the time it was terminated. The span between the first two readings may be called the courtship period although the proportion of time spent in actual courtship was variable. The data are shown in tables I and II. If copulation did not occur within 60 minutes, the courtship was considered incomplete even though longer observation showed that a few mating's did occur within the next hour. These few mating's are included in table II for the duration of copulation hut are listed as incomplete in table I. The data show that there was a characteristic courtship period for each type of female no matter which male was present. On the other hand, although the difference is not so striking, the duration of copulation is a characteristic of the male type. Both differences are statistically highly significant.

Sexual Behavior And Isolation In Drosophila. Ii. The Interspecific Mating Behavior Of Species Of The Willistoni Group

Abstract: Divers mechanisms reproductively isolate animals of a bisexual species from those of all other species. These may operate at various times in the life of the organism. Some are of greater potency than others, but usually complete isolation is dependent upon the sum total of the effects of several mechanisms. Within the Diptera, sexual isolation seems to be one of the most dynamic and efficient isolating barriers. Because many species of Drosophila can be maintained conveniently for study, they can serve as material for investigations of the manner in which sexual isolation operates and the effectiveness thereof utider laboratory conditions. There are three major methods (Patterson et al., -1947) by which this type of isolation can be studied, i.e., by multiplechoice experiments, by non-choice experiments (pair matings), afid by direct observations of the sexual behavior of the specimens. The last method is laborious and ill suited for obtaining sufficient data for statistical analysis; however, it yields qualitative data that are unavailable by the other two methods. The multiple-choice and ion-choice methods have been used extensively (see Patterson et al., 1947). Direct observation of interspecific crosses, however, has been infrequent. Stalker (1942) watched the interspecific mating behavior of D. virilis and D. americana; Mayr (1946) observed that of D. pseudoobscura and D. persimilis; Wallace and Dobzhansky (1946) watched that of D. subobscurai, D. persimilis, and D. pseudoobscura. Previously the intraspecific or "normal" mating behavior of the willistoni group had been explored by the same method (Spieth, 1947). The present paper is concerned with the interspecific behavior of the zvillistoni group, and its primary aim has been to identify, if possible, some of the s5exual isolating mecha'nisms that operate within the group. Th'e data were obtained by Idirect observations. Of the seven species described for this group (fumipennis Duda, nebulosa Sturt., sucinea Patt. and Main., capricorni Dobz. and Pavan, willistoni Sturt., equinoxialis Dobz. and paulista Dobz. and Pavan), all were available for study except paulista.

Serological evidence on lagomorph relationships.

Abstract: What are the affinities of the Lagomorpha (hares, rabbits,' conies) ? Are they allied to the rodents, or are the adaptive similarities between the two groups the result of parallel evolution? Are the lagomorphs more closely related to any other mammalian order than they are to Order Rodentia? This paper adds serological evidence to the morphological evidence which has been marshalled in attempts to answer these questions. The' investigation for-ms part of a program of serological research devoted largely to rodents' (Levine and Moody, 1939; Moody, 1940, 1941, 1948; Moody and Itzkowitz, 1943). The taxonomic position to be assigned to the hares and rabbits has long been a subject for disagreement., An historical summary is given 'by Simpson (1945); details will not be repeated here. Traditionally the Order. Rodentia has been divided into two suborders: Duplicidentata (hares, rabbits, conies), characterized by two pairs of upper incisor teeth, and Simplicidentata, characterized by one pair of upper incisors. This systematic arrangement is still not uncommon, particularly among European authors (cf. Parker and Haswell, 1940). Evidence has gradually accumulated, however, to indicate that dissimilarities between the groups outweigh similarities. In 1912 Gidley summarized the evidence -and separated the hares and rabbits into a distinct Order Lagomorpha. According to this author lagomorphs differ from rodents in the fQllowing ways. Lagomorphs have four functional upper incisors, -six being present in young indi-

Essay-review of recent works on evolutionary theory by rensch, zimmermann, and schindewolf

Abstract: For some ten years Central Europe and the West have been separated by a highly effective intellectual isolating mechanism, only now beginning to break down. This barrier to interthinking virtually stopped the interchange of knowledge and ideas between the respective populations, within each of which the development of evolutionary theory continued independently of that within the other. The phenomenon is in some ways analogous to the results of cessation of inter-breeding, stopping the interchange of genes between populations and resulting in their independent biological evolution. Explicitly, aspects of parallelism, convergence, and divergence occur among both the intellectual and the biological results of isolation. It is the purpose of this essay-review to examine some of these phenomena in the field of evolutionary theory and also to help to end the isolation by making more widely known some of the recent German work in this field. Such work continued with diminished and yet with considerable vigor during the period of isolation. It is not proposed here to consider all of this work, but rather to concentrate attention on three publications which seem representative of the best recent German thought on the subject, which are outstanding studies that should become known to all students of evolution, and which exemplify in their different ways evolutionary phenomena of intellectual isolation. These selected publications also represent three major fields of evolutionary study: zoology (Rensch), botany (Zimmermann), and paleontology (Schindewolf). Isolation has, of course, worked both ways; western students have been isolated from German scholars as much as the reverse. It is, however, assumed that readers of EVOLUTION are by now quite familiar with the main trends in western evolutionary studies during this period. These trends have been surprisingly uniform. Although using, at times, radically different data and working in widely distinct fields, western students have in almost all cases tended toward a synthesis the core of which is the action of natural selection on genomes in populations. Of the three German works here under review, two also arrive at essentially the same, or certainly closely congruent, conclusions. Rensch (1947) was actively participating in this trend before the war. He carried it forward during the period of isolation in remarkably dose parallelism to work going on elsewhere, unknown to him. Rensch, himself, points this out in a brief appendix referring to three publications (Huxley, 1942; Mayr, 1942; Simpson, 1944) received after his book was in type. Zimmermann (1948) shows no evidence of acquaintance with these or other recent non-German publications, and indeed makes little reference to any non-German work, new or old. He departs from -premises quite different from those of the modern synthesis, in which Rensch participates, and yet his views converge toward this and are in general compatible with it. As may be expected of convergence as opposed to parallelism, Zimmermann's agreement with this synthesis is, however, neither so close nor so extensive as in the case of Rensch. The third work under review here, that of Schindewolf (1945), is in radical disagreement and exemplifies the phenomenon of divergence in intellectual evolution. Schindewolf, like Rensch and unlike Zimmermann, departed, in the main, from somewhat the same basis that has produced the synthesis and he was, indeed, a pioneer in one aspect of it (Schindewolf, 1936, an attempted synthesis of genetical and paleontological data). Divergence began, however, even in this earlier work and Schindewolf has now developed a general theory of evolution profoundly different from that of Rensch, Zimmermann, most English and American students, and many in other countries. Rensch has long been one of the great authorities on speciation in a narrow sense, on what he calls \"intraspecific evolution,\" and his fundamental work in this field (especially Rensch, 1929) is known to all students of evolution. In his new book, he summarizes this subject only briefly and turns his attention, rather, to \"transspecific evolution,\" to the origin of higher categories and to the broader, more long-continued aspects of the history of life. After relatively short, but extraordinarily interesting and valuable, discussions on the lack of an over-all directional control in evolution and on rates of evolution, he divides transspecific evolution into two main aspects for which he proposes the names \"cladogenesis,\" splitting or 'branching of phyletic lines, and \"anagenesis,\" progressive change (without, or regardless of, branching). He thus pictures the whole pattern of evolution as analyzable into three main aspects, which are analogous but not identical with the three aspects

Evolutionary changes in the sex chromosomes of Coleoptera; wood borers of the genus Agrilus.

Abstract: A review of the literatture (Harvey, 1916, 1920; Suomalainen, 1940a) and an extensive sampling of species representing 18 families of beetles establishes the fact that there are at least four types of sex-determining mechanisms operating in the Coleoptera. These are: (1) haploid(S)-diploid (?), known only in Micromiwlthus debilis Lec. (Scott, 1936) ; (2) XO(&)-XX(?) and (3) XY(S)-XX (T), as exemplified by the pioneer work of Stevens (1905, 1906, 1908, 1909), and more recently that of Suomalainen (1940a and b, 1947) ; and (4) a multiple condition involving five chromosomes in Blaps latsitanica Hbst. (Nonidez, 1920; Wilson, 1925). An XXO(3) system, resembling that of spiders (Painter, 1914; Revel, 1947; Patau, 1948), has been reported for various species of Cicindela by Goldsmith (1919), but his account is very unsatisfactory and conflicts with that of Stevens (1906, 1909). Of the species known to the writer 21 fall into the XO category and 82 into the XY. Of the latter only eight were known prior to the present study to have a Y chromosome which approaches the X chromosome in size. Three are scarabaeids (Prowazek, 1902; Hayden, 1925) and one a coccinellid, Cleis hudsonica Csy. (unpub.), in all of which there is no heteromorphisml in the sex pair; the fifth is a lagriid, Arthromiacra aenea (Say), with a slight but consistent inequality between the X and Y (unpub.), and the remainder are buprestids in which both X and Y are large but definitely unequal in size (Asana, Makino, and Niiyama, 1942). Otherwise, striking differences in the sizes of the X and Y chromosomes result

Avian relicts and double invasions in peninsular india and ceylon

Abstract: One of the most interesting evidences of speciation is in connection with double invasions. Double invasions occur when two closely related populations arrive in an isolated area at different times. For some reasons, genetic and ecological, they are able to maintain their separate identity and coexist without interbreeding. A number of such cases, particularly for oceanic islands or other isolated areas, have been cited by Mayr (1942). Double invasions are more difficult to study on or about continental areas. Here auxiliary factors must play their part in order to create the necessary isolation. Geological changes may occur and in their wake bring secondary ecological shifts. The peninsula of India and the island of Ceylon are such areas, and I have been able to collect specimens and study the faunas of both places during the war (Ripley, 1946), and during the winter of 1946-47. The peninsula of India and Ceylon have long been noted for their peculiar and isolated faunas. Salim Ali (1935) has made an interesting survey of the south Indian states of Travancore and Cochin, in which he points out the similarities of the fauna of these states to that of Assam, Burma, and Malaysia, over a thousand miles away. He describes the climatic similarities of the two zones, separated by a relatively arid intervening area. Earlier authors, noting the close resemblance of the faunas of these zones, had suggested that a bridge of some sort must once have existed between south India and Ceylon and the Malay peninsula. Recent work tends to discredit this land-bridge thesis and to look instead for the explanation in recent physiographic changes in peninsular and central India. There are three main factors which have created the isolated biota of south India and Ceylon. The first is geologic. By Pliocene time the main backbone or watershed of India, as Wadia (1944) calls it, had arisen. This consisted of the Vindhyan Hills and the Kaimur Ridge running east to Rajmahal, across the gap of alluvial deposits connected with the present course of the Ganges and Brahmaputra rivers, and on to the Khasia and other central hill ranges of Assam (fig. 1). This mountain chain undoubtdely served as a faunal link between the mountains of southwest peninsular India and the Himalayas, a bridge by which the forms of one area could be dispersed continuously throughout the other. There has been considerable recent discussion on the age and configuration of the Rajmahal gap alluded to above, as it provides an important break in the backbone. Hora (1944) postulates that the gap was formed in mid-Miocene time, was closed in Pliocene time by block-faulting, and finally reopened in the Quaternary. He maintains that such a series of events would have had to occur in order to allow for the migration of freshwater torrential fish from Assam into the peninsula. Aside from these torrential fish, whose potentialities for secondary adaptation are not considered in Hora's paper, I would assume that the majority of the flora and fauna which had migrated from the north would be able to cross the Rajmahal gap during the pluvial periods. Presumably the gap was much narrower in late Tertiary or early Quaternary time, and in the absence of any sound geological evidence to the contrary, it would seem wiser to assume that it has existed continuously since its formation. The

Bromus inermis and b. pumpellianus in north america

Abstract: Since the introduction of the grass Bromus inermis Leyss. from Europe (Dwinelle, 1884) significant changes have occurred in populations of the North American native B: pumpellianus Scribn. This preliminary study is concerned with an evaluation of the apparent introgression of the two species of grasses which has followed. B. inermis, grown extensively on ranges and pastures in the United States and Canada, may -also be found along roadsides, railroad right-of-ways, and old fence lines rather generally throughout the range as indicated in figure 1. B. pumpellianus, the closest North American counterpart of B. inermis, ranges northward from the Intermountain Region into Canada and Alaska as far as the Seward Peninsula as indicated by herbarium specimen sources in figure 2. In recent years more aggressive introduced grasses, including B. inermis, have largely replaced B. pumpellianus and certain other native

Interspecific matings between Colias eurytheme and Colias philodice in wild populations.

Abstract: The act of copulation between the sexes of two different species has not often been actually observed in wild populations. More often hybridization has been assumed from the presence of intermediates or hybrids in the respective populations. Owing to the rarity of data showing the actual rather than the assumed hybridization, the following data may be of interest. The two butterflies Colias eurytheme and Colias philodice apparently hybridize in most areas where their ranges are in contact or overlap. This hybridization however does not result in the expected elimination of both species in favor of an intermediate type. Rather, the two species maintain themselves year after year in the same areas despite constant exchange of genes from one species to the other. The result of the hybridization is the production of F1 hybrids between the two parental species which are intermediate in their phen6typical characteristics. But the hybridization does not stop there. Backcross progeny and F2 hybrids are also produced, thus leading to -a population having individuals ranging from the one parental form to the other with no sharp line of demarkation. It has been found that these intermediate types rarely exceed 10 per cent of the entire population numbers (Hovanitz, 1943; Gerould, 1946). Interspecific crosses in the laboratory have been observed by Gerould (1943) and Hovanitz (1944) and in wild populations by Hovanitz (1943). However, no quantitative data have ever been published which would indicate the relative proportions of these hybrid crosses as compared with the intraspecific matings. The following data were obtained on October 26, 1947 in an alfalfa field near Celina, Ohio (for map see *Hovanitz, 1948). The characteristics of this field were those favoring the presence of a considerable number of copulating pairs. Males were swarming over the field but females were relatively scarce. Both males and females were in the emerging cycle. Copulating pairs were collected as given in table 1. Grades of orange pigment from 8 to 1(0 are considered to be Colicas eurytheme and grade 1 to be Colias philodice; grades 2 to 7 are considered to be hybrid products.

Sources of genetic variation in Parthenium argentatum Gray (Compositae).

Abstract: variant evolutionally. With appropriate data and their statistical treatment, predictions as to the probabilities of survival, or the chances of a given type becoming dominant, may be made. But it remains largely for the paleobotanist and comparative morphologist to chart the actual course of a given evolutionary sequence. Experimental methods of attack on such problems are limited principally by the time factor in nature which cannot be duplicated or predicted. However, experimental procedures may be effectively employed to show how natural evolutionary events most probably took place in many instances. Both experimental and comparative methods have been used individually and in combination to elucidate the facts concerning Parthenium arqeniatum. which follow. In this species, there is a combination of characteristics which establishes its area of origin and permits revealing studies on several factors important in its evolution. Though the species has been worked with in some detail, much remains to be done. Most of the needed data must come from future field studies in Mexico. SOURCES OF GENETIC VARIATION IN PARTHENJUM ARGENTATUM GRAY (COMPOSITAE)

The selection index and its test of significance.

Abstract: Under experimental conditions in which equal numbers of individual animals of two types, A and B, are exposed to selection by a predator with the result that the numbers of individuals taken are a and b and the numbers not taken are c and d, respectively, making a combined total of n, then we may measure the strength of selection by the selection index: (a b)/(a + b). Should a greater number of type A than of type B individuals be taken (selection against A), then the selection index will be positive, while if more of B than of A are taken (selection against B), the index will be negative. The formula which I earlier proposed (Dice, 1947: 3) for the calculation of the chi-square of the difference between the numbers of A and of B taken under such conditions, unfortunately, is inappropriate. This has been pointed out to me by Don W. Hayne, who also has given other aid with this problem. The formula for chi-square earlier given, (a-b)2/(a+ b), is correct for a 1:1 ratio, as stated, but that formula takes account only of the successes in each of the two classes and neglects the failures in the same classes. The chi-square appropriate for testing the significance of the deviation index from zero is that for a 2 x 2 table: