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Showing papers in "Evolution in 1965"


Journal ArticleDOI
TL;DR: It was found that there is no equilibrium in either case short of complete fixation locally, in spite of the linear increase in number of different ancestors with increasing number of ancestral generations, in contrast to systems (half first cousin or second cousin) in which this increase is more than linear and a steady state is rapidly attained with respect to heterozygosis.
Abstract: Kimura and Crow (1963b) have recently made an interesting comparison between two classes of systems of mating within populations of constant size: ones in which there is maximum avoidance of consanguine mating and ones in which all matings are between close relatives around an unbroken circle. These are illustrated in Figs. 1 and 2 in populations of eight. The rate of decrease of heterozygosis in the former class had, as they note, been found long before to approach 1/(4N) asymptotically with increasing size of population, N (Wright, 1921, 1933a). Two cases with patterns of mating similar to those of Kimura and Crow's second class, except that the matings were between neighbors along infinitely extended lines instead of around a circle, had also been considered in these papers. These systems consisted of exclusive mating of half-sibs or of first cousins, otherwise with a minimum of relationship. It was found that there is no equilibrium in either case short of complete fixation locally, in spite of the linear increase in number of different ancestors with increasing number of ancestral generations. This was in contrast to systems (half first cousin or second cousin) in which this increase is more than linear and a steady state is rapidly attained with respect to heterozygosis. Kimura and Crow were surprised to find that the limiting rates of decrease of heterozygosis in their circular systems are much less than under maximum avoidance approaching [v/(2N + 4)]2 in the case of half-sib matings and [7/ (N + 12)]2 under first-cousin matings with large N. Maxi-

3,305 citations


Journal ArticleDOI
TL;DR: With the advent of relatively objective classifications, such as the phenetic classifications produced by the operational techniques of numerical taxonomy, it was inevitable that biologists would wonder what phylogenetic conclusions could be drawn from them and with what reliability.
Abstract: With the advent of relatively objective classifications, such as the phenetic classifications produced by the operational techniques of numerical taxonomy (Sokal and Sneath, 1963), it was inevitable that biologists would wonder what phylogenetic conclusions could be drawn from them and with what reliability. If these phenetic taxonomies did not reflect all of the elements of phyletics (Sokal and Camin, 1965), could techniques be devised for deducing the latter? For example, could operational methods be devised for deducing the cladistic relationships among taxa, so that, given the same initial information, different investigators would obtain the same results? By cladistic relationships we mean the evolutionary branching sequences among taxonomic units without regard to phenetic similarities among them or to an absolute time scale. There is no question that phylogenies could probably be reconstructed without error for any taxonomic group if complete fossil sequences for that group were available. However, can cladistic reconstructions be carried out with any degree of

710 citations


Journal ArticleDOI
TL;DR: The current revival of morphology is heralded by a flourish of studies in functional anatomy with the general result being a renewed focus of interest in the problem of organic form.
Abstract: The current revival of morphology is heralded by a flourish of studies in functional anatomy with the general result being a renewed focus of interest in the problem of organic form. Recent morphological studies are characterized by considerations of the functional properties of structure and of the interrelationships between the structure and the environment of the organism although the traditional considerations of pure morphological description and of the phylogenetic change of morphological form are not ignored. These studies have established a broader base for morphological inquiry and have permitted a far better understanding, albeit largely theoretical, of all factors influencing the observable shape of morphological features. It may be possible, in the near future, to partition these factors and to determine the influence of function, of surrounding structures, of phylogeny, and so forth in the molding of anatomical features. The importance of functional anatomy in the recent upsurge of morphological studies is eclipsed by its basic contribution to a deeper appreciation of biological adapta-

642 citations


Journal ArticleDOI
TL;DR: The results of an analysis of bill length of 46 bird families inhabiting temperate, subtropical, and tropical zones are presented, and several models are proposed to explain interfamilial, regional, and intrafamilial differences.
Abstract: Recently, Hutchinson (1959), Klopfer and MacArthur (1961), and Klopfer (1962) have introduced the use of the quantitative comparison of bill size differences among groups of sympatric congeneric species of birds in the study of the evolution of niche overlap and size. In his analysis of character displacement, Hutchinson (1959) has shown that the ratio of the size of the larger to smaller trophic appendages of congeneric species generally falls between 1.2 and 1.4 where they are sympatric, but is less where they are allopatric. Klopfer and MacArthur (1961), using Ridgway's data, found a much smaller ratio for certain associations of sympatric tropical birds. This paper presents the results of an analysis of bill length, in which representatives of 46 bird families inhabiting temperate, subtropical, and tropical zones are compared. Several models are proposed to explain interfamilial, regional, and intrafamilial differences. Finally, the implications of this study for the concepts of niche overlap and behavioral stereotypy are discussed.

311 citations


Journal ArticleDOI
TL;DR: Attempts to interpret the greater bill-length and winglength of island birds have been unable to interpret, and even if a size trend among island birds can be demonstrated, its significance has yet to be explained satisfactorily.
Abstract: Species of animals living on islands may have morphological characteristics not possessed by their mainland counterparts, a fact which was recognized by Wallace (1881). He remarked that in the Celebes: "Nearly thirty species of butterflies, belonging to three different families, have a common modification in the shape of their wings by which they can be distinguished at a glance from their allies in any other island or country whatever, and all these are larger than the representative forms inhabiting most of the adjacent islands." Among birds there is an apparently undisputed tendency for island forms to have a plumage more drab than that of their most closely related mainland forms (Murphy and Chapin, 1929; Murphy, 1938; Amadon, 1953; Grant, 1965a). It has also been reported that island birds have long bills (Murphy, loc. cit.; Chapman, 1940; Amadon, loc. cit.) and long wings, the latter being interpreted as an indication of large body-size (Mayr and Vaurie, 1948; Amadon, loc. cit.). Yet these trends of large size in island birds are not universal, since there are reports of opposite trends (Hesse et al., 1937; Bourne, 1955). For instance Bourne (loc. cit.) has pointed out that many of the birds of the Cape Verde islands have smaller dimensions than their mainland counterparts. He considered this to be a "character of sedentary species which is often particularly well shown by insular forms." Furthermore, even if a size trend among island birds can be demonstrated, its significance has yet to be explained satisfactorily. Attempts to interpret the greater bill-length and winglength of island birds have been unable to

217 citations


Journal ArticleDOI
TL;DR: Self-fertilization may also result from temporary or local environmental changes as in Narthecium ossifragum, Stipa, and Danthonia (Stebbins, 1957), however, the mechanism is apparently less subject to environmental influences.
Abstract: The evolutionary effects of outbreeding and inbreeding systems have caused considerable interest and speculation over the years and their influence in changing gene frequencies has been formalized mathematically (see Fisher, 1949; Wright, 1921). A large body of data is available on the genetical basis of the sex and incompatibility mechanisms promoting outcrossing in plants but similar information on the devices favoring self-fertilization is much more scanty. Self-pollinating races or species occur widely throughout the angiosperms and there seems little doubt that they have been derived from outcrossing relatives (Stebbins, 1957). In self-compatible plants, self-pollination is often limited either by temporal or physical separation of the mature anther and stigma or by some combination of these two types of mechanism. Automatic self-pollination is effected by the juxtaposition of the mature anther and stigma. It can be the result of regular environmental changes as in Viola (Evans, 1956; Bergstrom, 1939; Bergdolt, 1932) where open, cross-pollinated flowers occur early in the season and cleistogamous flowers are produced during the longer, warmer days of summer; a comparable seasonal sequence occurs in Bromus carinatus (Harlan, 1945). Self-fertilization may also result from temporary or local environmental changes as in Narthecium ossifragum, Stipa, and Danthonia (Stebbins, 1957). In the majority of self-pollinating plants, however, the mechanism is apparently less subject to environmental influences. Stebbins (1950, pp. 176-179) has suggested that self-pollination is most likely to arise

158 citations


Journal ArticleDOI
TL;DR: Proper understanding of Mayr's definition, of the phenomenon of hybrid zones, and even of the speciation process itself, depends upon the clarity of the understanding of the words "reproductively isolated."
Abstract: The interpretation of hybrid zones remains one of the most difficult of taxonomic problems, despite the striking advances in systematics of recent decades. Mayr (1963: 496) selected sexual isolation, hybrid fertility, and developmental compatibility as the only criteria that will render possible unequivocal decisions (one species or two) in the case of peripheral isolates, and the same criteria should apply to hybrid zones. In hybrid zones, sexual isolation is always imperfect or absent, the hybrids are often fertile, and developmental compatibility is sufficient to produce hybrids. All three criteria break down in hybrid zones without always rendering unequivocal decisions possible, and similar difficulties may be encountered in the experimental analysis of peripheral isolates. These are the anomalies that make decisions based on Mayr's well-known species definition difficult. The difficulties are due in part to the gradual process of evolution itself, but they may also be due to a misunderstanding of the definition, which states rather emphatically, at the very outset, "Species are groups of actually or potentially interbreeding natural populations. ." In every hybrid zone, interbreeding is actual and this opening phrase of the definition promotes a natural tendency to consider the question as settled once interbreeding has been established. This feature of the definition diverts attention from the more essential remainder: ". . . which are reproductively isolated from other such groups." This is the essence of Mayr's definition. Mere interbreeding does not necessarily indicate an absence of reproductive isolation. Two good species will remain reproductively isolated however many sterile hybrids they might produce. The production of sterile or inviable hybrids may even be the best possible evidence of full species status, as Mayr himself has pointed out. Proper understanding of Mayr's definition, of the phenomenon of hybrid zones, and even of the speciation process itself, depends upon the clarity of our understanding of the words "reproductively isolated." Mayr (1959a) defines reproductive isolation in terms of "the protective devices of a well-integrated and harmoniously coadapted gene pool against pollution by other gene pools," and he regards speciation as essentially completed when geographical isolation can be removed without resulting in "genetic swamping of the new daughter species by the parental species." Dobzhansky (1950) omits the word "interbreeding" from his species definition, and stresses the reproductive community of the gene pool, and later (1951:

154 citations


Journal ArticleDOI
TL;DR: The present problem centers on the development of one type of premating isolating mechanism -ethological isolation -and whether it arises incidentally along with other adaptive processes operating in allopatric populations, or whether through the direct action of selection for the isolating effect itself.
Abstract: The geographic ranges of two closely related southeastern Australian anurans, Hyla ewingi and H. verreauxi,' overlap in part so that continuously distributed allopatric and extensive sympatric populations of each species exist, without any hybridization in the latter situation. The existence of a second contact of the two forms north of the Victoria-New South Wales border is suspected but this relatively inaccessible area awaits detailed investigation. In addition, disjunct allopatric populations of H. ewingi occur as late Pleistocene isolates on three adjacent continental islands: Tasmania, Flinders Island, and King Island (Fig. 1). This distribution pattern appears suitable for a study of geographic variation in actual or potential reproductive isolating mechanisms, since within the species complex it may be possible to assess the applicability of the two main tenets advanced to account for the origin of reproductive isolation (reviewed by Mayr, 1963). The present problem centers on the development of one type of premating isolating mechanism (Littlejohn, 1957, 1959; Mecham, 1961)-ethological isolation -and whether it arises incidentally along with other adaptive processes operating in allopatric populations, or whether through the direct action of selection for the isolating effect itself. Differences in mating call structure may operate as important ethological isolating mechanisms between sympatric species of anurans (Blair, 1955a, 1958; Littlejohn and Michaud, 1959; Littlejohn et al., 1960; Littlejohn, 1961; Michaud, 1962). Since this behavioral characteristic can be objectively analyzed and compared by conventional electronic means it was selected for detailed study. Other possible premating isolating mechanisms

133 citations


Journal ArticleDOI
TL;DR: While very gradual changes theoretically can account for the known evolutionary history of organisms, rapid changes are reasonably frequent in nature, it is not clear whether these rapid changes tend to oscillate or to occur only sporadically, so that the long-term effects may be similar to those which would result from relatively constant slow selection.
Abstract: The relative contributions of various forces to evolutionary changes in natural populations remains one of the central questions of population biology. Many of the studies bearing on this question have been initiated because of the unusual characteristics of certain populations. For instance, they have dealt with extreme interpopulation variability in Cepaea and striking differences in composition of young and adult samples of Natrix. In other cases, marked changes in populations through time (as in Biston betularia) have attracted the attention of biologists. These investigations generally have led to the conclusion that rapid evolutionary changes are taking place, and in particular they have led to the detection of high selection pressures. It now seems that while very gradual changes theoretically can account for the known evolutionary history of organisms, rapid changes are reasonably frequent in nature. It is not clear, however, whether these rapid changes tend to oscillate or to occur only sporadically, so that the long-term effects may be similar to those which would result from relatively constant slow selection. The sampling of microevolutionary situations represented by these studies is neither large enough nor sufficiently unbiased to permit reasonable statements about patterns of microevolutionary rates, or about the relative magnitude of factors contributing to these rates. One partial solution to this sampling problem is to investigate populations chosen neither because of some unusual characteristic, nor

133 citations


Journal ArticleDOI
TL;DR: Drosophila paulistorum is indeed a superspecies composed of six races or subspecies or incipient species, and this situation is of interest precisely because these six may be considered about equally legitimately as very distinct races or as very closely related species.
Abstract: Dobzhansky and Spassky (1959) first suggested that Drosophila paulistorum was a cluster of species in statu nascendi, a borderline case of uncompleted speciation. Their suggestion has been borne out by much laboratory work since then. It is indeed a superspecies composed of six races or subspecies or incipient species. This situation is of interest precisely because these six may be considered about equally legitimately as very distinct races or as very closely related species. Each race inhabits a geographic area different from the others, but the distributions of some of the races do overlap. Where two or more races share a common territory (four is the maximum number of races occurring sympatrically), they seem not to interbreed, and thus they behave like full-fledged species (Dobzhansky et al., 1964). Sexual (or ethological or behavioral) isolation wherein potential mates meet but do not mate, is a most efficient isolating mechanism; it does away with the wastage of gametes, food and space for developing hybrids, etc. This sexual isolation which makes matings between the females and males of different D. paulistorum races much less likely to occur than matings within the races, is due to polygenes scattered in every one of the three pairs of chromosomes which the species possesses. These polygenes efficiently control the sexual preferences of their carriers. Their effects seem to be simply cumulative (Ehrman, 1961). A female of hybrid origin which carries a majority of the chromosomal material derived from a given race

132 citations


Journal ArticleDOI
TL;DR: It will be shown that even though these genotypes are counted one generation apart at exactly the same stage of the life cycle, it is not possible to obtain estimates of net fitness unless very special and unlikely conditions hold.
Abstract: The purpose of this note is to examine a method in current use for estimating fitnesses in experimental populations. This method had been used by Polivanov (1964) and evidently by Jain and Allard (1960). The procedure involves comparing the frequencies of genotypes in one generation with the frequencies of the same genotypes at the same stage of development in the next generation. The objective is to estimate the set of fitnesses which brought about the change. In what follows, it will be shown that even though these genotypes are counted one generation apart at exactly the same stage of the life cycle, it is not possible to obtain estimates of net fitness unless very special and unlikely conditions hold. The difficulties arise from the fact that the selection process may be only partially completed at the stage of development when the genotypic frequencies are determined within each of the two generations. In order to investigate such a situation, a model was constructed which has an arbitrary stage of incomplete selection inserted between the time of fertilization and the time when selection is finished. The case of two alleles at one locus with random mating will be examined in detail. However, the conclusions derived probably apply in a general way to non-random mating systems and to genotypes formed by segregation at several loci. The model which permits the investigation is set forth in Table 1. This model divides the life cycle within one generation into three stages: (1) a preselection stage when the genotypes are in Hardy-Weinberg proportions due to the random mating of their parents; (2) a partially selected stage when there has been some differential survival which is measured by the "early" fitness components El, and E22; and (3) a post-selection stage when selection is completed. The "late" fitness components, L1i and L22, operate between the last two stages. Net fitnesses W1l and W22 will be defined in terms of the early and late components as follows:

Journal ArticleDOI
Albert E. Wood1
TL;DR: All recently proposed classifications of the rodents adopt the multiplicity of major groups postulated by Miller and Gidley (1918) or Winge (1924), and agree that the three classic suborders are not monophyletic clades, but rather, taken as a whole, represent a grade.
Abstract: As has been pointed out many times, the rodents are the most abundant and successful mammalian order. Their evolution has been channeled into a single major direction by the development, as an initial modification, of ever-growing, gnawing incisors, with associated changes in skull and jaw muscles. Subsequent evolution has involved a great deal of parallelism within the order, making it very difficult to disentangle the convergent and parallel changes from those that are truly indicative of phyletic relationship. The similarity in complexity of the evolutionary pathways among rodents to those among actinopterygians, and particularly teleosts, has also been pointed out. Work by various authors has indicated that the evolution of the actinopterygians consists of the sequential attainment of a series of morphological stages, or grades (as in Huxley, 1958), each of which has been derived from the preceding one several independent times by a series of parallel trends. The classification of actinopts at the supraordinal level involves a series of taxa that are currently agreed to represent such polyphyletic grades rather than monophyletic units or clades (Schaeffer, 1956, p. 202). The rodents were, classically, divided into three suborders on the basis of the structure of the jaw musculature and associated osteological differences-the Sciuromorpha, Myomorpha, and Hystricomorpha (Simpson, 1945). All recently proposed classifications of the order (Lavocat, 1956; Schaub, 1958, p. 691-694; Simpson, 1959: and Wood, 1955a and 1959), adopt the multiplicity of major groups postulated by Miller and Gidley (1918) or Winge (1924), and agree that the three classic suborders are not monophyletic clades, but rather, taken as a whole, represent a grade

Journal ArticleDOI
TL;DR: It appears to be a general rule that where two or more of the incipient species are sympatric in the same territory, no hybrids between them are formed spontaneously, and crosses can be made in laboratory experiments only with considerable difficulty, always yielding sterile hybrids.
Abstract: The superspecies Drosophila paulistorum Dobzhansky and Pavan consists of six races or incipient species which exhibit various degrees of reproductive isolation from each other. The two principal isolating mechanisms involved are sexual (ethological) isolation and sterility of hybrid males (Dobzhansky and Spassky, 1959; Dobzhansky et al., 1964). Sexual isolation is almost certainly the more effective of the two mechanisms in preventing gene exchange between the incipient species in their natural habitats. It appears to be a general rule that where two or more of the incipient species are sympatric in the same territory, no hybrids between them are formed spontaneously, and crosses can be made in laboratory experiments only with considerable difficulty, always yielding sterile hybrids. On the other hand, socalled transitional strains are found in some territories inhabited by only one race. Thus, only the Andean-Brazilian race is found in northeastern Brazil, and some of the strains derived from the populations of that region cross to some strains of the allopatric Amazonian race and produce fertile hybrid sons (Malogolowkin, 1962, 1963; Malogolowkin et al., 1964). On the other hand, various degrees of sexual isolation have been encountered in crosses between strains of the same race from geographically remote localities (Carmody ct al., 1962). Much of the experimental work on sexual isolation in D. paulistorum has used the so-called \"male choice\" method. Groups of virgin females belonging to two different strains or races are confined for a certain time interval with males of one of the strains; all of the females are then


Journal ArticleDOI
TL;DR: Insects have been the most successful animals in colonizing isolated islands as mentioned in this paper, and they are the only animals that have occupied islands and speciated there, and only lands snails approach insects in the degree to which they have occupied and occupied islands.
Abstract: Insects have been the most successful animals in colonizing isolated islands. Only lands snails approach insects in the degree to which they have occupied islands and speciated there. Abundant evidence shows that many islands have arisen directly from the ocean bottom and were never connected with continents. It is also clear that some ancient islands have existed which are now represented only by atolls or submarine mountains. Such relict islands must have been important in the evolution of the present fauna of isolated island groups and some probably formed stepping stones to existing islands. Correlation of results of trapping at sea and in the air with types of insects native on oceanic islands strongly indicates dispersal across open ocean from source areas primarily by air currents. The study of the distribution and speciation of insects on Pacific islands may be an important key to the knowledge of the history of islands, of the nature of dispersal, and of the evolution of species in the insular environment.

Journal ArticleDOI
TL;DR: Comparisons of symbiotic algae from diverse hosts and freeliving algae with respect to their infectivity into particular host species, and to the characteristics of the symbiotic association which they form, ought to provide at least a partial answer to these questions.
Abstract: Chlorella or Chiarella-like algae are found as intracellular symbionts in several invertebrate phyla, including the Protozoa (both Sarcodina and Ciliata), Porifera, Platyhelminthes, and Coelenterata. (See review of Droop, 1963 for additional details.) The evolutionary history of these associations is of interest because the algal symbionts from these diverse hosts seem closely related to one another and to freeliving strains of the genus Chlorella. To what degree have the symbiotic algae become adapted to their intracellular mode of existence, and how specific is the association between particular host species and potential symbionts? Comparisons of symbiotic algae from diverse hosts and freeliving algae with respect to their infectivity into particular host species, and with respect to the characteristics of the symbiotic association which they form, ought to provide at least a partial answer to these questions. The symbiotic association of the ciliated protozoan Paramecium bursaria with Chlorella is especially suited for such studies. The protozoan host can be rid of its naturally occurring intracellular symbionts, and the resulting aposymbiotic strains are readily cultured. Moreover, many of the symbiotic algal strains can be cultured axenically. The host reinfects easily with its own or other symbiotic algae (Siegel, 1960; Karakashian, 1963) and is small and unicellular, which simplifies the quantification of the infectivity of the algae. We have recently examined the affinity

Journal ArticleDOI
TL;DR: According to the present concepts, sustained intensive directional selection would decrease and eventually exhaust this residual store of genetic variance, yet the situation is never brought to this extreme since evolution, even at its most rapid pace, is slow compared to changes produced by artificial selection.
Abstract: Information amassed by animal breeders has aided considerably the understanding of the genetic changes that accompany phenotypic population changes through time. In spite of genetic inferences from these artificial selection experiments, there are few studies of genetic and phenotypic changes in characters evolving under natural conditions. Because of the scarcity of statistically adequate series of fossils and the incompleteness of knowledge of phylogenetic patterns, the contributions of paleontology to the understanding of evolutionary dynamics have been far below its potential. However, as phylogenies become better known and series are emphasized rather than types, it is increasingly possible to study the detailed behavior of evolving characters. Findings of these studies, in turn, permit a more critical evaluation of our theoretical models. One of the critical areas of evolutionary research is the behavior of the intrapopulational variation of a character when it is undergoing change. An understanding of the changes in genetic variation as the population moves from one mean to another is central to any investigation involving evolutionary mechanics. Lerner (1955) listed as one of the significant landmarks of population genetics the discovery of the great genetic reserves in natural populations, yet this high potential genetic variation is usually associated with relatively low phenotypic variation. According to our present concepts, sustained intensive directional selection would decrease and eventually exhaust this residual store of genetic variance. In reality the situation is never brought to this extreme since evolution, even at its most rapid pace, is slow compared to changes produced by artificial selection. However, the problem of the elimi-

Journal ArticleDOI
TL;DR: Attention was directed toward one of the interesting cases of partial isolation, the cross between Peromyscus maniculatus, the deer mouse, and P. polionotus, the oldfield mouse, which appeared to offer promising possibilities for studies of evolving barriers to reproduction, particularly with respect to fertility and disrupted size inheritance.
Abstract: The North American cricetid genus Peromyscus has long been recognized as a useful group for evolutionary and genetic studies (Sumner, 1932; Dice, 1940; Blair, 1950). The diversity of species and subgeneric taxa (Hall and Kelson, 1959), together with the adaptability of these mice to laboratory conditions, makes this genus especially useful for investigations of experimental hybridization between races or species. Dice (1940) concluded from hybridization studies that within the genus there exists a spectrum of genetic isolation, ranging from complete fertility between most races of the same species, through several grades of partial isolation between species of the same species group, to complete intersterility among species of different species groups or subgenera. In the present study attention was directed toward one of the interesting cases of partial isolation, the cross between Peromyscus maniculatus, the deer mouse, and P. polionotus, the oldfield mouse. This cross appeared to offer promising possibilities for studies of evolving barriers to reproduction, particularly with respect to fertility and disrupted size inheritance. Sumner (1930) and Dice (1933) demonstrated in a limited number of attempts that successful crosses between these species could be achieved and viable hybrids produced. Watson (1942) conducted more extensive investigations of this cross. With 23 matings each of the reciprocal types, using various races of each species, she first showed that the cross in which P. polionotus is maternal succeeds much less frequently than when P. maniculatus is used as the female. These results were subsequently confirmed for other races by Liu (1953a). Liu (1953b) further showed that much of the difference in fertility in the reciprocal crosses was due to pronounced fetal mortality after midgestation in the female P. polionotus x male P. maniculatus cross. This was sometimes accompanied by maternal mortality. Watson (1942), Blair (1944), and other workers demonstrated that F1 hybrids from the interspecific cross were fertile, although Blair and Howard (1944) using the races P. maniculatus blandus and P. polionotus leucocephalus noted partial male hybrid sterility. Watson (1942) reported that F2 and backcross generations were also successfully produced.

Journal ArticleDOI
TL;DR: It is hoped that the diversifying (disruptive) selection, combined with migration between the experimental populations of Drosophila, may to some extent serve as a model of certain genetic processes which take place also in human populations having certain types of social structures.
Abstract: Hirsch and his students (Hirsch and Erlenmeyer-Kimling, 1961; ErlenmeyerKimling, et al., 1962; and other publications) have shown that Drosophila melanogaster responds readily to selection for positive and for negative geotactic behavior. So does D. pseudoobscura (Dobzhansky and Spassky, 1962). In both species, the geotactic behavior is under polygenic control. Also in both species, the genetic basis of the geotaxis displays the phenomenon called by Lerner (19 S4) genetic homeostasis; when the selection is relaxed, the selected strains relapse partly towards the original, preselection states. The loss of the selection gains is even more rapid if the direction of the selection is reversed. Using the apparatus devised by Hirsch (a classification maze), the geotactic behavior becomes a trait lending itself readily and conveniently to selection and other genetic experiments. After the exploratory experiments referred to above, Dobzhanskyand Spassky have initiated (in 1962) a study of the effects of selection for positive, or for negative, geotaxis in pairs of populations of very unequal size (one population 10 times larger than the other) , which in every generation exchange a fixed number of migrants. It is hoped that the diversifying (disruptive) selection, combined with migration between the experimental populations of Drosophila, may to some extent serve as a model of certain genetic processes which take place also in human populations having certain types of social structures. Be that as it may, these experiments have yielded some completely unanticipated adventitious results

Journal ArticleDOI
TL;DR: Wahlund (1928) has shown that in a population divided into many small subpopulations, within each of which random mating is made, the frequency of heterozygotes for a locus with two alleles decreases by the amount equal to twice the variance of gene frequency amongSubpopulations compared with that expected in a single random mating population.
Abstract: Wahlund (1928) has shown that in a population divided into many small subpopulations, within each of which random mating is made, the frequency of heterozygotes for a locus with two alleles decreases by the amount equal to twice the variance of gene frequency among subpopulations compared with that expected in a single random mating population, while the frequency of homozygotes increases by the same amount. If, however, there are more than two alleles, this simple rule does not hold in the strict sense. Further, if multiple alleles are present, the differentiation of subpopulations may be investigated in terms of the covariation as well as the variation of gene frequencies.

Journal ArticleDOI
TL;DR: The authors believe that Antarctica during the Mesozoic and early Tertiary was an austral center of evolution and radiation, or at least an important route of migration, and that the groups displaying transantarctic relationships are the recent members of a once more extensive austral biota which has been broken up by geological and climatic causes.
Abstract: Among the problems raised by the distribution of plants and animals in the south temperate zone, there is none which takes a more central position and is more stimulating to the imagination than the problem of transantarctic relationships. We have before us the broken circle of southern lands-southern South America, South Africa, Tasmania-Australia, New Zealandseparated by wide stretches of ocean, but populated by a biota containing numerous groups whose disjunct elements are more closely related to one another than to any other group. In the center of the scene we are faced with the dormant Antarctic Continent, hiding its secrets beneath a mighty ice cap. It is, indeed, easily understandable that distribution patterns displaying such immense disjunctions have been interesting to scientists and that the question has been raised as to the biogeographic and evolutionary role played by the Antarctic Continent. More than 110 years have passed since the eminent botanist Sir Joseph Hooker outlined that great problem. Surveying the comprehensive literature which has appeared on the subject after Hooker, we can state that the answer to the problem has been searched for along very different lines and that opinions still are surprisingly divergent. Many, especially among the botanists, agree in principle with Hooker in his earlier papers and accept dispersal over former land bridges between Antarctica and the surrounding southern lands, while others are firm believers in long distance dispersal and permanence of the main geographical features. There are even different opinions as to the age and origin of the discussed groups which are conceived by some as forming a very old "Antarctic element," coming from an Antarctic center of evolution. Opponents have supposed a northern origin and migration southward of the ancestors of the austral groups. Many present-day scientists, however, accept the confinement of numerous plant and animal groups to the circumantarctic land areas as indicating nothing more than that they are relicts of formerly cosmopolitan groups. A special camp among the believers in dispersal over land is formed by the old and new adherents of the theory of displacement of the once united Gondwana continents. This short survey may be sufficient to show how unsettled and confused the situation really is. Yet, that is how matters stand in respect to our interpretation of most of the problems of great disjunctions. It is, nevertheless, important to note that biogeographers like C. Skottsberg among the botanists and J. W. Evans among the entomologists, who base their conclusions on extensive fieldwork in the south, who really look upon the problems from the south, and who know the world flora and world fauna of their respective groups, have arrived at a conception of the problems which is essentially unitary. They believe that Antarctica during the Mesozoic and early Tertiary was an austral center of evolution and radiation, or at least an important route of migration, and that the groups displaying transantarctic relationships are the recent members of a once more extensive austral biota which, in the words of Hooker (1853), "has been broken up by geological and climatic causes." 1This work was supported partly by Swedish funds, and partly by grants from the Rockefeller Foundation (GA NSA 52104, GA BMR 58129) and the American Philosophical Society (No. 1650, Penrose Fund). 2 The paper was read on July 13, 1964, in the section "Geographical Distribution" at the XIIth International Congress of Entomology (London).

Journal ArticleDOI
TL;DR: It is perfectly true, as these authors have noted in somewhat different ways, that selection for generally favorable alleles will improve population fitness over what it would have been without such alleles, but it does not follow from the fact that a change will be good for a population, that it will happen.
Abstract: Five related terms are in current use to describe the effects of selection on a population: the load, cost, intensity, pressure, and coefficient of selection. The cost of selection (Haldane, 1957, 1960; MacArthur, 1962) is an aspect of the genetic load, and has been called the "substitutional load" by Kimura (1960). "Cost" and "load" represent (with the qualification on density-dependence below) the reduction in fitness of a population below the level it would attain if composed entirely of the optimum genotype or combination of genotypes in it at the time measured. This statistic, like most ranges, can be investigated experimentally, and estimates of it on the phenotypic level are given below for several characters of a horse. The load is the price a population pays for at least much of its genetic variance, and is presumably balanced on the average, through interspecific selection, by a greater probability of extinction (cf. Lewontin, 1961) with a smaller genetic variance. The complete redefinition of load by Dobzhansky and Spassky (1963) is unnecessary. To the extent that population size is determined by density-dependent factors (as by the carrying capacity of the environment in relation to the present population genome), some fertility reduction or pre-reproductive mortality is however necessary. Mortality therefore need not be considered a burden on a population of stable size if it does not exceed the proportion of deaths caused by density-dependent factors (and similarly for fertility reduction). The occurrence within a population of genotypes with differential sensitivity to density makes the estimation of this proportion more difficult but does not affect the general conclusion on the effect of density-dependence. I no longer believe, however (contrary to Van Valen, 1963d; Mayr, 1963; and Brues, 1964), that this kind of argument allows a large increase in the rate of genotypic evolution. It is perfectly true, as these authors have noted in somewhat different ways, that selection for generally favorable alleles will improve population fitness over what it would have been without such alleles. But it does not follow from the fact that a change will be good for a population, that it will happen. This statement is obviously true, as I realized earlier (Van Valen, 1963d), for selection produced by a deteriorating environment. If in such an environment 10 independent allelic changes will be necessary for survival in a few generations and all the now favorable alleles have an initial frequency of 1 0-3, even with no dominance an enormous initial population size will be necessary for a reasonable probability of any survival through the crisis for a species with relatively little density-dependent wasting of immatures. Otherwise the population will become extinct before it can adapt itself. For general adaptations, not related to a deteriorating environment, the result is similar although less obvious. Any density-dependent wastage will permit some selection that is not a burden, the amount varying with the species or population. When this is used up (even ignoring random deaths and fertility differentials), simultaneous selection at two or more recombining loci produces a mutual interference, this interference increasing at a greater than linear rate with the number of loci and the mean selection intensity and resulting from biological limits on increase of population size. This interference (between loci, not with species survival) exists even though all replaceIFor J. B. S. Haldane (1892-1964), one of the greatest geneticists of this century.

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TL;DR: The purpose of this paper is to present a unique example in Chaenactis (Compositae) in which the cytogenetical evidence indicates beyond reasonable doubt that an extant species of relatively mesic habitats, C. glabriuscula DC.
Abstract: Very similar species are of great interest to students of evolution because such species probably have evolved recently, making it likely that much of the evidence of their origin can be reconstructed. In very few instances of closely similar non-polyploid plant species is there evidence to suggest that one is the ancestor of another. A notable example is the cytological evidence for the origin of Crepis fuliginosa (n-= 3) from C. neglecta (n = 4) or its near ancestor (Tobgy, 1943). Similarly, Sherman (1946) showed that Crepis kotschyana (n = 4) has been derived from an ancestor like C. foetida (n 5). Another convincing example of an aneuploid species with a living diploid ancestor was described by Lewis and Roberts (1956) in their studies of Clarkia biloba (n= 8) and C. lingulata (n -9). These two species are so similar that they can only be distinguished by petal shape and chromosome number. Cytogenetic analysis showed that C. lingulata has an additional chromosome composed of parts of two chromosomes of the C. biloba genome. The presence of duplicated chromosome material in C. lingulata clearly establishes it as a derivative of C. biloba. Recently, Jackson (1962) described a case in Haplopappus that also suggests the origin of one diploid species from another. What was considered Haplopappus gracilis was found to consist of two extremely similar species, one with a chromosome number of n = 4, the other with n = 2. Cytological evidence indicates in this instance, as in Crepis, that aneuploid reduction is the most probable explanation for the difference in chromosome number, with the n = 2 species the derived one. The purpose of this paper is to present a unique example in Chaenactis (Compositae) in which the cytogenetical evidence indicates beyond reasonable doubt that an extant species of relatively mesic habitats, C. glabriuscula DC., has given rise independently to two similar desert species, C. fremontii Gray, and C. stevioides H. & A., by aneuploid reduction in chromosome number. Chaenactis comprises approximately 24 herbaceous species, all endemic to western North America. There are at least nine annual species, and four of these, C. glabriuscula, C. stevioides, C. fremontii, and C. xantiana form a closely related assemblage,2 judging from their similarity and frequent natural hybridization. The first three will be considered in detail in this paper. The yellow-flowered C. glabriuscula is the most variable, and some of its intergrading variants have been treated as separate species.3 Chaenactis stevioides and C. fremontii have white flowers, but are otherwise very similar morphologically to certain populations of C. glabriuscula. Despite their overall similarity, the gametic chromosome number of C. glabrius-

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TL;DR: There is an immense amount of convergence in the evolution of cave-dependent characteristics of cave Collembola, and the relationship between these behavioral adaptions and the morphological changes is unclear.
Abstract: The study of evolution has always presented an exciting challenge and a series of very serious problems. Caves furnish an environment in which most of these problems can be avoided. First, the arthropod fauna of the caves is such that a living record often remains of virtually every stage in any evolutionary process which has occurred. Jeannel almost 50 years ago first noted this and said that caves were filled with living fossils. Second, caves furnish us with what is a natural evolutionary laboratory in which the evolutionary pressures are at the same time more simple and more compelling than those that we see in most surface areas. Thus caves furnish an ideal region for the study of the processes of evolution. For the past six years I have spent some time examining the evolution of one group of cave arthropods, the Collembola. Although the Collembola form an important part of the soil fauna they are poorly known among non-specialists. Fig. 1 gives an idea of the overall structure of the group concerned in this study. These animals are generally quite small. None of the forms I shall discuss here exceeds five mm. in length. The unique furcula at the end of the abdomen can be used for jumping, if the animals are hard pressed. Collembola are widely varied in both form and habitat, and are found from Antarctica to Nova Zemlya. They are common in caves and often occur in large numbers. In an earlier work (Christiansen, 1961), I showed how the distinctions between cave-dependent and cave-independent characteristics made it easy to map the phylogenies of the various groups with considerable confidence. One of the conclusions of this study was that there is an immense amount of convergence in the evolution of cave-dependent characteristics of cave Collembola. The conclusions drawn from the phylogenetic studies were amply reinforced by the data of zoogeography and relative dominance of the different forms. I also pointed out varying levels of preadaption in new cave invaders and the probable relationship between the modification of the foot complex and the increasing adaptation for locomotion over wet surfaces and standing water. Some questions were left unanswered by this work. First, what is the exact nature of behavioral evolution? Second, what is the relationship between these behavioral adaptions and the morphological changes? It is entirely possible that the cave species which are more primitive in morphological terms had compensated for this by extensive behavioral evolution. The present paper is an attempt to further clarify these problems in the subfamily Entomobryinae, where the changes involved are most extensive.

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TL;DR: It will be shown that the subspecies described by Dobzhansky and Mather are, in fact, two reproductively completely isolated species, D. serrata and D. birchii, which are sympatric at several localities and belong to a third sibling species, fully isolated from the other two.
Abstract: Drosophila serrata Malloch (1927) is a species belonging to the melanogaster section of the subgenus Sophaphora and was originally described from a locality (Eidsvold) in Queensland, Australia. A redescription, in the form now accepted in Drosophila systematics, was made by Mather in 1955. Dobzhansky and Mather (1961) collected living specimens in eight localities of New South Wales and Queensland in Australia, and in New Guinea and New Britain. They distinguished two subspecies, the southern D. serrata serrata in New South Wales and most of Queensland, and the northern D. serrata birchii from Cairns, Queensland, to New Guinea and New Britain. The southern subspecies was described as having females with darker abdomens and males with two pairs of strong bristles in the genital arch. The northern group was characterized as having lighter females and three pairs of bristles in the male genitalia. More recently, some 15 strains were collected by H. L. Carson and M. Wasserman. The present paper reports the results obtained from the study of all the available strains. It will be shown that the subspecies described by Dobzhansky and Mather are, in fact, two reproductively completely isolated species, D. serrata and D. birchii, which are sympatric at several localities. A strain collected in a locality of New Guinea (Madang) belongs to a third sibling species, fully isolated from the other two, which I have named Drosophila dominicana. A description of this new species has been published elsewhere (Ayala, 1965a).

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TL;DR: The method employed in the present study is that of the paternal half-sib analysis utilizing the hierarchal, or "nested," analysis of variance, which enables one to obtain an estimate of the heritability relatively free of non-additive genetic variance, to obtain the non-heritable maternal effect, if any, and to make all measurements on one generation.
Abstract: The currently popular synthetic theory of evolution has been forged from data and concepts contributed by both the neontologist and the paleontologist. In general, however, the organisms and characters studied by the former have little in the way of an historical record, while the groups and organ-systems of paleontological interest have rather infrequently (e.g., Olson and Miller, 1958; Berry, 1964) been subjected to modern microevolutionary analyses. Change in the molar teeth of mammals is one of our best historical evidences of evolution. Although dental morphology has received considerable attention from students of Recent and fossil mammals, knowledge of the hereditary and environmental components of dental variation remains meager in spite of the potential value for studies of evolutionary mechanisms. Study of the variation in the dentition of extant rodents would seem to offer a particularly attractive opportunity to subject to genetical analysis characters which do possess a reasonable fossil record. Many, if not most, of the traits utilized by the systematist are of a quantitative, i.e., continuously varying, nature. In recent years a fundamental approach to such characters has been to partition their phenotypic variation into a number of causal categories (Falconer, 1960). The primary step is, of course, the identification of the relative contributions of the heredity and the environment. These, in turn, may be divided into a number of subclasses of causation. In a recent study (Bader and Lehmann, 1965) the hereditary and environmental components were estimated for three dental traits in the house mouse by appropriate comparisons of the phenotypic variation in wild, randombred, inbred, and hybrid populations. The present study is concerned primarily with the heritability of these characters. The heritability is the proportion of the total phenotypic variance due to genetic factors acting in an additive fashion, or it may be regarded as the reliability of the phenotype as an indicator of the breeding value of the individual and the population (Falconer, 1960). The total hereditary, or genotypic, variance includes both the additive and non-additive genetic fractions. Estimation of the heritability of a trait depends essentially upon the measurement of the degree of phenotypic resemblance among animals of known genetic relationship. The most commonly used methods are those of regression of offspring on one or both parents and fullor half-sib correlation. The relative merits of the various analyses are examined in detail by Falconer (1960, 1963). The method employed in the present study is that of the paternal half-sib analysis utilizing the hierarchal, or "nested," analysis of variance. This design enables one to obtain an estimate of the heritability relatively free of non-additive genetic variance, to obtain an estimate of the non-heritable maternal effect, if any, and to make all measurements on one generation. It is more efficient, in the statistical sense, than the regression analysis when the heritability is low (Robertson, 1959a).

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TL;DR: The statistical criticisms of Olson and Miller's method given by Bock (1960) are in my opinion correct but not sufficiently serious to invalidate any general conclusions so far published.
Abstract: The book of Olson and Miller (1958), despite certain inadequacies, is in my opinion a landmark in our understanding of the interrelations of the parts of organisms. It includes the first demonstration that functionally related characters tend to be more highly correlated than others, and is the first study of several other aspects of integration. Because studies in this field are continuing by several investigators, I wish to discuss some points bearing on the methodology and interpretation. No important new evidence on integration is given in the present note; the main purpose of this note is the presentation of a simple and adequate method for the study of integration. The statistical criticisms of Olson and Miller's method given by Bock (1960) are in my opinion correct but not sufficiently serious to invalidate any general conclusions so far published. One of these criticisms is that the estimated value of the correlation coefficient r should be used rather than its lower confidence limit. These two statistics have the same ranking with equal sample sizes, and with different sample sizes the confidence limits reflect this difference as well as the correlation. If it is desired to exclude non-significant correlations, this can be done by simply ignoring all correlations below some common level at which some are not significantly different from zero, although it should be noted that the region of non-significance applies to each correlation individually and not to all considered together, and that an unknown number of real correlations will also fall in this region. The other criticism given by Bock is that individual correlations are frequently not significantly different from each other and that this fact invalidates the choosing of one pair of characters for special treatment rather than some other pair. This defect probably has relatively little influence on the general patterns found, as noted by Miller and Olson (1960). Olson and Miller (1958) discarded certain correlations (by a reproducible method) in the formation of their groups of integrated characters, and used as primary groups only those in which all members were mutually correlated above some fixed level. I regard this as unfortunate, as they did (p. 147), because the discarded correlations are as real as those retained, sampling error will form or eliminate a moderate number of groups, and part of the information in the value of the correlations is lost. The reason they used this procedure is that the complex pattern of intersection among groups was thereby much simplified. An alternative treatment, using all correlations, is the following. Consider the matrix of estimated absolute correlations, Irl. Normalize each r by the z-transformation (Fisher, 1958), corrected if necessary for sample size. Find the pair of characters with the highest lzl These two characters will now be treated as a single character and the means of their signed z correlations with each other character are used as the correlations of this combined character. Repeat this process as long as desired, when combining the correlations of a group using the original signed z correlations rather than those of the group, until sufficiently large groups have been formed or sufficiently low correlations are present between the groups or any characters still separate. At any level the groups are the mutually exclusive ones that maximize the mean of all the correlations among the components of these groups (more precisely, that minimize the mean

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TL;DR: Outcrossing data is provided with which to compare the breeding systems in Clarkia exilis and C. unguiculata and to estimate the frequency of outcrossing in these two closely related species.
Abstract: Flower structure suggests that selfpollination would be expected in Clarkia exilis and cross-pollination would be expected in C. unguiculata. In the former species, pollen shedding anthers are clustered around the receptive stigma at anthesis. Nevertheless, the surprisingly high estimate of 44.S per cent outcrossing was obtained from one natural population (Vasek, 1964). In contrast, C. unguiculata has long, exserted styles and is markedly protandrous. One would, therefore, expect this species to be cross-fertilizing. The present paper describes an experiment to estimate the frequency of outcrossing in C. unguiculata. It thereby provides outcrossing data with which to compare the breeding systems in these two closely related species.


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TL;DR: It is a widely recognized fact that in an evolving lineage, unit characters evolve at different rates, and in order to bring the rates of change of unit characters to a common denominator, Haldane (1949) suggested measuring the proportional changes of characters.
Abstract: Rates of evolution of morphological characters are usually expressed as dimensional changes referred either to the absolute time scale or to some other scale correlative with time. For example, uniform sequences of sediments, where these can be shown to have accumulated continuously and at an approximately constant rate, may provide a time scale for evolutionary events. Applicability of such "absolute temporal" and "correlative temporal" scales has been discussed by Simpson (1953). The rate of change of a single character will not, of course, reflect an "overall" rate of evolution just as change in only one character cannot in general be used to measure the differences between two groups. Simultaneous consideration of all the discernible morphological characters on the other hand can hardly be regarded as practical, and the term "character complex" refers only to the characters actually considered in a study of a lineage. The fact of variation in the dimensions of the individual characters within a population and the fact that most of the morphological characters are intercorrelated to a greater or lesser degree have to be taken into account in order to obtain a measure of a character complex amenable to a statistical treatment. It is a widely recognized fact that in an evolving lineage, unit characters evolve at different rates. If the rates of change are expressed in the conventional units of length, weight, etc., per unit time, or per generation, the results would not be comparable one with another. In order to bring the rates of change of unit characters to a common denominator, Haldane (1949) suggested measuring the proportional changes of characters. Thus, when the proportional increase in the dimensions of a character is (1/X) (dX/dt), the mean value of the proportional change in X in time t is ln X2 ln X1