Showing papers in "The American Naturalist in 1961"

The Paradox of the Plankton

Abstract: The problem that I wish to discuss in the present contribution is raised by the very paradoxical situation of the plankton, particularly the phytoplankton, of relatively large bodies of water. We know from laboratory experiments conducted by many workers over a long period of time (summary in Provasoli and Pintner, 1960) that most members of the phytoplankton are phototrophs, able to reproduce and build up populations in inorganic media containing a source of CO2, inorganic nitrogen, sulphur, and phosphorus compounds and a considerable number of other elements (Na, K, Mg, Ca, Si, Fe, Mn, B, C1, Cu, Zn, Mo, Co and V) most of which are required in small concentrations and not all of which are known to be required by all groups. In addition, a number of species are known which require one or more vitamins, namely thiamin, the cobalamines (B or related compounds), or biotin. The problem that is presented by the phytoplankton is essentially how it is possible for a number of species to coexist in a relatively isotropic or unstructured environment all competing for the same sorts of materials. The problem is particularly acute because there is adequate evidence from enrichment experiments that natural waters, at least in the summer, present an environment of striking nutrient deficiency, so that competition is likely to be extremely severe. According to the principle of competitive exclusion (Hardin, 1960) known by many names and developed over a long period of time by many investigators (see Rand, 1952; Udvardy, 1959; and Hardin, 1960, for historic reviews), we should expect that one species alone would outcompete all the others so that in a final equilibrium situation the assemblage would reduce to a population of a single species. The principle of competitive exclusion has recently been under attack from a number of quarters. Since the principle can be deduced mathematically from a relatively simple series of postulates, which with the ordinary postulates of mathematics can be regarded as forming an axiom system, it follows that if the objections to the principle in any cases are valid, some or all the biological axioms introduced are in these cases incorrect. Most objections to the principle appear to imply the belief that equilibrium under a given set of environmental conditions is never in practice obtained. Since the deduction of the principle implies an equilibrium system, if such sys-

Animal Population Regulation by the Genetic Feed-Back Mechanism

Abstract: That a genetic feed-back mechanism functions to regulate populations of herbivores, parasites, and predators is supported by evidence from the biomathematics of population dynamics and studies of natural populations. The mechanism functions as a feed-back system through the dynamics of density pressure, selective pressure, and genetic changes in interacting populations. In a herbivore-plant system, animal density influences selective pressure on plants; this selection influences genetic make-up of plant; and in turn, the genetic make-up of plant influences animal density. The actions and reactions of interacting populations in the food chain cycling in the genetic feed-back mechanism result in the evolution and regulation of animal populations.

Some Parallels Between Gene Control Systems in Maize and in Bacteria

Abstract: In this article, McClintock attempted to link the regulation of controlling elements to observations made by Francois Jacob, Jacques Monod, and Andre Lwoff on bacterial operons in 1960. In part, McClintock was trying to establish her preeminence in the study of gene regulation, but she also wanted to demonstrate that regulation was widespread among organisms. Many of the parallels she observed were not universally accepted and McClintock backed off most of her claims by 1968.

On the Causes of Tropical Species Diversity: Niche Overlap

Abstract: It is suggested that the major factor causing the tropical increase in numbers of bird species is neither increased complexity of habitat nor, solely, increased specialization, but an increase in the similarity of coexisting species, reflected in a reduced "character displacement." This implies a reduction in the size of the exclusive portion of the species niche and is thus in accord with some of the more general predictions we have previously made on the relation between niche size and climatic stability.

Experimental Sympatric Populations of Clarkia

Abstract: Clarkia lingulata is a tetrasomic derivative of C. biloba. It occurs locally at the southern limit of distribution of the parental species. They cross readily but the hybrids are essentially sterile. When first studied, they were found to occur in adjacent but not in mixed colonies. Since then, C. lingulata has extended its range and has become established at the margin of an adjacent colony of C. biloba. Experimental sympatric populations were established by sowing a mixture of the two species in selected sites south of their natural area of distribution. Populations in the driest sites died out rapidly and showed little or no reproduction; in other sites reproduction has been evident and the populations still persist after five generations. In these artificial populations, C. lingulata has consistently matured about two weeks earlier than C. biloba. This suggests that C. lingulata is adapted to a shorter growing season than C. biloba, and may explain the occurrence of C. lingulata at the margin of distr...

Population effects of natural selection

Abstract: This paper has two themes. The first is that the traditional approach to population ecology, via the fluctuation and interaction of species populations, is about the hardest possible. The second is that where natural selection proceeds along an unambiguous course, it controls population processes in a simple, readily understandable, way. The remarkably precise convergence of unrelated forms whose activities are similar shows how nice is the control exercised by selection over phenotype. The central difficulty in understanding populations as an outcome of natural selection arises from the fact that the good of the species is not always equivalent to the good of the individual genotype, and conversely. There is plenty of direct evidence that more fit genotypes replace less fit, and there is plenty of circumstantial evidence (mostly from the fossil record) that more fit species replace less fit. But the relative magnitudes of these processes and the equilibrium they reach are unknown and hence controversial. Only when what is good for the species coincides with what is good for the genotype is it easy to understand the population effects of natural selection. In what follows a collection of examples is presented, in no particular order, and with no pretense of reviewing the literature. The very nature of the predictions made requires that we consider fairly orderly populations. It seems much harder to make interesting predictions about the more erratic populations which are more appropriately treated by the methods of Andrewartha and Birch (1954). See also Birch (1960).

X-Ray-Induced Chromosome Aberrations and Reproductive Death in Mammalian Cells

Abstract: The kinetics of x-ray-induced chromosome aberrations in mammalian cells were found to be similar to that in plants. In particuiar, the shapes of dose curves for oneand two-break aberrations are similar in both types of material, as are the distributions per cell of the various types of aberration. It may be concluded that there is no threshold dose for aberrations and random samples of damage are obtained from cells sampled at the first postirradiation metaphase. The dose required to produce one break per cell in irradiated human diploid epithelioid cells in vitro was calonlated to be about 190 r. This dose is similar to the dose required to produce one break per cell in mammalian cells in vivo, but much higher than the value for flbroblasts in vitro. Studies of the x-ray survival genetics of a hypotetraploid human cell line similar to that used in other studies have shown that the survival of these cells better fits a compound curve containing both single- and multiple-hit oomponents than a simple sigmoidal curve. Data on the numbers of cells not having x-ray-induced 2hit chromosome aberrations show that these cells do not fit the quantitative pattern required by the hypothesis that x-ray-induced cellmore » killing is caused mainly by the production of visible 2-hdt chromosome aberrations. Although it is reasonable to assume that these aberrations cause a proportion of such killing, their relation to killing is obsource, and certainly other targets, beth oytoplasrnic and nuclear, should be taken into consideration. 31 references, (auth)« less

The edge effect in population genetics

Abstract: Many animal species have been observed in which populations living near the edge of the range differ from those more centrally placed. Sometimes different peripheral populations resemble each other more closely than they do the central ones. Such a distribution must be due either to more rapid change in central than in peripheral localities, or vice versa. In discussion of particular instances each alternative has its adherents. It is suggested here that in two examples from the European fauna (bank voles of the genus Clethrionomys and the snail Cepaea nemoralis) the distinct edge populations are less likely to be relicts than recent developments. Consideration of the response of populations of different sizes and breeding structure to the environmental conditions in various parts of the range suggests that we shall not know whether this is usually so, or whether peripheral populations play an important part in evolution, until the effects of coadaptation and interpopulation migration are better understood.

Preliminary ideas for a predictive theory of ecology

Abstract: We can all define ecology as the study of the interaction between organisms and their environment. Can we also state the goal of that study? This talk is essentially a presentation of my image of the goal of ecology with some discussion of the possible distance to that goal. I hope that ecological research will eventually permit the production of a relatively simple program for a very large computer. This computer would be able to start with any desired set of descriptive information about an ecological community and from that information define the class of possible future evolutionary states that such a community might have. It should be general enough to start with a geologist's picture of a preliving world and evolve life of some sort and from that evolve some recognizable ecological community. By a recognizable community I mean something as similar to any chosen terrestrial community as the forest of the Galapagos is to the Siberian Tundra or to the bottom of Block Island Sound. It should be sufficiently precise so that given a description of pre-rabbit Australia, it would predict not only that rabbit introduction would be successful but that rabbit introduction in Australia would lead to decimation of English rabbits by a virus disease. I am not able to construct such a program but -I have some general ideas of how it might be constructed. In particular, I think I can describe the kinds of information that would be important in such a program. It might be noted that even if such a program proves uninteresting the attempt is of value since one measure of our comprehension of ecology is our ability to explain it in a clear unequivocal way to a student with absolutely no prior information about nature or science. An electronic computer is the only such student available. Constructing the program will involve three processes. We must decide on the units to be used. We must also decide on the boundary conditions that define the class of realistic models of ecological communities. Finally, the operational procedures must be decided on. I will be concerned only with units and boundary conditions. The operational procedures will almost certainly be amplifications of the existing theories of population dynamics, population genetics and interspecific competition.

Effects of Immigration on the Evolution of Populations

Abstract: In each of two separate experiments, the responses of three lines of Drosophila melanogaster to selection for increased sternopleural chaeta number were studied for eight generations. In each experiment one line was isolated, a second received two unselected immigrants each generation, and a third received eight immigrants each generation. An additional unselected line was carried in each experiment to serve as a check and as a source of immigrants. In the first experiment, with selection maintained at the 90 per cent level, all three lines diverged significantly from the unselected line. Fertility decreased in the isolated line until the line went to extinction in the eighth generation. The line receiving two immigrants, which comprised one-fifth of the breeding population, showed little response to selection until the sixth generation. During the last three generations, the line made large gains and reached a significantly higher level than the isolated line. This change in response to selection probabl...

The Role of Chromosomal Translocations in Urodele Evolution and Speciation in the Light of Work on Grasshoppers

Abstract: Previous workers have interpreted multivalents present at meiosis in the spermatogenesis of hybrid Triturus as due to translocation heterozygosity. An alternative hypothesis according to which these individuals would be homozygous for reduplicated chromosome ends is put forward here.

Spontaneous Chromosome Aberrations in Human Tissue Culture Cells

Abstract: Spontaneous chromosomal aberrations may occur with a high frequency in human tissue culture cells, and the frequency increases with the age of the culture. In a series of experiments the frequency of aberrations found at anaphase and telophase increased from 3.6 per cent to 9.6 per cent as the culture aged over a period of three months. Most, or all, of the aberrations appear to be of the chromatid type, and must have originated at prophase of the nuclear cycle. There seems to be some correlation between the frequency of spontaneous chromosome aberrations and sensitivity to ionizing radiation in mammalian tissue culture cells.

Industrial melanism in north american moths

Abstract: (1) Within the last 100 years about 70 species of moth in Britain and Europe have developed melanic forms in and around industrial areas. These have increased in relative frequency through natural selection and in some areas more than 90 per cent of the population is now melanic. (2) Similar species have developed melanic forms in eastern North America. The first melanics appeared in and around industrial areas, such as New York City, Pittsburgh, Detroit, and Philadelphia. They have spread to other areas and may now comprise over 90 per cent of the population of some species.

Inbreeding, heterosis and information theory*

Abstract: Measurements of the ratio of hatched eggs to total in randomly breeding and inbred populations of Drosophila melanogaster show that the two differ strikingly and consistently. In the former the ratio is high and its variance low; in the latter the situation is reversed. The inbred embryo has a low and variable probability of developmental success even in a very constant environment. Recent attempts to apply information theory and cybernetics to biological phenomena suggest a new way of looking at these facts employing the concepts of redundancy, noise and equivocation with respect to developmental efficiency. Biological information is passed from one generation to another coded in the genetic material. It is generally agreed that DNA plays a major role in this coding and there is very good evidence that very slight changes in nucleotide sequences can produce phenotypic change. But there is also excellent evidence that the number of possible permutations of the DNA is far, far greater than the minimum nece...

Meiotic Drive in Natural Populations of Drosophila melanogaster. VI. A Preliminary Report on the Presence of Segregation-Distortion in a Baja California Population

Abstract: The phenomenon of segregation-distortion, originally discovered in Madison, Wisconsin, has now been detected in a natural population of Drosophila melanogaster from Baja California. It has been demonstrated that the second chromosome responsible for the aberrant segregation is an SD-bearing chromosome, because it shares the following properties with the Madison SD: (1) It is expressed in the male sex only. (2) It is inhibited by the Curly inversion. (3) The Baja California SD chromosome is not sensitive to the action of the Madison SD chromosome. (4) It is located near the centromere (closely linked to cn). This finding suggests that SD is not of recent origin.

Life Cycle and the Expression of Heterosis in Inversion Heterozygotes in Drosophila funebris and Drosophila pavani

Abstract: In three stocks of D. funebris and in two stocks of D. pavani it has been demonstrated that, in general, there is a higher frequency of inversion heterozygotes among adult flies than among larvae. Nevertheless, there exist some differences according to the species, the stock, or the chromosome concerned. In D. funebris, 100 day old flies contain a greater frequency of heterozygotes than "young" ten day old flies. In D. pavani, the incidence of second chromosome inversion heterozygotes is about the same in "young" and in "old" flies, but is significantly higher in these than in larvae. In this same species, the frequency of fourth chromosome inversion heterozygotes is about the same in larvae and in young adults, but, at least in one stock, it is significantly higher in 100 day old flies. The general conclusion which may be drawn is that there is differential mortality favoring the inversion heterozygotes. The selective pressures which confer a higher fitness on these heterozygotes seem to act both at the ...

The Contribution of Heterozygosity at Certain Gene Loci to Fitness of Laboratory Populations of Drosophila melanogaster

Abstract: Comparison of the fitness of three types of populations of Drosophila melanogaster has been made. These are (1) inbred monomorphic controls, (2) heterotic polymorphic populations, and (3) monomorphic populations reselected from the polymorphic ones. These reselected populations have the genetic background of the polymorphic populations but the phenotypes of the monomorphic controls. Fitness was compared by measuring the weights and numerical sizes of the populations when maintained under controlled environmental conditions where natural selection operates. The reselected populations have been found to closely parallel the original heterotic polymorphic populations in fitness. Their performance is only slightly below these heterotic populations but very significantly above the inbred monomorphic controls. These data indicate that the heterosis found originally by Carson in the polymorphic populations is largely the result of heterozygosity for unknown genetic factors, perhaps polygene blocks, and only due ...

Why Do Gull Chicks Peck at Visually Contrasting Spots? A Suggestion Concerning Social Learning of Food-Discrimination

Abstract: Young precocial birds often peck incessantly at small, visually contrastspots about them. This behavior enables domestic chicks (Gallus gallus) to discover seeds and other bits of food which contrast with the ground. The selective advantage of such a predilection in gull chicks (Larus spp.) is less obvious, since these chicks feed on relatively large pieces of regurgitated food held or dropped by the parent. Yet, it has been shown experimentally that gull chicks of at least three species do have a preference for pecking at contrasting spots: Herring Gull, L. argentatus (Tinbergen and Perdeck, 1950); Black-headed Gull, L. ridibundus (Weidmann and Weidmann, 1958; Weidmann, 1959); and Laughing Gull, L. atribilla (personal observation). In Laughing Gulls, for example, all of the nearly three-dozen chicks which I hand-reared in 1960 pecked repeatedly at contrasting spots (such as the black dot of the letter "i" and the white center of the letter "o" in headline-sized print) on the newspapers which lined their boxes. Goethe (1937) and, later, Tinbergen (Tinbergen and Perdeck, 1950; Tinbergen, 1953) correlated the red terminal spot on the adult Herring Gull's yellow bill with the chick's feeding. By pecking at this contrasting spot, the young bird finds the food held in the parent's bill. But adults of both the Black-headed Gull and the Laughing Gull have uniformly colored red bills, leading Tinbergen (1958, p. 232) to conclude, "thus the stimulus situation to which the chicks respond best does not fit the natural situation." Some of my observations on hand-reared Laughing Gulls suggest a possible solution to this apparent paradox. During June and July, 1960, I raised 32 chicks from the egg for experiments on the motor pattern of pecking (which will be published separately). I noticed that, when two or more chicks were placed in the same box, they often pecked at one another. Although I have no quantitative data, the relative order of preferred targets for pecking was: spots on newsprint > sibling's bill tip > sibling's feet > other objects. Furthermore, I noticed that the chick's bill is generally dark horn color, except for a very light tip. Such a contrasting bill tip is evident in Herring Gull chicks as well (personal observation; Tinbergen, 1953, plates 23 a, 24 a and 25 b). If pecking at another chick's bill is somehow of selective advantage, then the chick's preference for a contrasting spot may be

Chromosome Breakage in Structural Heterozygotes

Abstract: Natural populations of many Dipteran species are polymorphic for series of structural arrangements differing by simple or complex inversions. Several investigators have been struck by the non-random distribution of such "natural" inversions over the genome. In Drosophila pseudoobscura, D. persimilis and D. athabasca (Dobzhansky, 1944; Novitski, 1946) the structural polymorphism exhibits a pronounced preference for one member of the chromosome set; moreover, the inversion break points are clustered in certain sections of this chromosome. In other types, like D. robusta (Carson, 1958) D. willistoni (da Cunha, Burla and EDobzhansky, 1950) D. subobscura (Goldschmidt, 1956; Kunze-Muihl and MWller, 1958) and some Chironomids (Beermann, 1955; Rothfels and Fairlie, 1957) and Simuliids (Basrur, 1959), the polymorphic variation involves most or all of the chromosomes, but again, certain portions of the map stand out as distinctly favored break regions. Over ten years ago Novitski (1946) discussed the mechanisms which might be responsible for this non-random localization of naturally occurring inversions. Since the early radiation experiments (Helfer, 1941) had failed to show any agreement between the distributions of induced and natural break points, Novitski hesitates to ascribe the clustering of the latter to the intrinsic break sensitivity of certain chromosome regions.' He gives only brief consideration to the role of natural selections, which might have picked a particular assortment of inversions out of a random choice. He proposes, instead, that the different break events occurring in any population may be causally related. According to this hypothesis the presence of any inversion in a structural heterozygote will-favor the production of similar inversions with break points closely adjacent to those of the first. The structurally heterozygous bivalent is assumed to present the loop configuration, which is so well known in salivary chromosomes. In such a loop the inversion break points are marked by shorter or longer asynaptic regions, which may result from the change of direction in the pairing association of the partners. Novitski proposes that the twisted asynaptic portions of the homologues are subject to mechanical stress and therefore tend to break. Moreover, two breaks which have arisen in this manner are likely to produce a new inversion on healing, because of the close proximity of the two "wounds" in the loop configuration.

A remote coincidence

Abstract: From a re-examination of Weinstein's forty-year old data on recombination along the total length of the X-chromosome in Drosophila melanogaster, it is concluded that the number of chiasmata is greatly restricted. Over 90 per cent of the recombination was attributable to the effects of single chiasma bivalents and less than 0.5 per cent to bivalents with triple chiasmata. Each type of bivalent produced chiasmata in rather definite modal positions. A single chiasma was usually located in the mid-region of the chromosome. When double or triple chiasmata were formed, the proximal one was located closer to the centromere, and the others were crowded toward the distal end of the chromosome. It is pointed out that this pattern of chiasma location may have rather far-reaching consequences on genetic theory and, in particular, on interpretations of interference, relative map lengths, compensatory crossing-over both between and within chromosomes, temperature effects, and the apparent complexity of pseudo-alleles.

An Unsuccessful Attempt to Reduce Recombination by Selection

Abstract: It is many years now since Detlefsen and Roberts (1921) found that the amount of recombination between two genes in Drosophila melanogaster could be reduced by appropriately selecting the offspring which were to give rise to the next generation. A similar experiment had been tried by Gowen (1919), but for only six generations and with little effect. Since then this obviously important aspect of recombination has been neglected. Parsons (1958) in a letter to this journal showed that he had been able to increase the amount of recombination between certain genes, but this type of selection cannot be so effective because it must lead eventually to an increase in the number of double crossovers. I therefore repeated the experiment, using genes on another chromosome, to see whether the striking reduction achieved by Detlefsen and Roberts in the amount of recombination could be reproduced. Detlefsen and Roberts used the genes w and m; in the course of ten generations they reduced the recombination frequency in one line from 27 per cent to 2 per cent, and in another line from 29 per cent to 6 per cent in 29 generations. Apart from the smaller number of matings used, I followed Detlefsen and Roberts' technique closely; they had an average of 40 and 61 matings per alternate generation in the two lines, I an average of 24, using instead a stock of cn vg and a wild-type stock formed by haphazardly mixing Florida 4 and Oregon K stocks about ?0 generations previously. The mixed stock was used in the hope of increasing the amount of variability which would be available for selection. In the experiment, each even-numbered generation was a mass-mating of non-crossover offspring from the selected pair in the previous generation. In the following list, the first number is the recombination percentage of the total population in each odd-numbered generation; the second number is the recombination percentage of the single pair in this generation which was selected to give rise to the next generation.

Rare Parthenogenesis in Drosophila robusta

Abstract: 1. By screening large quantities of unfertilized eggs laid by unmated females in the laboratory, 14 parthenogenetically-produced individuals of Drosophila robusta were found. All were females. It is suggested that they arose through automictic fusion of haploid products of meiosis in the unfertilized egg. 2. Each of the initial seven females came from a different wild strain, five from Nebraska and two from Missouri. The rate of parthenogenesis in these strains was .93 adult females per million unfertilized eggs laid. 3. Virgin females with one or more parthenogenetic females in their ancestry show a rate of parthenogenesis about 21/2 times that of the original strains. 4. A majority of the parthenogenetic females show reduced fertility, complete sterility and/or morphological abnormalities. 5. The evidence indicates that rare parthenogenesis such as is found in D. robusta, D. parthenogenetica, and D. polymorpha is genetically based and may serve as an example of an evolutionary stage through which the ob...

Crossing Over Within the Macromelanophore Gene in the Platyfish, Xiphophorus maculatus

Abstract: The platyfish, Xiphophorus maculatus, exhibits a high degree of pigmentary polymorphism, consisting in part of a series of patterns composed of large black pigment cells, called macromelanophores. The results from hundreds of crosses made in this and other laboratories have indicated that the appearance and position of these patterns on the body and fins are controlled by a series of dominant multiple alleles. These include Sd (spotteddorsal), Sr (striped-side), Sp (spotted-side), N (black-sided), and Sb (spottedbelly). They are located on the sex chromosomes. In this species, some races exhibit a heterogametic male (XX female, XY male) type of sexdetermining mechanism, while others exhibit a heterogametic female (WY female, YY male) type (Gordon, 1951). Two exceptions to this inheritance as classic alleles have been found. In a cross between a WY female and a YRSp YN male (R, for red color, is closely linked to Sp and N) of a \"domesticated\" strain, Gordon (1937) obtained a R female and concluded that N and Sp were not allelic; however, the possibility that the expression of Sd or N was surpressed could not be ruled out. Recently, another exception occurred when an XX female was crossed to a YSdySr male, resulting in 15 Sd males, 28 Sr males, and one SdSr male. The appearances of the Sd and Sr patterns were identical in the exceptional fish and its siblings. When it was crossed to an XspXsp female. ten Sp females and 15 SpSdSr males (triple dominants) were produced. Further crosses confirmed that the Sd and Sr factors were inherited as a unit. As Pontecorvo (1958) has pointed out, recent studies have shown that when a sufficiently detailed analysis is made, crossing over within a genic locus is found to occur. In this case, no test of complementarity can be made, since the factors controlling macromelanophore patterns are dominant. Following Pontecorvo's nomenclature, these factors may be described as \"sites of mutation, \" within a macromelanophore gene or as very closely linked genes. Semantics aside, this situation represents a grouping of elements separable by crossing over but closely related functionally.

X-Ray Induced Sex-Linked Recessive Lethals and Visibles in Sciara coprophila

Abstract: A study was made of the sex-linked recessive lethal and visible mutations induced by x-rays in the sperm and oocytes (first meiotic prophase) of S. coprophila. For sperm irradiated at 2000 r, 3000 r, and 4000 r the lethal rates obtained were 0.0136, 0.0303, and 0.0348 respectively. Sperm and oocytes irradiated at 4000 r gave approximately the same lethal rate, 0.0348 versus 0.0437. Two sex-linked recessives were recovered from the experiments on irradiated sperm (2/447 = 0.0044), an appreciable visible rate in view of the fact that only five sex-linked traits have been recovered heretofore in this species. On the basis of the experimental data the following interpretations have been made: (1) Sciara is not resistant to the mutagenic effects of x-rays. The low yield of visible mutations obtained repeatedly in this genus can be attributed to a number of factors, including the unusual mode of inheritance and sex determination found in these flies. (2) There is no correlation between the induction of chromoso...

A Developmental Study of Recessive Lethals from Wild Populations of Drosophila melanogaster

Abstract: Samples of Drosophila melanogaster from three populations of common origin were examined for their load of recessive lethals. Two of the samples were collected from wild populations in Berea, Kentucky, on successive years and a third sample was obtained from a laboratory population, initiated with flies from a wild population and maintained for an eight month period. These samples were tested for frequency of second chromosome recessive lethals, for allelism rate and for ontogenetic distribution of lethality of the lethal factors. The frequency of recessive lethals in the samples from wild populations in 1959 and 1960 were 31.4 per cent and 37.4 per cent, respectively, and that of the laboratory population was 23.4 per cent. The allelism rates for the lethal factors in the three populations were 0.9 per cent, 2.4 per cent and 3.1 per cent, respectively. The ontogenetic distributions of lethal action of the mutants in the three Berea samples, in an earlier sample from a Wisconsin wild population, and in a ...