Conrad A. Istock

THE EVOLUTION OF COMPLEX LIFE CYCLE PHENOMENA: AN ECOLOGICAL PERSPECTIVE

It seems reasonable to expect ecological theory to supply clear statements about the dominant forces which influence sequences of evolutionary change. This idea proceeds simply from the recognition that natural selection is identical with the interactions between organisms or populations and their secular environments. Such interactions determine probabilities of survival and reproduction as well as population size. This paper contains a theoretical treatment of the ecological and evolutionary exigencies imposed by a complex life cycle. For the purposes of this paper a complex life cycle is one which involves passage through two or more ecologically distinct phases. This means that there does not exist any overlap among the factors which limit abundance in each phase. Each phase thus has its own set of predator-prey, competitive, physical-environmental, and resource interactions. The following notation (adapted from Cole, 1954) will be convenient in designating important junctures in the life history of any species which exhibits two ecologically distinct life cycle phases:

Bacillus sonorensis sp. nov., a close relative of Bacillus licheniformis, isolated from soil in the Sonoran Desert, Arizona.

Eight Bacillus strains isolated from Sonoran Desert soil were shown to belong to a previously unidentified species, for which the name Bacillus sonorensis sp. nov. is proposed. The type strain is strain L87-10T (= NRRL B-23154T). On the basis of phenotypic and genetic data, B. sonorensis is most closely related to Bacillus licheniformis. B. sonorensis can be distinguished from B. licheniformis by salt tolerance, pigmentation, multilocus enzyme electrophoresis, reassociation of genomic DNA and sequence differences in protein-coding genes and 16S rRNA.

Sexuality in a natural population of bacteria–Bacillus subtilis challenges the clonal paradigm

Reproduction by binary fission necessarily establishes a clonal genotypic structure in bacterial populations unless a high rate of genetic recombination opposes it. Several genetic properties were examined for a wild population of Bacillus subtilis in the Sonoran Desert of Arizona to assess the extent of recombination in a natural population. These properties included allozyme variation revealed by multilocus enzyme electrophoresis, phage and antibiotic resistance, and restriction fragment length polymorphism with Southern hybridization. Evidence of extensive genetic recombination was found along “with evidence of modest clonal structure. Recombination must be frequent relative to binary fission in this population. This mixed population structure provides broader options for bacterial evolution than would a purely clonal structure.

ECOLOGY AND EVOLUTION OF THE PITCHER-PLANT MOSQUITO. 2. THE SUBSTRUCTURE OF FITNESS

The relations between structural gene variation, chromosomal organization and polymorphism, life-history variation, and the ecology of natural selection remain unclear despite much study of natural populations. One might even claim, with Lewontin (1974), that these relations seem less clear after extensive recent analysis of electrophoretic and immuno-variants and the subsequent juxtaposition of neutralist and selectionist interpretations. The adaptive response of heritable variation in fitness characters under selection is fairly easily demonstrated (Falconer, 1960) . Part of this paper will show this for development time, a quantitative fitness trait, and for the tendency to diapause, a qualitative fitness trait, using laboratory populations of the pitcher-plant mosquito Wyeomyia smithii. Much accumulated experience with quantitative variation and adaptation in the selective breeding of plants and animals strengthens the conviction that variation in quantitative characters is the immediate raw material of most evolutionary change. From such observations, we may anticipate that natural selection acting on the life-history or fitness traits of a species in a given environment might be observed, interpreted, and even predicted with some accuracy (Falconer, 1960; Slatkin, 1970; Dingle, 1974). The contrasting experiences with molecular variation vs. quantitative variation in the study of selection and adaptation suggest that the genetic basis of selective differences may have a deep structure more complex than the additive, multiplicative, or linkage models now used. It is about the same thing to say that selective differences at the level of the structural genes are usually too small to be resolved with present techniques, or so weak as to be overcome by genetic drift. The question of how the palpable and selectable fitness variations of quantitative genetics are produced remains! (In two cases, the red blood cell sickling of man and alcohol dehydrogenase variants in Drosophila melanogaster studied by Day et al., 1974, structural gene variants do correspond to phenotypic fitness variations.) Retreating from discrete structural genes as the basis of fitness variation and adaptation to discrete regulatory genes or gene arrangements mayor may not help to resolve the problem (Wilson et al., 1974a, b; Prager and Wilson, 1975). In the present setting of population biology the predominantly ecological and biometrical approach to the study of fitness complements the molecular approach. The former starts with firm information about heritable phenotypic variation in fitness characters and responses to known selection regimes. Such work explores the shallow structure of fitness and establishes the framework within which the expression of genetic variation at lower levels of organization must emerge if it is to be part of selection and adaptation. We employ the life-history descriptors of fitness and its components familiar in ecology (survivorship, la;; births ma;; mean generation time, r ; age of first reproduction, a; age of last reproduction, w; net reproductive rate, Ro ; intrinsic rate of increase, r). These are quantitatively varying characters amenable to ecological and genetic analysis in at least some laboratory and field situations. Their mathematical

ECOLOGY AND EVOLUTION OF THE PITCHER-PLANT MOSQUITO: 1. POPULATION DYNAMICS AND LABORATORY RESPONSES TO FOOD AND POPULATION DENSITY

It is commonplace to emphasize that ecological and evolutionary processes are related and ought to be studied together, but analyses of natural populations which unravel the interrelations of historical evolutionary factors, current population processes, and the prevailing effects of natural selection are not common. This is the direction of our studies of the pitcher-plant mosquito, Wyeomyia smithii. In this paper we present: the natural history and population dynamics of this species in a single bog; an experiment to determine if the larval part of the population saturates its resources at a critical time preceding metamorphosis; and studies of laboratory responses of the species to food and population density. The field data show that larval abundance is more tightly regulated (i.e., steadier) than the abundance of other parts of the life cycle. Our laboratory data show how the components of fitness are influenced by different food levels and larval density. Central in these studies is completion of the life cycle. We are trying to understand the factors which encourage or discourage completion of the life cycle and influence the average fitness of individuals under different ecological circumstances. Study of W. smithii was undertaken for a variety of conceptual and practical reasons. The ecological and evolutionary consequences of a complex life cycle (two or more distinct ecological stages) are of some interest (Istock, 1967, 1970). The species

Population dynamics of bacteriophage and Bacillus subtilis in soil

The dynamics of the interaction of populations of Bacillus subtilis and a temperate bacteriophage in soil microorganisms were examined. The purpose of the study was to investigate these dynamics in a structured habitat approaching that of their natural habitat in soil, in contrast to previous investigations in broth or chemostats. We addressed three main questions. What are the population dynamics of a phage-bacteria interaction in soil? What role might the heterogeneity of the soil play in shaping the interaction? Are the dynamics controlled more by the population biology of the phage or of the bacteria? The phage used was isolated from Arizona desert soil in which B. subtilis is common. Phage and bacteria were grown separately or together in sterile soil microcosms consisting of autoclaved soil rich in organic material. Densities of phage and bacteria were estimated through repeated sterile sampling of the microcosms by spreading precise dilutions of sampled soil suspensions onto plates of microbiological media. The plates for estimation of phage densities contained in addition a lawn of host bacteria. While the principal focus is on temperate phage ecology, the dynamics of a single example of interaction between B. subtilis and virulent phage is also presented. In all cases an initial epidemic of phage occurred, followed by stable equilibria lasting weeks to months. A threshold host density for phage outbreak occurs at 5 x 106 colony-forming units per gram of soil. At equilibrium the phage, both temperate and virulent, were much less abundant than the bacteria. The temperate phage did not depress the equilibrium host density, while the virulent phage lowered it by a factor often. The acidic soil of these experiments caused rapid and permanent inactivation of free phage, making the continuous interaction of phage and host essential for persistence of phage. Low levels of phage resistance (superimmunity or genetic) were typical of host populations. Most properties of the interaction between B. subtilis and phage in soil are quite different from those observed with chemostat populations of Escherichia coli and virulent phages. Potential doubling times and rates of increase in soil for phage and B. subtilis were cal- culated. At equilibrium, soil slows the interaction of phage and host relative to population growth in broth culture, and possibly also makes it heterogeneous in space and time. The life-history features of B. subtilis and temperate phage are considerably more complex than those of non-sporeforming bacteria and virulent phage. The biotic relationship of temperate phage and host may be distinct from predation or parasitism. Patterns in the ecology and evolution of temperate and virulent phage were explored, leading to the expectation that temperate forms will be more common among the phages of soil bacteria.

FINE-SCALE GENETIC AND PHENOTYPIC STRUCTURE IN NATURAL POPULATIONS OF BACILLUS SUBTILIS AND BACILLUS LICHENIFORMIS: IMPLICATIONS FOR BACTERIAL EVOLUTION AND SPECIATION

The genetic and phenotypic structure of sympatric populations of wild bacteria traditionally identified as Bacillus subtilis and B. licheniformis was analyzed. Small soil samples were taken from a single, tiny site in the Sonoran Desert of Arizona, USA, to provide a true population analysis, in contrast to many analyses of genetic structure using bacterial strain collections of widely heterogeneous origin. Genetic analyses of isolates used multilocus enzyme electrophoresis, mismatches in restriction fragment length polymorphism, and variants from Southern hybridization with B. subtilis DNA probes. Phenotypic analyses of isolates used the API test system for detection of growth and acid production on specific carbon sources. The two species were distinct both phenotypically and genetically, despite their known potential for genetic exchange in laboratory experiments. Genic and genotypic diversity were high in both species, and only 16% of observed allozyme variants might possibly be common to both species. Hence, there is probably modest genetic exchange, if any, between the species in nature. Clear hierarchies of population‐genetic structure were found for both species. Different types of genetic data yield concordant population structures for B. subtilis. For both species, two‐locus and multilocus statistical analyses of linkage demonstrated modest to strong disequilibrium at the species level but truly panmictic subunits within each species. The evidence for extensive genetic recombination within these fine‐scale subdivisions is unequivocal, indicating that the sexuality of these bacteria can be well expressed in nature. The relation of these results to processes of bacterial evolution and speciation is discussed.

ECOLOGY AND EVOLUTION OF THE PITCHER-PLANT MOSQUITO. 3. RESOURCE TRACKING BY A NATURAL POPULATION

If a population exploits or tracks a limiting resource perfectly through time, it will be constantly under density-dependent population regulation and density-dependent natural selection. Such complete resource use underlies many theoretical formulations in ecology and population genetics, particularly the so-called logistic models (Roughgarden, 1971; Charlesworth, 1971; MacArthur, 1972; May, 1974). Often the limiting resource is food, which is equivalent to energy. Occasionally space is the limiting resource, but for plants, microbes, or sessile animals space limitation may be equivalent to energy limitation. Perfect density-dependence does not occur when predators, parasites, pathogens, demographic lags, resource fluctuations or abiotic factors rarify a population relative to its resources leaving no resource in absolute short supply. In discussions of natural selection, density-dependence is usually about the same as K-selection and density-independence is the same as r-selection, where K and r are the usual terms of the logistic equation (MacArthur and Wilson, 1967). These dichotomies of ecological and evolutionary theory are too simple. First of all, at a theoretical level, K and r are not independent. The solution of the logistic equation for either must include the other. Hence, at population densities between zero and some maximum, there is an interrelation of K and r through which we try to describe selection in such models (King and Anderson, 1971). Second, selection always operates through birth, developmental, and death events which are the components of r, hence selection for any adaptation, K-related or not, proceeds by the same mechanism as selection for rrelated adaptations. What varies is the degree to which realized r is positive or negative. The selection occurring when r is negative for substantial periods may be formally equivalent to the selection occurring when r is positive (Istock, 1970). We are left not with a simple dichotomy between density-independence and densitydependence, but with the more cumbersome continuum between them. Since many aspects of an individuals ecology can be influenced by the activities of other co-occurring, conspecific individuals (mating, nesting, cannibalism, group, foraging, etc.) even when the population is well below K, we may assert that selection is always densitydependent to some extent. What we really want to assess is the degree of density-dependence. One way to do this, though not a perfect one, is to measure how close a population is to K at any given time. Measurement of the closeness of a natural population to its K may reveal imperfect resource tracking. Experimental results presented in this paper will show highly imperfect tracking in a natural population of the pitcher-plant mosquito Wyeomnyia smithii. Small deviations from K may not seriously challenge the assumptions or even the realism of logistic models (MacArthur, 1972). Major deviations such as recorded here do. If it turns out that highly imperfect resource tracking is widespread, many of our models of intraand interspecific competition, predation, community organization and natural selection may require revision. Imagine the following specialized and oversimplified, though probably not uncom-

Population Characteristics of a Species Ensemble of Waterboatmen (Corixidae)

The spatial and temporal distributions of 12 species of waterboatmen in a 1.2 ha pond in Northern Michigan were studied for three consecutive summers. Enclosure experiments involving the growth of one, two, and three corixid species were performed to test for interspecies competitive interactions and to determine whether the dominant species reach the upper limits to population growth. The experimental data indicate that interspecific competition between the two dominant species, Hesperocorixa lobata and Sigara macropala, in enclosures led to a significant reduction in the population size of H. lobata but not of S. macropala. From the enclosure experiments it appears that lobata failed to grow to its maximum limit because of competition, while macropala did reach its limit, at least in late summer. The experiments also show that the early summer and late summer breeding habits of Hesperocorixa lobata and Sigara macropala respectively, are not physiologically (genetically) fixed properties of these species...

Bacterial species and evolution: Theoretical and practical perspectives

A discussion of the species problem in modern evolutionary biology serves as the point of departure for an exploration of how the basic science aspects of this problem relate to efforts to map bacterial diversity for practical pursuits—for prospecting among the bacteria for useful genes and gene-products. Out of a confusing array of species concepts, the Cohesion Species Concept seems the most appropriate and useful for analyzing bacterial diversity. Techniques of allozyme analysis and DNA fingerprinting can be used to put this concept into practice to map bacterial genetic diversity, though the concept requires minor modification to encompass cases of complete asexuality. Examples from studies of phenetically definedBacillus species provide very partial maps of genetic population structure. A major conclusion is that such maps frequently reveal deep genetic subdivision within the phenetically defined specles; divisions that in some cases are clearly distinct genetic species. Knowledge of such subdivisions is bound to make prospecting within bacterial diversity more effective. Under the general concept of genetic cohesion a hypothetical framework for thinking about the full range of species conditions that might exist among bacteria is developed and the consequences of each such model for species delineation, and species identification are discussed. Modes of bacterial evolution, and a theory of bacterial speciation with and without genetic recombination, are examined. The essay concludes with thoughts about prospects for very extensive mapping of bacterial diversity in the service of future efforts to find useful products. In this context, evolutionary biology becomes the handmaiden of important industrial activities. A few examples of past success in commercializing bacterial gene-products from species ofBacillus and a few other bacteria are reviewed.