Selection In Natural Populations Research Articles

Selection in Nature: Experimental Manipulations of Natural Populations

Numerous studies have documented evolution by natural selection in natural populations, but few are genuine selection experiments that are designed and then executed in nature. We will focus on these few cases to illustrate what can be learned from field selection experiments alone or field and laboratory selection experiments together that cannot be learned from laboratory experiments alone. Both types of study allow us to evaluate cause and effect relationships because a planned experiment can be accompanied by a more direct evaluation of the factors that cause evolution. A unique benefit of field experiments is that they give us the opportunity to measure the rate and magnitude of selection in nature. We have found that this rate is far greater than one might imagine based on observations of the fossil record. A combination of field and laboratory selection experiments has revealed the importance of population size and structure in shaping the genetics of adaptation. For example, laboratory selection experiments on insecticide resistance tend to attain resistance though polygenic inheritance. The evolution of insecticide resistance in nature often eventually yields to single genes of large effect that are rare but, once they arise, represent a higher fitness solution to resistance and spread among populations. Finally, field studies enable us to test evolutionary theory in a context in which all of the tradeoffs associated with a trait are realized; in the laboratory, organisms may be shielded from the fitness tradeoffs associated with the evolution of a trait. For example, we have compared the patterns of senescence in guppies from high and low mortality rate environments in the laboratory and in the field. In the laboratory, guppies from high predation environments had delayed senescence relative to those from low predation environments. In the field the apparent relationship is the opposite. One hypothesis for this difference is that a tradeoff associated with the evolution of the high predation life history is a decrease in the investment in the immune system. Such a sacrifice would be evident in nature where there is exposure to disease and parasites but less so in the laboratory, which is relatively disease and parasite free.

GENETIC DRIFT WITHIN A PROTECTED POLYMORPHISM: ENIGMATIC VARIATION IN COLOR-MORPH FREQUENCIES IN THE CANDY-STRIPE SPIDER, ENOPLOGNATHA OVATA

The candy-stripe spider, Enoplognatha ovata, exhibits a striking color polymorphism comprising three morphs. A number of lines of evidence strongly suggest that this polymorphism is maintained by natural selection: its presence in a sister species, E. latimana; the physical nature of the variation; the virtual lack of monomorphic populations; the highly consistent rank-order of morphs within populations; and the presence of large-scale clines associated with climatic variables. However, the absence of selection is equally strongly suggested by very local surveys of morph frequencies over space and time, perturbation experiments, and a variance in morph frequency between populations that is virtually independent of spatial scale. In addition, local spatial patterns in one study site (Nidderdale, Yorkshire, England) have been explained in terms of intermittent drift over half a century ago, a hypothesis supported here by the distributions of four other genetic markers (two allozyme and two visible polymorphisms). A heuristic model is suggested that reconciles these apparently contradictory messages regarding the importance of drift and selection in this system. It is proposed that when allele frequencies of the color morph redimita lie between approximately 0.05 and 0.3, the deltaq on q plot is very shallow, so that within this region, where the majority of populations lie, selection is weak and drift is the major force determining local morph frequencies. However, outside this range of frequencies, powerful selection acts to protect the polymorphism. This model may apply to polymorphisms in other species and explain why evidence of selection in natural populations is often elusive.

COMPARING STRENGTHS OF DIRECTIONAL SELECTION: HOW STRONG IS STRONG?

The fundamental equation in evolutionary quantitative genetics, the Lande equation, describes the response to directional selection as a product of the additive genetic variance and the selection gradient of trait value on relative fitness. Comparisons of both genetic variances and selection gradients across traits or populations require standardization, as both are scale dependent. The Lande equation can be standardized in two ways. Standardizing by the variance of the selected trait yields the response in units of standard deviation as the product of the heritability and the variance-standardized selection gradient. This standardization conflates selection and variation because the phenotypic variance is a function of the genetic variance. Alternatively, one can standardize the Lande equation using the trait mean, yielding the proportional response to selection as the product of the squared coefficient of additive genetic variance and the mean-standardized selection gradient. Mean-standardized selection gradients are particularly useful for summarizing the strength of selection because the mean-standardized gradient for fitness itself is one, a convenient benchmark for strong selection. We review published estimates of directional selection in natural populations using mean-standardized selection gradients. Only 38 published studies provided all the necessary information for calculation of mean-standardized gradients. The median absolute value of multivariate mean-standardized gradients shows that selection is on average 54% as strong as selection on fitness. Correcting for the upward bias introduced by taking absolute values lowers the median to 31%, still very strong selection. Such large estimates clearly cannot be representative of selection on all traits. Some possible sources of overestimation of the strength of selection include confounding environmental and genotypic effects on fitness, the use of fitness components as proxies for fitness, and biases in publication or choice of traits to study.

Interlocus nonrandom association of polymorphisms in Drosophila chemoreceptor genes.

Some forms of multilocus selection with epistasis, such as truncation selection, can effectively reduce the mutation load [Kondrashov, A. S. (1988) Nature 336, 435-440]. Many quantitative characters, including complex genetic diseases, are likely to be subject to these types of selection. However, direct measurement of selection in natural populations is difficult and the effect of epistasis on within-species variations remains unclear. Epistatic interaction in the fitness effect can generate linkage disequilibrium (LD). Therefore, we may detect the action of natural selection from its amount and pattern. Here, we report a large number of interlocus nonrandom associations between polymorphisms in 98 Drosophila chemoreceptor genes. LD was examined in two fly samples collected at the same location, but in different seasons. The amount of LD was much larger in the spring sample than in the autumn one. The between-sample difference was much more striking for the replacement polymorphisms than for the silent polymorphisms. This difference between the replacement and silent polymorphisms could not be attributed to differences in the mean marker distances. We also found a significant excess of associations between one frequent and one less common allele for the replacement polymorphisms, but not for the silent polymorphisms. It is unlikely that a simple seasonal bottleneck could explain all these differences in the scale of LD between the samples and between the replacement and silent polymorphisms. Natural selection is suggested to play a significant role in shaping the pattern of LD observed in this study.

An Adaptive Pattern of Inversion Polymorphisms in Trimerotropis Pallidipennis (Orthoptera)

Correlation of a trait with environmental factors is a common means of measuring natural selection in natural populations. If the same correlation is seen in independent groups of populations over a wide area, selection is then a likely explanation. Natural populations of T. pallidipennis (2n = 23, X0) — polymorphic for 3-7 pericentric inversions in 4 medium-sized pairs — show altitudinal clines for 9 chromosome sequences over a wide area in Argentina. Joint analysis has shown correlations with latitude and longitude. In order to identify the climatic variables responsible for this clinal variation, 7 populations sited on an altitudinal gradient were scored for inversion frequency. Multiple regression analyses among chromosome frequencies and environmental variables do not only confirm the consistency of altitudinal gradients for all sequences but also identify minimum temperature (Tmin) as the main climatical variable responsible for this pattern, to such extent that populations with extreme Tmin values are monomorphic. In addition, some sequences are correlated with humidity. These results do not only reinforce the hypothesis of a selective origin for cline maintenance but are also a new example of monomorphism associated to marginal environments. Three inversions (4A1, 7SM2, 8SM4) correlate simultaneously with altitude, latitude, and minimum temperature; two other inversions (6M and 8SM4) correlated with longitude and humidity, because this variable decreases westward in Argentina. The identification of the causes of clinal variation leads to a predictive pattern of inversion distribution for still unexplored regions.

Patterns of ethnic diversity among the genes that influence AIDS.

Although HIV-1/AIDS emerged late in the last century, more than 42 million individuals have been infected and 25 million have died worldwide, making AIDS, like malaria, a strong selective force for disease-associated genetic factors. Many of the genes that mediate immune response or that are co-opted by HIV-1 for completion of its lifecycle show differences in allele frequencies, as a result of drift, migration or selection. Here we show that the majority of AIDS candidate genes and AIDS restriction genes show significant differences in allele frequencies, possibly the result of historic selective pressures. These genes are undergoing present day natural selection in populations with high AIDS prevalence.

COMPARING STRENGTHS OF DIRECTIONAL SELECTION: HOW STRONG IS STRONG?

The fundamental equation in evolutionary quantitative genetics, the Lande equation, describes the response to directional selection as a product of the additive genetic variance and the selection gradient of trait value on relative fitness. Comparisons of both genetic variances and selection gradients across traits or populations require standardization, as both are scale dependent. The Lande equation can be standardized in two ways. Standardizing by the variance of the selected trait yields the response in units of standard deviation as the product of the heritability and the variance-standardized selection gradient. This standardization conflates selection and variation because the phenotypic variance is a function of the genetic variance. Alternatively, one can standardize the Lande equation using the trait mean, yielding the proportional response to selection as the product of the squared coefficient of additive genetic variance and the mean-standardized selection gradient. Mean-standardized selection gradients are particularly useful for summarizing the strength of selection because the mean-standardized gradient for fitness itself is one, a convenient benchmark for strong selection. We review published estimates of directional selection in natural populations using mean-standardized selection gradients. Only 38 published studies provided all the necessary information for calculation of mean-standardized gradients. The median absolute value of multivariate mean-standardized gradients shows that selection is on average 54% as strong as selection on fitness. Correcting for the upward bias introduced by taking absolute values lowers the median to 31%, still very strong selection. Such large estimates clearly cannot be representative of selection on all traits. Some possible sources of overestimation of the strength of selection include confounding environmental and genotypic effects on fitness, the use of fitness components as proxies for fitness, and biases in publication or choice of traits to study.

EXPERIMENTAL DEMONSTRATION OF A CAUSAL RELATIONSHIP BETWEEN HETEROGENEITY OF SELECTION AND GENETIC DIFFERENTIATION IN QUANTITATIVE TRAITS

Comparisons of estimates of genetic differentiation at molecular markers (FST) and at quantitative traits (QST) are a means of inferring the level and heterogeneity of selection in natural populations. However, such comparisons are questionable because they require that the influence of drift and selection on QST be detectable over possible background influences of environmental or nonadditive genetic effects on QST-values. Here we test this using an experimental evolution approach in metapopulations of Arabidopsis thaliana experiencing different levels of drift and selection heterogeneity. We estimated the intensity and heterogeneity of selection on morphological and phenological traits via selection differentials. We demonstrate that QST-values increased with increasing selection heterogeneity when genetic drift was limited. The effect of selection on QST was thus detectable despite significant genotype-by-environment interactions that most probably biased the estimates of genetic differentiation. Although they cannot be used as a direct validation of the conclusions of prior studies, our results strongly support both the relevance of QST as an estimator of genetic differentiation and the role of local selection in shaping the genetic differentiation of natural populations.

Measuring nonlinear selection.

A recent comprehensive review of empirical studies that measured the strength of selection concluded that there was little evidence for strong nonlinear selection in natural populations (Kingsolver et al. 2001). The median quadratic selection gradient identified by Kingsolver et al. (2001) was only 0.1, and gradients consistent with stabilizing or disruptive selection were found at a similar frequency and to be of similar magnitude. Stabilizing selection in particular is an important premise in many areas of evolutionary biology (Travis 1989), so this finding challenges our current understanding of how selection may operate in the wild (Conner 2001; Kingsolver et al. 2001). There is already some evidence that the strength and frequency of nonlinear selection identified in the review of Kingsolver et al. (2001) has influenced how evolutionary biologists view the potential role of stabilizing selection (Barton and Keightley 2002; Zhang et al. 2002). Kingsolver et al. (2001) suggest a number of reasons why empirical studies may not find or may underestimate nonlinear selection, including an empirical bias toward selecting experimental systems that are likely to show directional selection. In addition, nonlinear selection gradients are often able to be estimated from data sets but are not published (Kingsolver et al. 2001), probably as a consequence of the paucity of significant individual gradients and the difficulty of interpreting the overall pattern of nonlinear selection from the large number of estimated gradients. Here, we demonstrate that the strength of non-

VARIATION IN VIABILITY SELECTION AMONG COHORTS OF JUVENILE RED SQUIRRELS (TAMIASCIURUS HUDSONICUS)

Selection will result in observable changes in traits only if it acts consistently in space and time, but few estimates of selection in natural populations have been temporally replicated. Here we estimate viability selection on nestling growth rates for 13 cohorts (1989-2001) of red squirrels (Tamiasciurus hudsonicus) from a natural population located in southwestern Yukon, Canada. Directional selection on nestling growth rates varied in magnitude and direction from one cohort to the next. The magnitude of directional selection was relatively weak in most years (median beta' = 0.24), but there were episodes of very strong viability selection (beta' > 0.5) in some cohorts. We found no evidence of significant stabilizing or disruptive selection on this trait. Examination of viability selection episodes over shorter time periods suggested that the strength of selection on juveniles in this population was positively related to the time scale over which selection was measured. Viability selection from birth to emergence from the natal nest (50 days of age) and from emergence to successful recruitment (100 days of age) were positively correlated, but were both independent of selection on nestling growth rates from recruitment to potential breeding age (one year). The strength of directional selection on growth rates prior to recruitment was negatively correlated with spring temperature whereas selection from recruitment to breeding was positively correlated with the abundance of spruce cones produced in the previous fall. Episodes of strong directional selection from birth to breeding age appear to be due to potentially rare combinations of environmental conditions. As a result, predicting the occurrence of very strong episodes of selection will be extremely difficult, but predicting the microevolutionary responses to observed selection on individual cohorts remains feasible.

Waking the Sleeping Giant: The Evolutionary Foundations of Plant Function

As students of ecological and evolutionary physiology, we often accept the adaptive nature of morphological and functional traits despite little or no evidentiary connection between function and fitness, or we accept hypotheses of homology or covergence between functional traits without strong phylogenetic hypotheses of ancestry and trait evolution. For more than four decades, the fields of plant ecophysiology and plant functional biology have evolved on the basis of fundamental assumptions that relate aspects of carbon and water balance to plant fitness or that define the causes of functional plasticity, physiological niche differentiation, and plant competition. On conceptual grounds, strong connections must exist between plant performance and fitness as well as between plant function and ecological pattern; plant growth, for example, is directly correlated with plant size, and plant size is, in turn, often correlated with reproductive output and competitive performance. While the scaling from ecophysiology to ecosystem function has made enormous advances, the integration of functional ecology with population and evolutionary biology is incomplete. There are too few theoretical and empirical studies of fitness consequences, heritability, genetic architecture, and phylogenetic distribution for traits that may be defined as physiological or functional. This lack of evolutionary rigor in the testing of hypotheses related to plant function is not for want of an evolutionary foundation. An outstanding collection of seminal studies is available as the cornerstone for the construction of paradigms in physiological adaptation and evolutionary ecology. We recognize four major traditions in the study of plant functional evolution, each with several distinct lines of research. We have also attempted to identify important papers in each of these areas as applied to the evolution of plant function—papers that have initiated, raised the standards, and synthesized their respective fields of study (table 1). The demarcation between these lines of research are not all clear cut, and our selection of papers is necessarily subjective (with input from several colleagues); we hope that the result provides a useful introduction to past and current research in this area. 1. The adaptive value of functional traits, evidence for the action of selection in natural populations, and evolution of functional strategies: The application of optimality theory and game theory to plant function, particularly in the context of physiological trade-offs and the allocation of limited resources; Analysis of the adaptive value of individual traits and the fitness consequences of phenotypic variation in natural

Comment on "Parasites as a Viability Cost of Sexual Selection in Natural Populations of Mammals"

Comment on "Parasites as a Viability Cost of Sexual Selection in Natural Populations of Mammals"

Sperm transfer through forced matings and its evolutionary implications in natural guppy (Poecilia reticulata ) populations

In species in which individuals alternate between mating strategies, males may respond to elevated predation risk by switching from conspicuous courtship displays to less risky or more profitable sneaky mating attempts. As a consequence, in such species female choice is likely to be undermined more frequently in relatively dangerous localities. We tested this prediction using the guppy, a species of fish in which individual males alternate between courtship (solicited) and forced (unsolicited) copulations according to prevailing levels of predation. We collected females at late stages of gestation from four high- and four low-predation populations in Trinidad and examined them for the presence of sperm in their gonoducts. Due to the patterns of sperm storage in guppies, sperm found in the gonoducts of such late-cycle females can only arise from unsolicited copulations. We anticipated that because female guppies are subject to greater sexual harassment in the form of forced mating attempts in high-risk localities, a higher proportion of females in these populations would contain sperm in their gonoducts arising from recent unsolicited copulations. Contrary to this prediction, only one of the four paired comparisons (from the Quaré River) revealed a significant difference in the proportion of females recently inseminated through forced copulations. The paired comparisons for the remaining three rivers revealed no significant differences in the proportion of females with recoverable sperm in their gonoducts. However, overall, we found that 44.5% (±4.3 SE) of females had sperm in their gonoduct arising from sneaky mating, a figure three times higher than previously reported for this species. We discuss these findings in relation to recent predictions concerning the strength of sexual selection in natural populations.

COMMUNITY ECOLOGY AND THE GENETICS OF INTERACTING SPECIES

panding the synthesis of ecology and genetics. Ecology 84: 545-558. Odum, E. P. 1969. The strategy of ecosystem development. Science 164:262-270. Palumbi, S. R. 2001. Humans as the world's greatest evolutionary force. Science 293:1786-1790. Pimm, S. L. 1982. Food webs. Chapman and Hall, London, UK. Roughgarden, J. 2002. Evolution reduced to ecology: history of conflict and cooperation between disciplines. Ecological Society of America 2002 Annual Meeting Abstracts: 45. Sober, E. 1984. The nature of selection. MIT Press, Cambridge, Massachusetts, USA. Wade, M. J. 1978. A critical review of the models of group selection. Quarterly Review of Biology 53:101-104. Whitfield, J. 2002. Neutrality versus the niche. Nature 417: 480-481. Whitham, T G., W. P. Young, G. D. Martinsen, C. A. Gehring, J. A. Schweitzer, S. M. Shuster, G. M. Wimp, D. G. Fischer, J. K. Bailey, R. L. Lindroth, S. Woolbright, and C. R. Kuske. 2003. Community genetics: a consequence of the extended phenotype. Ecology 84:559-573. Wilbur, H. M. 1972. Competition, predation, and the structure of the Ambystoma-Rana sylvatica community. Ecology 53: 3-21. Williams, G. C. 1992. Natural selection: domains, levels, and challenges. Oxford University Press, Oxford, UK. Wilson, D. S. 1980. The natural selection of populations and communities. Benjamin/Cummings, Menlo Park, California, USA.

VARIATION IN VIABILITY SELECTION AMONG COHORTS OF JUVENILE RED SQUIRRELS (TAMIASCIURUS HUDSONICUS)

Selection will result in observable changes in traits only if it acts consistently in space and time, but few estimates of selection in natural populations have been temporally replicated. Here we estimate viability selection on nestling growth rates for 13 cohorts (1989–2001) of red squirrels (Tamiasciurus hudsonicus) from a natural population located in southwestern Yukon, Canada. Directional selection on nestling growth rates varied in magnitude and direction from one cohort to the next. The magnitude of directional selection was relatively weak in most years (median β′ = 0.24), but there were episodes of very strong viability selection (β′ > 0.5) in some cohorts. We found no evidence of significant stabilizing or disruptive selection on this trait. Examination of viability selection episodes over shorter time periods suggested that the strength of selection on juveniles in this population was positively related to the time scale over which selection was measured. Viability selection from birth to emergence from the natal nest (50 days of age) and from emergence to successful recruitment (100 days of age) were positively correlated, but were both independent of selection on nestling growth rates from recruitment to potential breeding age (one year). The strength of directional selection on growth rates prior to recruitment was negatively correlated with spring temperature whereas selection from recruitment to breeding was positively correlated with the abundance of spruce cones produced in the previous fall. Episodes of strong directional selection from birth to breeding age appear to be due to potentially rare combinations of environmental conditions. As a result, predicting the occurrence of very strong episodes of selection will be extremely difficult, but predicting the microevolutionary responses to observed selection on individual cohorts remains feasible.

Unapparent virus infection and host fitness in three weedy grass species

Summary The effects of pathogens on natural plant populations are poorly understood, in part because many cause few overt symptoms. I investigated the correlations between the incidence of barley yellow dwarf viruses (BYDVs) and total panicle length, a fitness proxy, in three apparently unaffected hosts, Bromus hordeaceus (Pooideae), Setaria viridis and Setaria lutescens (Panicoideae). Whereas B. hordeaceus individuals did not differ in fitness with respect to infection, average fitness of infected S. viridis was approximately half that of uninfected individuals, and in one out of two years, infected S. lutescens had approximately 25% higher fitness. Variability among hosts appears to contribute more than variability among BYDV serotypes to the pattern of infection outcomes. These correlations suggest that the relationship between BYDVs and these co‐occurring host species ranges from antagonism to mutualism. If so, BYDVs can act as agents of selection in natural populations in which the symptoms of infection are unapparent. The results also demonstrate that infections can be unapparent due to large trait variance rather than small mean effects. Because high trait variances are widespread in many natural populations, unapparent infections may be common in wild plant communities.

Parasites as a Viability Cost of Sexual Selection in Natural Populations of Mammals

Sexual selection in mammals has resulted in the evolution of sexual size dimorphism (SSD), with males usually being the larger sex. Comparative analyses indicate that the evolution of SSD is associated with the evolution of male-biased mortality, suggesting a possible causal link between the two. Here, we use a comparative approach to investigate the possible role of parasites in generating this relation. We show that there is a robust association between male-biased parasitism and the degree of sexual selection, as measured by mating system (monogamous or polygynous) and by the degree of SSD. There is also a positive correlation, across taxa, between male-biased mortality and male-biased parasitism. These results are consistent with the hypothesis that parasites contribute to the observed association between SSD and male-biased mortality.

DEMOGRAPHIC AND EVOLUTIONARY RESPONSES TO SELECTIVE HARVESTING IN POPULATIONS WITH DISCRETE GENERATIONS

Mathematical models are used to explore the evolutionary and demographic consequences of size-selective harvesting and natural selection in populations with discrete, nonoverlapping generations. We specify the harvesting intensity and size selectivity in terms of both the phenotypic criterion for harvesting and the demographic goal of harvesting. The phenotypic criterion is defined by a threshold that specifies the minimum size of individuals harvested, while the demographic goal defines the desired catch or escapement from the harvest. Models use either a constant phenotypic threshold, or a variable phenotypic threshold that moves each year to remain a fixed distance from the mean size prior to harvest. Demographic goals are defined as a desired yield or escapement, measured in abundance or biomass. We find that all combinations of phenotypic criteria and demographic goals can produce evolutionary changes in equilibrium mean size and abundance when harvesting is size selective. Reductions in mean size are more severe under harvesting scenarios for which it is not possible to fully satisfy the demographic goals of harvesting at equilibrium. Models with a variable phenotypic threshold always produce a characteristic equilibrium for mean size and abundance for a given set of harvesting and natural selection parameters, regardless of the initial population characteristics. For some combinations of parameters, constant-threshold models produce alternative equilibria for the mean size and abundance. Our results suggest that the evolutionary effects of selective harvesting warrant consideration in long-term conservation and management planning.

Demographic and Evolutionary Responses to Selective Harvesting in Populations with Discrete Generations

Mathematical models are used to explore the evolutionary and demographic consequences of size-selective harvesting and natural selection in populations with discrete, nonoverlapping generations. We specify the harvesting intensity and size selectivity in terms of both the phenotypic criterion for harvesting and the demographic goal of harvesting. The phenotypic criterion is defined by a threshold that specifies the minimum size of individuals harvested, while the demographic goal defines the desired catch or escapement from the harvest. Models use either a constant phenotypic threshold, or a variable phenotypic threshold that moves each year to remain a fixed distance from the mean size prior to harvest. Demographic goals are defined as a desired yield or escapement, measured in abundance or biomass. We find that all combinations of phenotypic criteria and demographic goals can produce evolutionary changes in equilibrium mean size and abundance when harvesting is size selective. Reductions in mean size are more severe under harvesting scenarios for which it is not possible to fully satisfy the demographic goals of harvesting at equilibrium. Models with a variable phenotypic threshold always produce a characteristic equilibrium for mean size and abundance for a given set of harvesting and natural selection parameters, regardless of the initial population characteristics. For some combinations of parameters, constant-threshold models produce alternative equilibria for the mean size and abundance. Our results suggest that the evolutionary effects of selective harvesting warrant consideration in long-term conservation and management planning.

Genetic diversity and relationship between Bradyrhizobium strains isolated from blackgram and cowpea

The genetic diversity of bradyrhizobial strains associated with blackgram and cowpea grown in two different agricultural soils (non-saline and saline) along the coastline of Tamil Nadu has been analysed. Phenotypically indistinguishable isolates were analysed for DNA polymorphism using random amplification of polymorphic DNA (RAPD) and polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) of 16S rDNA and nifD. Although these bacteria belong to a group with a broad host range, RAPD analysis showed a considerable level of genetic diversity among the strains isolated from different host plants. Soil pH and salinity seem to have an effect on the selection of natural populations as revealed by PCR-RFLP of 16S rDNA. A combination of PCR-RFLP genotyping with nodulation studies indicates that monocropping of blackgram and the salinity of the soil have made ineffective rhizobia the dominant genotype, thereby creating an ecological burden on their other compatible hosts. A group of strains and a type strain sharing three different 16S PCR-RFLP types were shown to have the same set of symbiotic genes as inferred from the PCR-RFLP pattern of nifD. Another group of cowpea rhizobia that were found to be effective nitrogen fixers and sharing distinct 16S profiles were found to have a different set of symbiotic genes.