REPRODUCTIVE CHARACTER DISPLACEMENT IN PHLOX

Plants of the red- and pink-flowered P. drummondii were introduced into a natural population of P. cuspidata. Their seeds were progeny-tested, and the percentage hybrid seed determined for each variant. Thirty-eight percent of the seed from the pink variant were hybrid compared to 13% of the red. The difference in hybrid production is best explained by corolla color. The self- and cross-compatibility of nine sympatric and nine allopatric populations were studied in the greenhouse. On the average, sympatric populations are more self-compatible than allopatric populations. Pollen germination is 13.7% vs. 7.3%, the difference being statistically significant. The self-compatibility differential was accompanied by a self seedset differential. On the average, sympatric populations are more cross-compatible with P. cuspidata than are allopatric populations. The germination of P. cuspidata pollen on sympatric P. drummondii stigmas was 13.3% vs. 9.8% on allopatric stigmas. However, the difference is not statistically significant. The presence of P. cuspidata has promoted reproductive character divergence in P. drummondii. The shift in corolla color and the increase in self-compatibility reduce the potential for gametic wastage and interspecific hybridization.     

A PHYLOGENETIC ANALYSIS OF CHARACTER DISPLACEMENT IN CARIBBEAN ANOLIS LIZARDS

Twenty-seven islands in the Lesser Antilles contain either one or two species of Anolis lizards. On nine of the ten two-species islands, the species differ substantially in size; 16 of the 17 one-species islands harbor an intermediate-sized species. Two processes could produce such a pattern: size adjustment (or character displacement), in which similar-sized species evolve in different directions in sympatry; and size assortment, in which only different-sized species can successfully colonize the same island together. Previous analyses implicitly have assumed that size is evolutionarily plastic and determined solely by recent ecological conditions, and consequently have tested the hypothesis that character displacement has occurred on each of the ten two-species islands. Other studies have focused only on size assortment. By analyzing such patterns in a phylogenetic context, I explicitly consider historical effects and can distinguish between size adjustment and size assortment. Using a minimum evolution algorithm, I assess evidence for size adjustment by partitioning changes in size along branches of the phylogenetic tree. Size evolution appears rare (a minimum of 4-7 instances of substantial size evolution). In the northern (but not the southern) Lesser Antilles, size change was significantly greater when a descendant taxon occurred on a two-species island and its hypothetical ancestor occurred on a one-species island, thus supporting the size adjustment hypothesis, though size adjustment might have occurred only once. The relative rarity of size evolution suggests that size assortment might be responsible for nonrandom patterns. In both the northern and southern Lesser Antilles, a null model of no size assortment is convincingly rejected. Closely related taxa, however, are usually similar in size, and hybridization between species has been reported. Consequently, similar-sized species might not coexist because they interbreed and coalesce into one gene pool. A null model that only allows species from different “clades” to co-occur is rejected for the northern Lesser Antilles, but is ambiguous with regard to the southern Lesser Antilles. Thus, competitive exclusion is probably responsible for the pattern of size assortment in the northern Lesser Antilles; both competitive exclusion and interbreeding of closely related species of similar size might be responsible for the patterns evident in the southern Lesser Antilles.     

MODELS OF CHARACTER DISPLACEMENT AND THE THEORETICAL ROBUSTNESS OF TAXON CYCLES

The appropriateness of the techniques used in modeling character displacement has been the focus of vigorous debate. In this paper, the three competing methods (the coevolutionarily stable community (CSC), the evolutionarily stable strategy (ESS), and quantitative genetic recursion (QGR)) are compared in models using a common ecological setting. Specific predictions of the CSC model have been used to understand features of character displacement among Cnemidophorous lizards on islands off Mexico, Anolis lizards in the Lesser Antilles and Galápagos finches. Nonetheless, the validity of the approach has been repeatedly questioned. Conceptually the three formalisms vary in the degree to which within species variability is allowed in the models. The predictions of the CSC are found not to be robust to even small violation of its fundamental assumption of absolute species monomorphy. We show by simulation and analytical observations that the CSC is not valid under frequency dependent selection, and that the ESS is the limiting case of QGR as intraspecific phenotypic variation goes to zero. Thus the ESS and the QGR models agree closely when the between-phenotype component (BPC) of the niche width is small. However, as the BPC increases, quantitative discrepancies between ESS and QGR predictions increase, although model behavior remains qualitatively similar. A fourth approach, termed “Quantitative Genetic Optimization” (QGO) analysis, is suggested, combining advantages of both the ESS and QGR. Although all approaches support the possibility of taxon cycles, the cycle patterns predicted are qualitatively different and strongly model dependent.     

EVOLUTIONARY DIVERGENCE AND CHARACTER DISPLACEMENT IN TWO PHENOTYPICALLY-VARIABLE,COMPETING SPECIES

Theoretical studies of character displacement lead to the view that evolutionary divergence depends primarily on incomplete utilization of available resources. Those models which incorporate constraints preventing complete utilization of resources, even in the absence of competitors, all predict character displacement. Those models which allow greater flexibility of resource use within a species predict correspondingly less divergence. Indeed, Matessi and Jayakar (1980, 1981) based their conditions for occurrence of character displacement on underutilization of resources. I extend a model used by Slatkin (1980, 1983) and Taper and Case (1985) which allows each species to fully utilize its resources in the absence of competitors. I concentrate on the biologically reasonable case in which the species, though similar, differ in their ecological characteristics. As a result of this greater biological realism, I arrive at a different conclusion regarding the conditions which lead to character displacement. The presence of a variety of biological differences between species—including as a subset those which result from resource underutilization—leads to divergence with respect to a quantitatively inherited character, due to interspecific competitive interactions. The resulting displacement can be large and depends little on the parameters chosen. The only exception, involving a character with very low heritability, occurs when the non-interactive phenotypic differences are much greater than those associated with studies of character displacement in natural populations. Thus, under conditions comparable to those encountered in the field, involving similar yet not identical species, evolutionary divergence is a consequence of interspecific competition.     

A MATERNAL EFFECT MEDIATES RAPID POPULATION DIVERGENCE AND CHARACTER DISPLACEMENT IN SPADEFOOT TOADS

Despite long-standing interest in character displacement, little is known of its underlying proximate causes. Here, we explore the role of maternal effects in character displacement. We specifically investigated whether differences in maternal body condition mediate divergence in the expression of resource-use traits between populations of spadefoot toads ( Spea multiplicata ) that occur in sympatry with a heterospecific competitor and those that occur in allopatry. In sympatry, S. multiplicata is forced by its competitor onto a less profitable resource. As a result, sympatric females mature in poorer condition and invest less into offspring. Consequently, their offspring produce a resource-use phenotype that minimizes competition with the other species and that also differs from the phenotype produced in allopatry. These differences in trait expression between allopatry and sympatry disappear once mothers are equilibrated in body condition in the laboratory. Thus, a condition-dependent maternal effect mediates population divergence and character displacement. Such effects potentially buffer populations from extinction (via competitive exclusion) while genetic changes accumulate, which produce divergent traits in the absence of the maternal effect. Maternal effects may therefore often be important in determining the initial direction and rate of evolution during the early stages of character displacement.     

INTRASPECIFIC REPRODUCTIVE CHARACTER DISPLACEMENT IN A POLYMORPHIC POISON DART FROG,DENDROBATES PUMILIO

Divergence in male mating signals and associated female preferences is often an important step in the process of speciation. Reproductive character displacement, the pattern of greater divergence of male signals and/or female preference in sympatry than in allopatry, has been observed in a variety of taxa with different degrees of postzygotic isolation. A number of selective processes, including reinforcement, have been proposed to cause such a pattern. Cases in which reproductive character displacement occurs among intraspecific variants are especially informative for understanding how selection acting within a species can lead to the evolution of reproductive barriers and speciation. This study tested the hypothesis that female strawberry poison dart frogs (Dendrobates pumilio) in polymorphic populations of the Bocas del Toro archipelago of Panama show stronger mating discrimination than do females from monomorphic populations, exhibiting an intraspecific pattern of reproductive character displacement. Our results contribute important insights into understanding selection's role in generating the striking diversity of Bocas del Toro's D. pumilio and provide a snapshot of what could be the early stages of reproductive isolation and speciation.     

SPITE VERSUS CHEATS: COMPETITION AMONG SOCIAL STRATEGIES SHAPES VIRULENCE IN PSEUDOMONAS AERUGINOSA

Social interactions have been shown to play an important role in bacterial evolution and virulence. The majority of empirical studies conducted have only considered social traits in isolation, yet numerous social traits, such as the production of spiteful bacteriocins (anticompetitor toxins) and iron‐scavenging siderophores (a public good) by the opportunistic pathogen Pseudomonas aeruginosa, are frequently expressed simultaneously. Crucially, both bacteriocin production and siderophore cheating can be favored under the same competitive conditions, and we develop theory and carry out experiments to determine how the success of a bacteriocin‐producing genotype is influenced by social cheating of susceptible competitors and the resultant impact on disease severity (virulence). Consistent with our theoretical predictions, we find that the spiteful genotype is favored at higher local frequencies when competing against public good cheats. Furthermore, the relationship between spite frequency and virulence is significantly altered when the spiteful genotype is competed against cheats compared with cooperators. These results confirm the ecological and evolutionary importance of considering multiple social traits simultaneously. Moreover, our results are consistent with recent theory regarding the invasion conditions for strong reciprocity (helping cooperators and harming noncooperators).     

EVOLUTION OF CHARACTER DISPLACEMENT IN SPADEFOOT TOADS: DIFFERENT PROXIMATE MECHANISMS IN DIFFERENT SPECIES

Character displacement occurs when two species compete, and those individuals most dissimilar from the average resource‐use phenotypes of the other species are selectively favored. Few studies have explored the sequence of events by which such divergence comes about. We addressed this issue by studying two species of spadefoot toads that have undergone ecological character displacement with each other. Previous research revealed that phenotypic shifts between sympatric and allopatric populations of one species, Spea multiplicata, reflect a condition‐dependent maternal effect. Here, we show that analogous shifts in the other species, S. bombifrons, cannot similarly be explained by such a maternal effect, and that these shifts instead appear to be underlain by allelic differences. We hypothesize that these two species have evolved different mechanisms of character displacement because they differ in duration in sympatry. Specifically, because they occur at the edge of a range expansion, populations of S. bombifrons have been exposed to S. multiplicata for a longer period. Consequently, S. bombifrons have likely had more time to accumulate genetic changes that promote character displacement. Generally, character displacement may often progress through an initial phase in which trait differences are environmentally induced to one in which they are constitutively expressed.     

DIVERSIFICATION OF CONSPECIFIC SIGNALS IN SYMPATRY: GEOGRAPHIC OVERLAP DRIVES MULTIDIMENSIONAL REPRODUCTIVE CHARACTER DISPLACEMENT IN FROGS

Theoretical models suggest that geographic overlap with different heterospecific assemblages can promote divergence of mate recognition systems among conspecific populations. Divergence occurs when different traits undergo reproductive character displacement across populations within a contact zone. Here, I tested this hypothesis by assessing patterns of acoustic signal divergence in two- and three-species assemblages of chorus frogs ( Pseudacris ), focusing in particular on P. feriarum and P. nigrita . In addition, I tested one criterion for reinforcement, by examining the evolution of female P. feriarum preferences in the contact zone. Patterns of signal evolution indicated that in each of the four sympatric populations studied, only the rarer species displaced substantially ( P. feriarum in three cases and P. nigrita in one instance). Moreover, the three displaced P. feriarum populations diverged in different signal traits across the contact zone, evolving in directions that increased the energetic cost of calling relative to the allopatric call, and in ways that maximized differences from the particular heterospecific assemblage present. Consistent with reinforcement, divergence of female preferences in sympatry was estimated to reduce their propensity to hybridize by 60%. Together, signal and preference data suggest that interactions between species can promote diversification within species, potentially contributing to reproductive isolation among conspecific populations.