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Sexual dimorphism and adaptive speciation: two sides of the same ecological coin.

Models of adaptive speciation are typically concerned with demonstrating that it is possible for ecologically driven disruptive selection to lead to the evolution of assortative mating and hence speciation. However, disruptive selection could also lead to other forms of evolutionary diversification, including ecological sexual dimorphisms. Using a model of frequency-dependent intraspecific competition, we show analytically that adaptive speciation and dimorphism require identical ecological conditions. Numerical simulations of individual-based models show that a single ecological model can produce either evolutionary outcome, depending on the genetic independence of male and female traits and the potential strength of assortative mating. Speciation is inhibited when the genetic basis of male and female ecological traits allows the sexes to diverge substantially. This is because sexual dimorphism, which can evolve quickly, can eliminate the frequency-dependent disruptive selection that would have provided the impetus for speciation. Conversely, populations with strong assortative mating based on ecological traits are less likely to evolve a sexual dimorphism because females cannot simultaneously prefer males more similar to themselves while still allowing the males to diverge. This conflict between speciation and dimorphism can be circumvented in two ways. First, we find a novel form of speciation via negative assortative mating, leading to two dimorphic daughter species. Second, if assortative mating is based on a neutral marker trait, trophic dimorphism and speciation by positive assortative mating can occur simultaneously. We conclude that while adaptive speciation and ecological sexual dimorphism may occur simultaneously, allowing for sexual dimorphism restricts the likelihood of adaptive speciation. Thus, it is important to recognize that disruptive selection due to frequency-dependent interactions can lead to more than one form of adaptive splitting.

Adaptation, Biological↗

Female choice for spot asymmetry in the Trinidadian guppy.

The effect of spot asymmetry on female mate choice was investigated in the Trinidadian guppy, Poecilia reticulata. Spot asymmetry arises because whole pattern elements are missing from one side or are located in a different position on either side. Pairs of males were selected that differed in melanic spot asymmetry. In other respects the pair of males were matched for body size, display rate and other aspects of colour pattern. Two experiments were carried out, one in which asymmetry was caused by missing spot elements, the other in which asymmetry was caused by misaligned spots. The latter experiment does not confound differences of pigment area with differences in asymmetry. In both cases females strongly preferred the symmetric male. This was not sensitivity to fluctuating asymmetry (small differences in the size of spot areas) but to asymmetry at the gross level of spot pattern.Copyright 1997 The Association for the Study of Animal Behaviour

Journal Article↗

Interrelationships of alcohol consumption, actions of alcohol, and biochemical traits.

Voluntary alcohol consumption, acute tolerance, and central nervous system (CNS) sensitivity to ethanol are potentially informative measures concerning human alcoholism. Little is understood regarding the associations among these parameters or between these traits and neurochemical processes such as brain protein or brain enzyme activities. A powerful strategy is to assess a large number of characteristics simultaneously on all individuals as a heterogeneous sample. This permits rapid screening of a large number of variables with respect to their interrelationships. Identification can thus be made of those variables that are elements of the caudal nexus, and subsequent experimental research can attack the problem of identifying mechanisms. The present study employed mice from the HS/Ibg stock which is maintained by systematic random mating to assure genetic heterogeneity. The results demonstrate that voluntary ethanol consumption and acquisition of acute tolerance to ethanol were positively associated, whereas these measures were not significantly related to CNS sensitivity to ethanol. In addition, ethanol preference was inversely related to soluble brain protein. The activities of the soluble enzymes from brain, aldehyde reductase and glucose-6-phosphate dehydrogenase, were not significantly associated with ethanol preference, acquisition of acute tolerance, or CNS sensitivity to ethanol. Unexpectedly, more than 30 percent of the variance in voluntary alcohol consumption could have been predicted from the measurements of acquisition of acute tolerance, and vice versa.

Alcohol Drinking↗

Female soldier beetles display a flexible preference for selectively favored male phenotypes.

In Georgia (USA) the soldier beetle, Chauliognathus pennsylvanicus (Coleoptera; Cantharidae), exhibits clinal variation in the length of the spot on its elytron. This suggests that the viability of phenotypes varies by habitat. Evidence of viability selection comes from within-site changes in the spot length distribution across a breeding season. When males with spots of intermediate length became less frequent, they became disproportionately less likely to mate, consistent with either a loss of vigor among remaining males or female rejection of disfavored phenotypes. Persistent, daily courtship by males provides females with the opportunity to track changes in male phenotype frequency and to exercise choice for phenotypes favored under natural selection. A laboratory experiment in which the frequency of one spot morph (long) or the other (short) was increased from 25% to 75% over a period of 30 days revealed that females possess a flexible preference that leads them to prefer whichever spot type has become more common over time. A haploid genetic model demonstrates that a flexible female preference for the locally favored male phenotype can be selected for when different viability alleles, genetically correlated with the male trait, are favored in different habitats that are linked by gene flow. Thus, migration between different kinds of habitat patches of a metapopulation could maintain the variation in male quality. This variation favors female choice for any trait that is directly or indirectly favored by natural selection. Such choice imparts positive frequency-dependent selection that could rapidly fix traits pleiotropically linked to viability. Rapid fixation would cause differentiation between populations of colonizing species as females exercise choice for mates favored under new ecological conditions.

Animals↗

Reimpressed selective breeding for lateralization of handedness in mice.

Eleven generations of bidirectional selection for lateralization produced 2 lines of mice that differ markedly in degree of asymmetry for hand preference. The foundation population was derived from 6 distantly related inbred strains and 2 stocks of wild mice, M. castaneus. HI line matings were made using mice that exhibited consistent right or left paw use in a food reaching task and LO line matings were made using mice with little overall paw preference. All matings were made without regard to the expressed directions of asymmetry. Line differences emerged at the third generation and increased thereafter. Selection was relaxed at generation 12 and the lines were maintained by random within-line mating. At generation 28 selective breeding was reimpressed for 3 generations. Results indicated that between-line divergence in degree of lateralization had remained high during 17 generations of relaxed selection. Mice of the HI line are more strongly lateralized than mice of the unselected HET population. Mice of the LO line are more weakly lateralized than controls. The selected lines may provide a useful mammalian genetic resource for studying the neurobiology of cerebral lateralization.

Animals↗

Mate choice, monogamy and mutational load.

The classical Haldane mutational load for dominant mutations is 1 - e-alpha (where alpha = the mutation rate of the genome). This is independent of the fitness of the mutant heterozygotes. As alpha may be between 0.9 and 1.8 this may be too high for low fecundity species to bear. The load may be greatly reduced for mildly deleterious mutations if female choice (for males with few or no mutations) is present in a polygamous species. It is shown here that if females and males prefer partners with few or no mutations then the load may also be reduced in monogamous species. As in the polygamous model the effect is greatest if mutations are mildly deleterious.

Animals↗

Koinophilia.

This paper is concerned with the evolution of the species phenotype (morphology and behaviour) when sexual organisms identify mutant traits by their unusual (or rare) appearance. Mate-seeking individuals are assumed, however, to have no means of distinguishing the occasional beneficial mutation from the others. We show that the resulting preference for mates with predominantly common traits (koinophilia) transforms the prevailing phenotype (as perceived by conspecifics) into an evolutionary stable strategy (ESS), as defined by Maynard-Smith (1974). This has far reaching evolutionary implications, permitting, in particular, the evolution of true "group adaptations". A subsidiary finding shows that koinophilia always has a substantial, immediate, selective advantage over panmixis, rendering koinophilia, itself, an ESS with respect to panmixis. This is particularly pronounced when the mutation rate is high, but beneficial mutations with high fitnesses are rare. The mean fitness of mutations has only a minor effect on the relative advantage of koinophilia.

Adaptation, Physiological↗

Role of ferutinin in the impairment of female sexual function induced by Ferula hermonis.

In the present study, we evaluated the effects of single components of Ferula hermonis extract on female rat sexual behaviour. Ovariectomized rats hormonally primed with estradiol benzoate (1.5 or 10 microg/rat s.c.) and progesterone (500 microg/rat s.c.) were acutely treated by oral gavage with ferutinin, teferin and teferdin (2.5 mg/kg). Thereafter they were tested for: a) partner preference, b) receptivity, c) proceptivity, d) paced mating behaviour. In the partner preference test, the choice of the female for a sexually active male was not influenced by the different treatments. Similarly, during the paced mating test, the contact-return latencies as well as the percentage of exits from the male compartment were not different in control and treated rats. Therefore none of the three compounds showed the capacity to alter sexual motivation. On the other hand, ferutinin, but not teferin and teferdin, significantly inhibited female receptivity. These results suggest a primary role of ferutinin in the impairment of sexual behaviour elicited by F. hermonis extract in hormone primed-female rats.

Analysis of Variance↗

The effect of transient food stress on female mate preference in the stalk-eyed fly Cyrtodiopsis dalmanni.

The effect of transient nutritional stress (sucrose culture medium) on female mate preference was investigated in the stalk-eyed fly Cyrtodiopsis dalmanni. Two experimental treatments were run in parallel. In the first, female preference was initially tested on corn (standard medium) and subsequently on sucrose (novel medium). In the second, female preference was tested in the reverse order: sucrose then corn. Females fed on corn had stronger preferences for large-eyespan males than females fed on sucrose. Female age, experience or the order of exposure to culture media had no effects on the strength of preference. An additional experiment demonstrated that the sucrose diet reduced the number of developing and mature eggs. We discuss the adaptive significance of changes in female preference due to transient nutritional stress, and the effect of changes in female preference on the strength of sexual selection.

Animals↗

Inbreeding in Littledale's whistling rat Parotomys littledalei.

Despite its rarity in nature, inbreeding is sometimes evident in species occupying ephemeral, unpredictable habitats, and which occur at low densities. One such species is Littledale's whistling rat, Parotomys littledalei, a murid rodent endemic to the south-west arid region of South Africa. Using a captive population of P. littledalei, I studied mate choice for kin and nonkin, and the reproductive performance of inbred and outbred pairs. In choice tests, estrous females presented with either odors or actual males showed a preference for siblings or half-siblings to unrelated males. Males did not discriminate between the odor of estrous kin and nonkin. In breeding studies, inbred (mother-son; brother-sister) and outbred (proven female and an unrelated young male and nonsiblings) pairs had a similar reproductive output, although the sex ratio favored males in inbred litters. The development of inbred young was indistinguishable from outbred young. The results indicate that female P. littledalei prefer to inbreed, but there are no apparent advantages to inbreeding over outbreeding.

Animals↗

Reproductive character displacement of male stickleback mate preference: reinforcement or direct selection?

Reproductive isolation can evolve between species as a byproduct of adaptation to different niches, through reinforcement, and by direct selection on mating preferences. We investigated the potential role of direct selection in the reproductive isolation between sympatric species of threespine sticklebacks. Each sympatric pair consists of a small-bodied limnetic species and large-bodied benthic species. We compared the mate preferences and courtship behavior of males from one sympatric limnetic population and two allopatric populations. We used limnetic-like allopatric populations to control for the effects of ecological character displacement and adaptation to different niches on mate preferences. The sympatric limnetic males preferred the small limnetic females, whereas the allopatric limnetic-like males preferred the large benthic females, suggesting that adaptation to the limnetic niche does not automatically confer a preference for small limnetic females. This reproductive character displacement of male preference is consistent with the predictions of both reinforcement and direct selection on mate preferences. To test for direct selection, we assessed a prediction of one proposed mechanism: predation by benthic females on eggs guarded by limnetic males. The allopatric males come from populations in which there is no egg predation. Sympatric limnetic males were more aggressive toward benthic females than toward limnetic females, whereas the allopatric limnetic-like males did not treat the two types of females differently. The contrast in male behavior suggests that egg predation has shaped male preferences. Direct selection is potentially more effective than indirect selection via reinforcement, and it is likely that it has been important in building up reproductive isolation between limnetic and benthic sticklebacks.

Adaptation, Physiological↗

The effect of vegetation density on male mate guarding and extra-territorial forays in the yellow-breasted chat (Icteria virens).

Extra-pair paternity is widely documented in birds, but the behaviors that lead to extra-pair copulations (EPCs) have been less well studied, particularly in territorial songbirds. We studied the behaviors associated with extra-territorial forays (ETFs) and male mate guarding in a socially monogamous, but genetically promiscuous, neotropical migrant passerine, the yellow-breasted chat (Icteria virens). Focal observations and radiotelemetry revealed that both males and females engaged in ETFs. 65% of the females in our study engaged in at least one foray onto a neighboring territory. 50% of males also were observed engaging in ETFs, but males were much more conspicuous during these intrusions compared to females. Females preferred to remain close to the ground in areas where vegetation was the densest. Female behavior was difficult to observe during ETFs but females sometimes interacted with neighboring extra-pair males. Males attempted to guard their mates by remaining close to them and following them during extra-territorial intrusions. We observed instances in which a male attacked his mate and appeared to herd her back to his territory. However a male's ability to maintain close proximity to his mate was significantly and negatively correlated with vegetation density. Our results suggest that the behaviors which lead to extra-pair encounters are influenced by the behavior of all participants and are modified by the characteristics of the habitat.

Animals↗

The attractiveness of nonface averages: implications for an evolutionary explanation of the attractiveness of average faces.

Researchers have argued that humans' attraction to average faces reflects an evolved psychological mechanism to identify high-quality mates. If this direct-selection account is correct, there is no reason to expect a similar averageness bias for stimuli that are irrelevant to reproductive fitness. The current study, however, found a strong relationship between averageness and attractiveness for dogs, wristwatches, and birds. The most parsimonious explanation is that humans have a general attraction to prototypical exemplars, and that their attraction to average faces is a reflection of this more general attraction. We tested whether a general preference for familiar stimuli can account for the attractiveness of averageness. This account was not supported for dogs or birds, but could not be ruled out for watches.

Adult↗

Olfactory cues influence female choice in two lek-breeding antelopes.

Pronounced differences in mating success between males holding territories clustered on traditional mating grounds (leks) are commonly cited as evidence of female choice for male phenotypes, but female ungulates appear to prefer particular territories even when no other individuals are on the lek. Female choice of territories may be influenced by spatial features, but observations suggest that females may also be attracted to successful territories by olfactory cues in the soil. Here we report that transferring the topsoil between successful and unsuccessful territories on leks of two reduncine antelope species caused the numbers of females and matings on the unsuccessful territories to increase tenfold. Females were probably attracted to the soil by smells that had accumulated from heavy use by other females. Because of this attraction, stochastic process may play an important part in generating the variance in mating success between territory holders on leks.

Animals↗

Chemosensation and genetic individuality.

Numerous studies have shown that there are measurable behavioural consequences that can result from the olfactory recognition of alleles borne at the major histocompatibility complex (MHC). These consequences include simple individual recognition, disassortative mate preference, discrimination of kin from non-kin and whether a pregnancy is carried to term. Such a system, which can influence the reproductive behaviour of a species, will have profound effects on its genetic constitution and survival. The likely mechanism responsible for the production of MHC-related odours involves soluble MHC molecules that carry allele-specific odoriferous molecules from the blood via the kidneys into the urine, from where they are released into the environment. The ability of soluble MHC molecules to signal genetic individuality in this way may have evolved before the appearance of an acquired immune system in our immediate ancestors, the protochordates.

Animals↗

Paternity frequency and maternity of males in some stingless bee species.

In monogynous hymenopteran societies, the number of mates of a queen strongly influences the potential for conflict between workers and queens over the maternity of males. Queens always 'prefer' their own sons to sons of workers, regardless of queen mating frequency. When a queen mates once, workers are more closely related to, and therefore are expected to prefer, their own sons and then sons of sisters to sons of the queen. However, if effective paternity frequency exceeds 2, workers on average should prefer queen-produced males to males produced by their sisters. We studied the queen mating frequency of seven stingless bee species: the Mexican species Scaptotrigona mexicana, S. pectoralis and the Australian species Austroplebeia symei, Trigona clypearis, T. hockingsi, T. mellipes and T. sapiens. We then determined whether males arise from eggs laid by workers or queens in A. symei, T. clypearis, T. hockingsi and T. mellipes. We show that all seven species investigated are most likely singly mated and that the queen dominates reproduction. This indicates that the queen's mating frequency alone does not determine whether workers or the queen produces the males.

Alleles↗

Androstenedione therapy reinstates normal, not supernormal, song structure in castrated adult male zebra finches.

In previous studies, androstenedione (AE) replacement therapy restored the highest levels and intensities of courtship song displays in castrated male zebra finches of any hormone tested. Furthermore, female zebra finches responded strongly to AE-treated males and preferred intact males given small AE implants to unsupplemented males. In this study, we asked whether AE treatment might alter song structure, since male song is an important cue in mate choice by female zebra finches. Songs of adult males were recorded. The males were then castrated and given AE therapy and recorded again. No differences were found between the courtship or undirected songs males sang before castration and after AE treatment. As in previous studies, the structure of a male's courtship song differed significantly from his undirected song, and the structural differences between these two song types were not altered by AE treatment.

Androstenedione↗

Reduction of brain volume correlates with behavioral changes in queen ants.

The behavior of reproductive female ants distinctly changes during the transition from virgin to mature, egg-laying queen. A winged female ant flies only once during her lifetime when she engages in the nuptial flight. Once she is mated she sheds her wings, excavates a nest and starts laying eggs, the basis for her future colony. We show for two species of harvester ants that this transition is accompanied by changes in the performance of behavioral tests: flying virgins are positively phototactic and prefer open areas, whereas young queens prefer the dark, avoid open areas and, given the opportunity, dig into the soil. These behavioral changes coincide with morphological changes in the brain. The brains of mature queens are significantly smaller than those of virgin females at the time of their mating flight. A disproportionately large shrinkage occurs in the medulla and other parts of the visual system during the early adult life of the queen. The brain reduction appears to be adaptive as mature queens show reduced behavioral repertoires and live in the dark. In contrast to virgin females, they do not rely on vision and might increase their fitness by reducing metabolically costly neural tissue.

Animals↗