Result Of Sexual Selection Research Articles

Sexual Selection in Plants: Perspective and Overview

Sexual selection has a long history of controversy, which shows little sign of waning. The more recent application of the concept to plants continues the historical trend. In recent years, the number of articles on sexual selection has grown; in journals that regularly accept articles on both animals and plants, the frequency of those on sexual selection in plants has grown dramatically. However, articles that examine processes and outcomes related to sexual selection in plants often do not use the term sexual selection, a tendency that indicates the aura of controversy in the field. In the decade since the last broad review of sexual selection in plants, considerable progress has been made in understanding both process and outcome of sexual selection in the botanical world. I have argued that sexual selection can occur at any phase of the reproductive process, from "courtship" to postnatal stages, in plants as well as animals. Most of this work has addressed events or traits before fertilization, but postfertilization aspects of sexual selection are receiving increased attention. As work continues, variation emerges in both pattern and process, which provides a critical investigative tool. The dynamic interplay of concept and results provides evidence of a lively, vigorous field of research.

THE EVOLUTION OF PLUMAGE BRIGHTNESS IN BIRDS IS RELATED TO EXTRAPAIR PATERNITY.

A positive association between plumage brightness of male birds and the degree of polygyny may be the result of sexual selection. Although most birds have a socially monogamous mating system, recent paternity analyses show that many offspring are fathered by nonmates. Extrapair paternity arises from extrapair copulations which are frequently initiated by females. Not all females will be able to mate with a male of the preferred phenotype, because of the mating decisions of earlier paired females; extrapair copulations may be a means for females to adjust their precopulation mate choice. We use two comparative analyses (standardized linear contrasts and pairwise comparisons between closely related taxa) to test the idea that male plumage brightness is related to extrapair paternity. Brightness of male plumage and sexual dimorphism in brightness were positively associated with high levels of extrapair paternity, even when potentially confounding variables were controlled statistically. This association between male brightness and extrapair paternity was considerably stronger than the association between male brightness and the degree of polygyny. Cuckoldry thus forms an important component of sexual selection in birds.

Sexual behavior, sexual swelling, and penile evolution in chimpanzees (Pan troglodytes)

Functional relationships between the penis, vagina, and cervix during copulation in the chimpanzee were studied. In 11 adult males, penile length during full erection ranged from 10.0-18.0 cm (mean +/- SD = 14.4 +/- 2.02). In 19 parous adult females examined during the early follicular phase of the cycle, or during lactation, vaginal depth (from the introitus to the os cervix) ranged from 9.8-16.5 cm (mean +/- SD = 12.6 +/- 1.69). However, when the sexual skin was maximally swollen, vaginal depth increased by up to 52% in some cases and ranged from 15.2-20.1 cm (mean +/- SD = 16.9 +/- 1.68). Under such conditions males vary in their ability to achieve maximal depth of intromission. Observations made on 6 females after natural matings revealed that in some cases males had deposited copulatory plugs in contact with the cervix, while in others plugs were lower down in the vagina. Direct observations under fluoroscopy using an artificial "penis" loaded with radio-opaque medium showed that the filiform tip can make contact with the os cervix. However, expulsion of radio-opaque medium under these conditions failed to demonstrate any immediate transfer of fluid through the cervix into the uterus. These studies indicate that, aside from its functions as a visual signal, the female chimpanzee's sexual skin swelling adds considerably to the distance males must negotiate during copulation to place spermatozoa at the cervical os. Evolution of the male's elongated, filiform penis may therefore be the result of sexual selection, to negotiate the long vagina of the female and to penetrate copulatory plugs deposited during previous copulations.

Absence of Trade‐Offs Between Sexual Size Dimorphism and Early Male Emergence in a Butterfly

Protandry, here defined as the earlier emergence of males, is a common feature in life histories and could be the result of sexual selection on males to maximize matings, or alternatively and incidental by—product of other selection pressures on the sexes. If protandry is selected for per se, theory predicts that it should be associated with seasonal environments where there is little overlap between generations. The degree of protandry should be insensitive to environmental conditions. Moreover, on the assumption that males and females grow at the same rate as larvae, a trade—off between development time and size is expected to result in a strong association between protandry and female—biased sexual size dimorphism. These predictions were tested by a combination of comparative and experimental studies on five populations of the speckled wood butterfly, Pararge aegeria, from central and south Sweden, England, Spain, and the island of Madeira. Protandry was associated with seasonal environments, as it was only exhibited in the three northernmost populations. Protandry in these populations remained largely constant in a variety of temperatures, both under direct development, when protandry results from a sex difference in development time through the egg, larval, and pupal stages, and under diapause development, when it results from a sex difference in pupal development time only. These results indicate that protandry is selected for per se through sexual selection in seasonal environments. Similar female—biased size dimorphism occurred in protandrous and non—protandrous populations alike, and hence sexual size dimorphism in P. aegeria is not a result of selection for protandry, nor the causal factor behind protandry. Protandry and sexual size dimorphism appear to be largely decoupled traits in the life history evolution of P. aegeria. This is achieved by means of variation in pupal developmental time and variation in the relative growth rates of the sexes. Variation in growth rates is likely to be a general phenomenon and may make possible independent optimization of size and development time (age at sexual maturity), and accordingly influence expected patterns of size—related trade—offs.

Is beauty in the eye of the beholder?

Theories of beauty were evaluated by requiring subjects to “evolve” a beautiful female face using a Genetic Algorithm. In this procedure, a computer program generated a small population of faces (first generation of phenotypes) from a set of random binary strings (genotypes). Genotypes specified the shapes and soft tissue anthropometrics of facial features. Each of the first generation of faces was rated by a subject (relative fitness measure) for beauty. The fittest genotypes then bred in proportion to their fitness, with crossover and mutation of the binary strings, to produce offspring which were again rated by the subject. This process continued until the most beautiful face, for that subject, was evolved. Forty Caucasian subjects (20 M, 20 F) were required to evolve their idealized beautiful female face using this procedure. The features and soft tissue anthropometrics of their final composites were compared to population norms. Also, the final composites, and different faces generated from the same data base, were rated for beauty by independent judges. The results support the conclusion that the concept of facial beauty is the result of sexual selection, and a beautiful female face has features and proportions indicative on high fertility.

Functional morphology of the human chin

AbstractThe protruding chin is an attribute that defines modern Homo sapiens to the exclusion of all other primates, including fossil hominids. The functional significance of the chin has been contemplated for most of the 20th century, but as yet no compelling functional argument for its evolution has withstood careful scrutiny. Consequently, the human chin is often cited as an example of a nonadaptive trait. Past attempts to explain the chin in a functional or mechanical context have failed, largely as a result of an incomplete understanding of in vivo masticatory biomechanics. When the morphology of the chin is considered in light of experimental data on mastication, its evolution can be interpreted as a consequence of recent changes in mandibular proportions that have altered the relative importance of different masticatory stresses. Hypotheses proposing that chin morphology is the result of sexual selection or spatial constraints may be untestable. As with arguments that posit no functional role for the chin, the credibility of these hypotheses has depended, to a large degree, on the refutation of previous biomechanical explanations.

Evidence for sexually selected infanticide in captiveCercopithecus mitis, Cercocebus torquatus, andMandrillus leucophaeus

Adult resident males of one-male-multi-female primate groups housed at the Hannover Zoo exhibited aggression, when confronted with nonadult individuals, which were fathered by other males: (1) a new adult resident male in a group of blue monkeys killed a 5.8-month-old female infant: (2) a new adult resident male in a group of white collared mangabeys injured a 24.0-month-old female and an 18.9-month-old male severely; they would have died without veterinary care; and (3) the resident male of a group of drills threatened an 1.8-month-old foreign female infant seriously; efforts to introduce the infant were discontinued. Pathological explanations are unlikely because the adult males showed no aggression towards own nonadult offspring under the same captive conditions. By and large, the events support the theory that infanticide is the result of sexual selection among males.

A biomechanical perspective on the use of forelimb length as a measure of sexual selection in frogs

AbstractDifferences in forelimb length between male and female frogs and between amplexing and non‐amplexing males have been interpreted to be the results of sexual selection on forelimb length. The causal feature of the forelimb that has been posited to cause such selection is the observation that non‐amplexing males attempt to disrupt breeding by prying amplexing males from females. A biomechanical model of forelimb function suggests that total length per se may not be the most appropriate measure to use. There are more functionally significant aspects of forelimb morphology, such as lever arm lengths, that should influence amplexing ability and may make measures of overall forelimb length misleading. This example highlights the relevance of functional analysis to current questions in evolutionary biology that rely on postulated roles for morphological structures under selection.

A phylogenetic interpretation of sexual dimorphism in body size and ornament in relation to mating system in birds

AbstractSexual dimorphism in body size and ornament is commonly assumed to be a result of sexual selection. Therefore, a pattern among species is expected where the type of mating system correlates with the degree of sexual dimorphism. This was analysed using comparative data on birds using an autocorrelation model. Mating system did correlate with sexual dimorphism in ornament (tail), but was only weakly correlated with sexual size (tarsus) dimorphism. Polygynous species were significantly larger than monogamous species. After correcting for size, there was no difference in size dimorphism between polygynous and monogamous species. The reverse was true for tail dimorphism; polygynous species exhibited a larger degree of tail dimorphism, but there was no difference in absolute size of the tail. This pattern suggests that an evolutionary change in mating system is associated with a change in overall size, and in degree of dimorphism in secondary sexual traits.

The Colour-Sense

WITH reference to Dr. Pole's valuable papers on Homer's colour-blindness, it may interest your readers to learn that I have now nearly completed a work on “The Origin and Development of the Colour-Sense”, which will be shortly published by Messrs. Trübner and Co. In it I have endeavoured to show (inter alia) that the use of colour-terms in the Homeric poems is strictly analogous to that of other races, existing or extinct, at the corresponding stage of culture; and that both depend, not upon dichromic vision, but upon a defect of language closely connected with the small number of dyes or artificial pigments known to the various tribes. To establish this result I have sent a number of circular letters to missionaries, Government officials, and other persons having relations with native uncivilised races in all parts of the world; and their answers to my queries, framed so as to distinguish carefully between perception and language, in every case bear out the theory which I had formed. As my results will so soon be published elsewhere, I shall not burden your columns with them at present, but may add that my researches lead me to place the origin of the colour-sense far lower down in the animal scale, as evidenced both by the distinctive hues of flowers and fruits, and by the varied integuments of insects, birds, &c, so far as these are the result of sexual selection, or of mimicry and other protective devices.