Host-parasite coevolution has been shown to provide an advantage to recombination, but the selective mechanism underlying this advantage is unclear. One possibility is that recombination increases the frequency of advantageous genotypes that are disproportionately rare because of fluctuating epistasis. However, for this mechanism to work, epistasis for fitness must fluctuate over a very narrow timescale: two to five generations. Alternatively, recombination may speed up the response to directional selection by breaking up linkage disequilibria that decrease additive genetic variance. Here we analyze the results of a numerical simulation of host-parasite coevolution to assess the importance of these two mechanisms. We find that linkage disequilibria may tend to increase, rather than decrease, additive genetic variance. In addition, the sign of epistasis changes every two to five generations under several of the parameter values investigated, and epistasis and linkage disequilibrium are frequently of opposite signs. These results are consistent with the idea that selection for recombination is mediated by fluctuating epistasis. Finally, we explore the conditions under which an allele causing free recombination can spread in a nonrecombining host population and find general agreement between the predictions of a population genetic model of fluctuating epistasis and our simulation model.
Whether geographic variation in signals actually affects communication between individuals depends on whether discriminable differences in signals occur over distances that individuals move in their lifetimes. We measure the ability of song sparrows (Melospiza melodia) to discriminate foreign from local songs using foreign songs recorded at a series of increasing distances and compare the results with previous measurements of dispersal distances. We test discrimination in males using playback of songs on territories and measuring approach and in females using playback to estradiol-treated captives and measuring courtship display. Females fail to discriminate against foreign songs recorded at 18 km but do discriminate against foreign songs recorded at 34, 68, 135, and 540 km. Males fail to discriminate against foreign songs recorded at 18, 34, 68, 135, and 270 km but do discriminate against foreign songs from 540 km. Females are more discriminating, but even they do not discriminate at a distance three times the root-mean-square dispersal distance, as estimated from mtDNA variation. We suggest that female preference for local songs benefits females not because it allows them to reject foreign males but because accurate production of local song serves as a test of song-learning ability.
Colorful plumages are conspicuous social signals in birds, and the expression of these colors often reflects the quality of their bearers. Since mature feathers are dead structures, plumage color is often considered a static signal that does not change after molt. Feathers, however, can and do deteriorate between molts, and birds need to invest heavily in plumage maintenance. Here we argue that this need for preserving plumage condition and hence signaling content might have given rise to a novel type of sexual signal: cosmetic coloration. Cosmetic coloration occurs when the substances used for plumage maintenance change the color of the feathers, thereby becoming a signal themselves. Our review of cosmetic coloration in birds demonstrates that it is more widespread than currently realized, occurring in at least 13 bird families. Cosmetics have varied origins: they can be produced by the bird itself (uropygial and skin secretions, feather powder) or obtained from the environment (soil, iron oxide). Intraspecific patterns of cosmetic use (sex, age, and seasonal dimorphism) suggest that in many cases it may act as a sexual signal. However, more information is required on function, mechanisms, and costs to understand the evolution of cosmetic coloration and to confirm its signaling role.
The nature of the interaction among deleterious mutations is important to models in many areas of evolutionary biology. In addition, interactions between genetic and environmental factors may affect the predictions of such models. Individuals of unknown genotypes of Arabidopsis thaliana, ecotype Marburg, were exposed to five levels of chemical (EMS) mutagenesis and three levels of Pseudomonas syringae infection. Survival, growth and flowering characteristics of each individual were measured. The logarithm of fitness is expected to be a linear function of mutation number if mutations act independently. Furthermore, the expected number of mutations should be approximately a linear function of time of exposure to mutagen. Therefore, nonlinear effects of mutagen exposure on the logarithm of fitness characters would suggest epistasis between mutations. Similarly, if pathogen infection and mutation act independently of each other, their effects should be additive on a log scale. Statistical interactions between these factors would suggest they do not act independently; particularly, if highly mutated individuals suffer more when infected than do less mutated individuals, this suggests that pathogens and mutations act synergistically. Pseudomonas‐infected individuals were shown to have an increased probability of flowering under conditions of short day length, but to ultimately produce fewer flowers than uninfected individuals. This suggests a plastic response to stress and, despite that response, an ultimately deleterious effect of infection on fitness. Leaf rosette growth was negatively and linearly related to the expected number of mutations, and the effects of mutation on different life‐cycle stages appeared to be uncorrelated. No significant interactions between pathogen and mutation main effects were found. These results suggest that mutations act multiplicatively with each other and with pathogen infection in determining individual fitness.
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