CAN REINFORCEMENT OCCUR WITH A LEARNED TRAIT?

We use birdsong as a case study to ask whether reinforcement can occur via the spread of a genetically determined female preference for a socially inherited (learned) male trait. We envision secondary contact between two neighboring populations with different song dialects. An individual's ability to learn song is confined by a genetic predisposition: if predispositions are strong, there will be no phenotypic overlap in song between populations, whereas weak predispositions allow phenotypic overlap, or \"mixed\" song. To determine if reinforcement has occurred, we consider if an allele for within-population female mating preference, based on song, can spread, and whether population specific songs can concurrently be maintained at equilibrium. We model several scenarios, including costs to mating preferences, mating preferences in hybrids, and hybrids having the ability to learn pure songs. We find that when weak predispositions are fixed within a population reinforcement based on song cannot occur. However, when some individuals have strong predispositions, restricting phenotypic overlap between populations in the trait, reinforcement is only slightly inhibited from a purely genetic model. Generalizing beyond the example of song, we conclude that socially learned signals will tend to prohibit reinforcement, but it may still occur if some individuals acquire trait phenotypes genetically.     

Female choice for male motor skills

Sexual selection was proposed by Darwin to explain the evolution of male sexual traits such as ornaments and elaborate courtship displays. Empirical and theoretical studies have traditionally focused on ornaments; the reasons for the evolution of elaborate, acrobatic courtship displays remain unclear. We addressed the hypothesis that females choose males on the basis of subtle differences in display performance, indicating motor skills that facilitate survival. Male golden-collared manakins (Manacus vitellinus) perform elaborate, acrobatic courtship displays. We used high-speed cameras to record the displays of wild males and analysed them in relation to male reproductive success. Females preferred males that performed specific display moves at greater speed, with differences of tens of milliseconds strongly impacting female preference. In additional males, we recorded telemetrically the heart rate during courtship using miniature transmitters and found that courtship is associated with profoundly elevated heart rates, revealing a large metabolic investment. Our study provides evidence that females choose their mates on the basis of subtle differences in motor performance during courtship. We propose that elaborate, acrobatic courtship dances evolve because they reflect motor skills and cardiovascular function of males.     

The extent of variation in male song, wing and genital characters among allopatric Drosophila montana populations

Drosophila montana, a species of the Drosophila virilis group, has distributed around the northern hemisphere. Phylogeographic analyses of two North American and one Eurasian population of this species offer a good background for the studies on the extent of variation in phenotypic traits between populations as well as for tracing the selection pressures likely to play a role in character divergence. In the present paper, we studied variation in the male courtship song, wing and genital characters among flies from Colorado (USA), Vancouver (Canada) and Oulanka (Finland) populations. The phenotypic divergence among populations did not coincide with the extent of their genetic divergence, suggesting that the characters are not evolving neutrally. Divergence in phenotypic traits was especially high between the Colorado and Vancouver populations, which are closer to each other in terms of their mtDNA genotypes than they are to the Oulanka population. The males of the Colorado population showed high divergence especially in song traits and the males of the Vancouver population in wing characters. Among the male song traits, two characters known to be under sexual selection and a trait important in species recognition differed clearly between populations, implying a history of directional and/or diversifying rather than balancing selection. The population divergence in wing characters is likely to have been enhanced by natural selection associated with environmental factors, whereas the male genitalia traits may have been influenced by sexual selection and/or sexual conflict.     

The unusual inheritance pattern of the courtship songs in closely related grasshopper species of the Chorthippus albomarginatus-group (Orthoptera: Gomphocerinae)

Sibling species of the Chorthippus albomarginatus-group are exceptional among all European grasshopper species because they have an extremely elaborate courtship behaviour. Here, we present a genetic analysis of the courtship song differences between two closely related grasshopper species Ch. albomarginatus and Ch. oschei. Measurements of seven courtship song characters and one parameter of courtship visual display were compared among parentals, F(1) and F(2) hybrids, and backcrosses. Variation in one morphological character, the number of stridulatory pegs, was studied as well. The distributions of song phenotypes were more consistent with a type-III genetic architecture, which involves complementary or duplicate pairs of loci. We suggest that this type of architecture may provide the novel elements of the hybrid songs, even more complex than the parental ones, which may offer a new starting point for sexual selection. One parameter of the visual display appeared to be controlled by a single locus, but differences in other characters of visual display could be explained by a more complex inheritance pattern. Inheritance of the peg number differences was consistent with a simple polygenic additive model. Sex linkage and/or maternal effects were not detected for any trait. We found a disproportionate influence of the Ch. albomarginatus parent on most hybrid song characteristics and the visual display, and this may support a previously suggested idea that Ch. albomarginatus is an ancestral form and Ch. oschei is a derived species.     

Condition-dependent traits and the capture of genetic variance in male advertisement song

The occurrence of additive genetic variance (VA) for male sexual traits remains a major problem in evolutionary biology. Directional selection normally imposed by female choice is expected to reduce VA greatly, yet recent surveys indicate that a substantial amount remains in many species. We addressed this problem, also known as the 'lek paradox', in Achroia grisella (Lepidoptera: Pyralidae), an acoustic moth in which males advertise to females with a pulsed ultrasonic song. Using a standard half-sib/full-sib breeding design, we generated F1 progeny from whom we determined VA and genetic covariance (COVA) among seven traits: three song characters, an overall index of song attractiveness, nightly singing period, adult lifespan, and body mass at adult eclosion. Because A. grisella neither feed nor drink as adults, the last trait, eclosion body mass, is considered a measure of 'condition'. We found significant levels of VA and narrow-sense heritabilities (h2) for all seven traits and significant genetic correlations (= COVAi,j / radical (VA i x VA j)) between most pairs of traits (i, j). Male attractiveness was positively correlated with body mass (condition), adult lifespan, and nightly singing period, which we interpret as an energy constraint preventing males in poor condition from singing attractively, from singing many hours per night, and from surviving an extended lifespan. The positive genetic correlation (r = 0.79) between condition and attractiveness, combined with significant levels of VA for both traits, indicates that much of the variation in male song can be explained by VA for condition. Finally, we discuss the morphological and physiological links between condition and song attractiveness, and the ultimate factors that may maintain VA for condition.