Phenotypic differentiation is associated with divergent sexual selection among closely related barn swallow populations.

Sexual selection plays a key role in the diversification of numerous animal clades and may accelerate trait divergence during speciation. However, much of our understanding of this process comes from phylogenetic comparative studies, which rely on surrogate measures such as dimorphism that may not represent selection in wild populations. In this study, we assess sexual selection pressures for multiple male visual signals across four barn swallow (Hirundo rustica) populations. Our sample encompassed 2400 linear km and two described subspecies: European H. r. rustica (in the Czech Republic and Romania) and eastern Mediterranean H. r. transitiva (in Israel), as well as a potential area of contact (in Turkey). We demonstrate significant phenotypic differentiation in four sexual signalling axes, despite very low-level genomic divergence and no comparable divergence in an ecological trait. Moreover, the direction of phenotypic divergence is consistent with differences in sexual selection pressures among subspecies. Thus, H. r. transitiva, which have the darkest ventral plumage of any population, experience directional selection for darker plumage. Similarly, H. r. rustica, which have the longest tail feathers of any population, experience directional selection for elongated tail feathers and disruptive selection for ventral plumage saturation. These results suggest that sexual selection is the primary driver of phenotypic differentiation in this species. Our findings add to growing evidence of phenotypic divergence with gene flow. However, to our knowledge, this is the first study to relate direct measures of the strength and targets of sexual selection to phenotypic divergence among closely related wild populations.

Genome-wide differentiation in closely related populations: the roles of selection and geographic isolation.

Population divergence in geographic isolation is due to a combination of factors. Natural and sexual selection may be important in shaping patterns of population differentiation, a pattern referred to as 'isolation by adaptation' (IBA). IBA can be complementary to the well-known pattern of 'isolation by distance' (IBD), in which the divergence of closely related populations (via any evolutionary process) is associated with geographic isolation. The barn swallow Hirundo rustica complex comprises six closely related subspecies, where divergent sexual selection is associated with phenotypic differentiation among allopatric populations. To investigate the relative contributions of selection and geographic distance to genome-wide differentiation, we compared genotypic and phenotypic variation from 350 barn swallows sampled across eight populations (28 pairwise comparisons) from four different subspecies. We report a draft whole-genome sequence for H. rustica, to which we aligned a set of 9493 single nucleotide polymorphisms (SNPs). Using statistical approaches to control for spatial autocorrelation of phenotypic variables and geographic distance, we find that divergence in traits related to migratory behaviour and sexual signalling, as well as geographic distance, together explain over 70% of genome-wide divergence among populations. Controlling for IBD, we find 42% of genomewide divergence is attributable to IBA through pairwise differences in traits related to migratory behaviour and sexual signalling alone. By (i) combining these results with prior studies of how selection shapes morphological differentiation and (ii) accounting for spatial autocorrelation, we infer that morphological adaptation plays a large role in shaping population-level differentiation in this group of closely related populations.

The vastus medialis oblique muscle and its relationship to patellofemoral joint deterioration in human cadavers.

Patellofemoral joint deterioration (PFJD) is frequently seen in physical therapy clinics and represents a significant problem for both patients and rehabilitation clinicians. The vastus medialis oblique (VMO) muscle is reported to be the primary stabilizer of the patella during knee extension. Most studies and treatment protocols emphasize strengthening of the VMO as the nonsurgical treatment of choice for patients with PFJD. The purpose of this study was to determine whether any relationship exists between the morphology of the VMO and the presence and severity of PFJD in human cadavers. Dissection of 374 vastus medialis (VM) muscles and patellofemoral joints was performed on 229 human cadaver lower limbs to determine what relationships exist between gender, VMO features, and PFJD. Patellofemoral joint deterioration was determined by direct visual observation and assigned a score based on severity of joint deterioration present. Two-way chi-square tests were performed to determine the relationships between cadaver gender, the presence of VMO features, and the presence and severity of PFJD. Linear regression was performed to determine whether any correlation existed between the VMO fiber angle and the severity of PFJD. A one-way analysis of variance was performed to determine whether any differences existed between the VMO fiber angle and the PFJD groups. No statistically significant relationships, correlation, or differences existed in any of the tests performed between cadaver gender, VMO features, and presence or severity of PFJD. The presence or severity of PFJD in human cadavers is not related to either gender or VMO morphologic features. The results of this study do not support the premise that a more distal insertion of the VMO onto the patella of the VMO will have any effect on the presence or severity of PFJD.

Prevalence and morphology of the vastus medialis oblique muscle in human cadavers.

BACKGROUND: The vastus medialis (VM) muscle has been described as being composed of two separate divisions: the vastus medialis longus (VML) proximally and the vastus medialis oblique (VMO) distally. The VML is reported to directly contribute to knee extension, while the VMO provides medial stabilization of the patella during knee extension. Despite the prevalence of literature describing the morphology and function of the VMO as an individual muscle, very little literature exists which actually substantiates the existence of the VMO as a separate, distinct muscle from the VML. The purpose of this study was to examine a sufficiently large sample of human cadavers to quantify and substantiate the existence of the VMO as a separate, distinct muscle from the VML, and to establish a statistical parameter representative of a normal adult population. METHODS: Three hundred seventy-four adult human cadaver lower extremities were dissected, exposing the entire anterior thigh from the anterior superior iliac spine to the tibial tubercle. Examination of the cadavers included goniometric measurement of the fiber angles of the VML and VMO, determination of the existence and location of a fascial plane, and determination of the maximum VM fiber angle in all cadaver specimens. Descriptive statistics were performed on all fiber angle measurements and frequency of fascial plane presence. Analysis of variance was performed on the maximal VM fiber angle between muscles with and without a definitive fascial plane. Intrarater reliability tests were performed on all measures to ensure the reliability and increase the validity of all of the measurements taken. RESULTS: A statistical parameter for the appearance of VMO features as originally defined was set at 21.65% of the sample. No statistically significant differences existed in the maximal VM fiber angle between the groups demonstrating the presence or absence of a VM fascial plane. None of the cadavers possessed an aponeurotic sheet of epimysium anatomically separating the VMO from the VML. CONCLUSIONS: This study supports earlier research reporting a difference in fiber orientation between the proximal and distal VM fibers; however, contrary to statements in published literature, the VMO does not appear to be an anatomically separate structure from the VML inherent throughout the human population. The results of this study do not support the concept that the VMO and VML exist as anatomically separate structures in a sample of human cadavers.