Biology subjects

Brawn, J. D.

Publications and source records attributed to Brawn, J. D..

3 recordsLinked to original sources

bioRxiv2026

Can the Z:A ratio serve as a genomic index of sexual selection? An assessment across space and time in a genus of lekking birds

Understanding the role of sexual selection in shaping evolutionary trajectories is a key goal of biology, and thus developing a genomic measure of sexual selection strength is desirable. In systems with ZW sex determination, the ratio of nucleotide diversity ({pi}) on the male-biased Z chromosome to that on the autosomes (the Z:A ratio) is expected to decline as male effective population size decreases under strong sexual selection, offering one such measure. However, non-equilibrium processes other than sexual selection (e.g., hybridization, changes in population size) can influence levels of Z-linked diversity relative to the rest of the genome, complicating interpretation of this metric and necessitating a systematic evaluation of how Z:A ratios vary across known demographic and selective circumstances. We leveraged longitudinal sampling over a [~]25-year period in Manacus manakin populations in western Panama, spanning a mainland transect (including M. candei, M. vitellinus, and their hybrid zone) and insular populations of the Bocas del Toro Archipelago. On the mainland, Z:A ratios were temporally stable within populations, but varied across populations: (1) Z:A ratios were lower in M. vitellinus than M. candei, likely due to stronger sexual selection in the former; and (2) Z:A ratios increased at the hybrid zone center, likely due to fast-Z effects in parentals inflating Z:A ratios in recent hybrids. Island populations exhibited significantly lower Z:A ratios than mainland populations, consistent with theoretical expectations of Z:A reductions following population contractions as the islands became isolated. Insular Z:A ratios increased with proximity to other landmasses, suggesting gene flow mitigates these effects in this system. While the Z:A ratio behaved predictably in relation to known non-equilibrium processes, our results demonstrate the potentially strong effects of demographic history and admixture on Z-linked diversity, which should be considered in studies aiming to use the Z:A ratio as a measure of sexual selection.

evolutionary biology↗

bioRxiv2025

Reduced offspring viability is associated with long-term stability of a narrow avian hybrid zone

Fitness of hybrid individuals can shape the dynamics of hybrid zones and offer insight into speciation processes. Yet, accounts of hybrid fitness in natural hybrid zones are few, especially from tropical regions where species diversity is high, and speciation processes could contrast with those at higher latitudes. We investigated a hybrid zone between the white-collared manakin (Manacus candei) and the golden-collared manakin (M. vitellinus), two lek-breeding species characteristic of lowland forest habitat in Central America. Despite evidence of asymmetrical introgression and selection on male secondary sexual traits, ongoing sampling indicates that this hybrid zone is spatially stable with narrow clines, thus implying selection against hybrids. To evaluate hybrid viability, we estimated two components of hybrid fitness: survival of adults and egg hatching rates, and a possible selective pressure: prevalence of parasitism by vector-borne haemosporidian parasites. Estimated survival was similar between parental and hybrid populations and the prevalence of infections by Plasmodium spp. or Haemoproteus spp. parasites was uniformly low. Estimated rates of hatching success, however, were lower in nests from our hybrid population (one or two eggs failed to hatch in 70% of nests) compared to nests at the parental species (M. candei 28.6% and M. vitellinus 19.0%). Thus, despite extensive admixture and clear evidence of introgression of male plumage traits under sexual selection, partial infertility or elevated rates of developmental mortality in hybrid offspring may underlie long-term stability in this hybrid zone.

evolutionary biology↗

bioRxiv2023

Ongoing introgression of a secondary sexual plumage trait in a stable avian hybrid zone

AO_SCPLOWBSTRACTC_SCPLOWHybrid zones are dynamic systems where natural selection, sexual selection, and other evolutionary forces can act on reshuffled combinations of distinct genomes. The movement of hybrid zones, individual traits, or both are of particular interest for understanding the interplay between selective processes. In a hybrid zone involving two lek-breeding birds, secondary sexual plumage traits of Manacus vitellinus, including bright yellow collar and olive belly color, have introgressed asymmetrically [~]50 km across the genomic center of the zone into populations more genetically similar to Manacus candei. Males with yellow collars are preferred by females and are more aggressive than parental M. candei, suggesting that sexual selection was responsible for the introgression of male traits. We assessed the spatial and temporal dynamics of this hybrid zone using historical (1989 - 1994) and contemporary (2017 - 2020) transect samples to survey both morphological and genetic variation. Genome-wide SNP data and several male phenotypic traits show that the genomic center of the zone has remained spatially stable, whereas the olive belly color of male M. vitellinus has continued to introgress over this time period. Our data suggest that sexual selection can continue to shape phenotypes dynamically, independent of a stable genomic transition between species.

evolutionary biology↗