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Dispersal patterns can dictate genetic population structure and, ultimately, population resilience, through maintaining gene flow and genetic diversity. However, geographical landforms, such as peninsulas, can impact dispersal patterns and thus be a barrier to gene flow. Here, we use 13 375 genome‐wide single‐nucleotide polymorphisms (SNPs) to evaluate genetic population structure and infer dispersal patterns of the Western Grasswren Amytornis textilis textilis (WGW, n = 140) in the Shark Bay region of Western Australia. We found high levels of genetic divergence between subpopulations on the mainland (Hamelin) and narrow peninsula (Peron). In addition, we found evidence of further genetic sub‐structuring within the Hamelin subpopulation, with individuals collected from the western and eastern regions of a conservation reserve forming separate genetic clusters. Spatial autocorrelation analysis within each subpopulation revealed significant local‐scale genetic structure up to 35 km at Hamelin and 20 km at Peron. In addition, there was evidence of male philopatry in both subpopulations. Our results suggest a narrow strip of land may be acting as a geographical barrier in the WGW, limiting dispersal between a peninsula and mainland subpopulation. In addition, heterogeneous habitat within Hamelin may be restricting dispersal at the local scale. Furthermore, there is evidence to suggest that the limited gene flow is asymmetrical, with directional dispersal occurring from the bounded peninsula subpopulation to the mainland. This study highlights the genetic structure existing within and between some of the few remaining WGW subpopulations, and shows a need to place equal importance on conservation efforts to maintain them in the future. [ABSTRACT FROM AUTHOR]

Assisted colonisation, the introduction of species beyond their historical range, is increasingly necessary for conserving species. However, empirical evidence of the long-term genetic outcomes of assisted colonisation is grossly lacking. A risk associated with moving species beyond their native range is the possibility of interspecific hybridisation with a closely related species, potentially resulting in outbreeding depression or the genetic swamping of a parental species. Here, we use a combination of genome-wide Single Nucleotide Polymorphism (SNP) markers and mitochondrial DNA sequencing to determine the long-term genetic consequences of introducing the intertidal periwinkle Bembicium vittatum (a direct developer) beyond its native range and into the native range of its congener Bembicium auratum (a species with planktotrophic larval dispersal). We found novel evidence of natural, multigenerational hybridisation between marine invertebrates with different modes of development. Intriguingly, introgression was highly asymmetrical initially, but became more evenly bidirectional as the population became more admixed. There was a significant decline in the frequency of alleles from the introduced B. vittatum over time, providing evidence of genetic swamping. The present study also provides potential evidence of outbreeding depression, in the form of cytonuclear incompatibilities, leading to the observed pattern of asymmetrical introgression. This study reveals the potential for unexpected mixing between species when reproductive barriers are not well understood, resulting in failure of pure B. vittatum to persist at the translocation site, a major concern associated with assisted colonisation. Without long-term genetic monitoring interspecific hybridisation between B. vittatum and B. auratum would have gone undetected, highlighting the importance of long-term monitoring to detect unintentional negative consequences of conservation translocations. Successful assisted colonisation requires an understanding of the potential for interspecific hybridisation between the threatened species and closely related native species, to reduce the risk of adverse outcomes. [ABSTRACT FROM AUTHOR]