Lewis G. Spurgin

Small, isolated populations are often characterised by low levels of genetic diversity. This can result in inbreeding depression and reduced capacity to adapt to changes in the environment, and therefore higher risk of extinction. However, sometimes these populations can be rescued if allowed to increase in size or if migrants enter, bringing in new allelic variation and thus increasing genetic diversity. This study uses experimental manipulation of population size and migration to quantify their effects on fitness in a challenging environment to better understand genetic rescue. Using small, replicated populations of Tribolium castaneum experimentally evolved to different temperature regimes we tested genetic and demographic rescue, by performing large‐scale manipulations of population size and migration and examining fitness consequences over multiple generations. We measured fitness in high temperature (38°C) thermal lines maintained at their usual ‘small’ population size of N = 100 individuals, and with ‘large’ scaled up duplicates containing N ≈10,000 individuals. We compared these large lines with and without migration ( m = 0.1) for 10 generations. Additionally, we assessed the effects of outcrossing at an individual level, by comparing fitness of hybrid (thermal line × stock) offspring with within‐line crosses. We found that, at the population level, a rapid increase in the number of individuals in the population resulted in reduced fitness (represented by reproductive output and survival through heatwave conditions), regardless of migration. However, at an individual level, the hybrid offspring of migrants with native individuals generally demonstrated increased longevity in high temperature conditions compared with individuals from thermal selection lines. Overall, these populations showed no evidence that demographic manipulations led to genetic or evolutionary rescue. Following the effects of migration in individuals over several generations may be the next step in unravelling these conflicting results. We discuss these findings in the context of conservation intervention.

Patterns of pathogen prevalence are, at least partially, the result of coevolutionary host–pathogen interactions. Thus, exploring the distribution of host genetic variation in relation to infection by a pathogen within and across populations can provide important insights into mechanisms of host defence and adaptation. Here, we use a landscape genomics approach (Bayenv) in conjunction with genome‐wide data (ddRADseq) to test for associations between avian malaria ( Plasmodium ) prevalence and host genetic variation across 13 populations of the island endemic Berthelot's pipit ( Anthus berthelotii ). Considerable and consistent spatial heterogeneity in malaria prevalence was observed among populations over a period of 15 years. The prevalence of malaria infection was also strongly positively correlated with pox ( Avipoxvirus ) prevalence. Multiple host loci showed significant associations with malaria prevalence after controlling for genome‐wide neutral genetic structure. These sites were located near to or within genes linked to metabolism, stress response, transcriptional regulation, complement activity and the inflammatory response, many previously implicated in vertebrate responses to malarial infection. Our findings identify diverse genes – not just limited to the immune system – that may be involved in host protection against malaria and suggest that spatially variable pathogen pressure may be an important evolutionary driver of genetic divergence among wild animal populations, such as Berthelot's pipit. Furthermore, our data indicate that spatio‐temporal variation in multiple different pathogens (e.g. malaria and pox in this case) may have to be studied together to develop a more holistic understanding of host pathogen‐mediated evolution.