Shaped by Isolation: Genetic Load and Purging in NonNative Populations of Wall Lizards (Podarcis muralis)

Abstract

Small populations are prone to inbreeding depression due to the increased likelihood of mating between close relatives, resulting in the expression of deleterious recessive mutations. While this increases the risk of population extinction, high levels of inbreeding can initiate a purging process, where natural selection gradually eliminates strongly deleterious mutations. This purging effect has been observed in various natural populations that experienced bottlenecks and can play a crucial role in population recovery and growth. Purging can also explain the lag phase between establishment and population growth and range expansion in non-native species, since introduced populations are often small and isolated. However, to what extent purging occurs in non-native populations is poorly understood. Here, we study independently introduced populations of the common wall lizard (Podarcis muralis) to examine genetic purging relative to their native sources. We analysed whole-genome data from 12 native populations across France and Italy and 10 non-native populations in England. Our findings indicate that Italian-origin lizards experienced stronger inbreeding and exhibited classic signatures of purging: a reduced frequency of highly deleterious alleles and an increased frequency of mildly deleterious ones. The oldest extant introduction showed the strongest signs of purging, underscoring the importance of time in the process. In contrast, French-origin nonnative populations retain native-like diversity, exhibit moderate levels of inbreeding, and show limited evidence of purging, likely due to weaker bottlenecks and their introduction history. Our results highlight that purging can occur in non-native populations and may influence their population dynamics but also suggest that the likelihood of purging will depend on the severity on time since introduction and likely on the demographic contraction, and effective population size.

Popular Abstract

Shaped by Isolation: Genetic Purge in Wall Lizards

New populations of introduced animals, like wall lizards brought to England, often begin with only a few individuals. This sharp drop in numbers means close relatives may breed (inbreeding), which can expose harmful versions of genes that usually stay hidden. While too much inbreeding can impact their health, it also gives natural selection a chance to weed out the most damaging gene changes over time, a process known as genetic purging. Our study asked whether wall lizards (Podarcis muralis) introduced from France and Italy into small, isolated English sites have experienced this genetic clean-up.

To find out, we sequenced the entire genomes of 32 lizards: Using both individuals from their native ranges in France and Italy, and from six small and isolated English populations. We compared three things in each group: overall genetic variety, signs of inbreeding, and how common the most harmful gene changes were. By looking at these measures side by side, we could tell whether the worst gene variants had become less frequent in the English populations.

The results revealed a clear contrast between the two origins. Lizards of Italian origin in England, especially the population established in Ventnor around 1930 (The oldest in the country), carry far fewer high-impact gene changes than their Italian ancestors, suggesting that selective removal of those harmful variants has taken place over decades. In comparison, English groups of French origin, which arrived more recently and in larger numbers, show almost the same mix of gene versions as their source populations, with only a small drop in harmful variants at one site.

These findings matter for both conservation and invasion biology. They suggest that small, isolated groups can gradually lose some of their most damaging gene changes on their own, which could inform how we breed and relocate endangered species. They also offer a genetic perspective on why some introduced animals seem to stall at first before expanding rapidly; time and small founder sizes may allow harmful gene variants to decline, improving overall health conditions. In this way, isolation isn’t purely a risk: under the right conditions, it can help populations remove the worst gene changes and build a stronger genetic foundation. However, purging cannot be the only solution because it doesn’t solve all the problems. Less harmful gene changes might become more common, and some bad ones might not be eliminated. Looking ahead, understanding other factors in the establishment of new populations and the conservation of endangered ones will be crucial to developing effective strategies.

Master’s Degree Project in Biology, Conservation biology, 60 credits 2025
Department of Biology, Lund University
Advisors: Tobias Uller and Nathalie Feiner
Division of Biodiversity and Evolution

• author: Avila Quintero, Santiago Jose
• supervisor: Tobias Uller; Nathalie Feiner
• organization: Degree Projects in Biology
• course: BION03 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9212757
• date added to LUP: 2025-09-18 14:30:15
• date last changed: 2025-09-18 14:30:15