The growing field of urban evolutionary ecology studies the impact of urbanization on its resident species. Often, that involves comparisons between conspecific urban and non-urban populations. Jackson et al. took a different perspective in this study. They analyzed the divergence between populations living in adjacent but isolated city parks. They developed their study around a theory presented by Littleford-Colquhoun et al. in “Archipelagos of the Anthropocene.” Littleford-Colquhoun posited that city parks were urban analogs to oceanic archipelagos. Both are isolated with limited dispersal and contain much ecological variation despite existing in the same geographical area and climate. There has been much research into adaptive radiation on island systems, most notably Darwin’s finches. City parks offer a new and accessible system in which we can study the rapid evolution of species while continuing to unravel the impacts of urbanization on organisms.
The characteristics of city parks within a single urban area can vary greatly. For instance, most parks are not uniform in vegetation diversity and abundance, human foot traffic, impervious surface distribution, and landscape design. For species with limited dispersal ability, the urban infrastructure separating parks can isolate conspecific populations and prevent gene flow. This study aimed to explore the role of natural selection and plasticity in driving divergence in morphology between isolated populations of Eastern Water Dragons (Intellagama lesueurii) in Brisbane.
Jackson et al. used a statistical method (Qst – Fst) to determine if selection, rather than genetic drift or plasticity, is driving these morphological differences between populations. In this method, Qst represents the change in a quantified trait. Fst represents the change in neutral genes, which do not affect an organism’s fitness. The Qst – Fst method argues that if the calculated result is greater than zero, natural selection is occurring. Such that the divergence of a specific heritable trait is greater than the random neutral changes that constantly occur within an organism’s genome. While if Qst = Fst, the variation between populations is only due to plasticity or genetic drift. Jackson et al. hypothesized that city park populations would have a Qst > Fst, indicating that rapid evolution is occurring in city parks.
City parks and isolated native populations had similar magnitudes of divergence
Jackson et al. analyzed the divergence between pairs of populations using four city park populations and three native riparian populations. The city parks were all located in central Brisbane, with distances between pairs ranging from 0.9 km to 5.1 km. The distance between riparian populations ranged from 15 km to 115 km to prevent genetic connections between populations and ensure isolation. Eastern water dragons in native riparian populations separated by less than 5 km may interact and have gene flow. Thus, any divergence between the populations results from selection, plasticity, or genetic drift.
Jackson et al. evaluated seven traits and found two to be sufficiently heritable: jaw width and snout-vent length (SVL). These two morphological traits were used for the Qst – Fst analysis. Their calculations indicate that selection caused significant divergence between all pairings of native riparian populations and most pairings of city park populations. Only two pairings of city park populations did not have a significant Qst – Fst value. These results support the theory that the ecological variation in urban microhabitats may be driving differentiation between isolated urban populations. Despite the variance in relative isolation, the magnitude of divergence between city park populations was also very similar to the divergence observed between the riparian groups. For species with limited dispersal ability, the urban infrastructure appears to be as isolating as the dozens of kilometers of distance between non-urban populations.
Going forward
Despite the confirmation that selection may diversify isolated urban populations, this is only the first step toward expanding this theory. For eastern water dragons, researchers must investigate what specific environmental factors are impacting their morphology and the functionality of the variation. Jackson et al. speculate that variation in SVL and jaw width may be due to population density or resource availability. It is also critical to confirm whether this phenomenon is unique to Brisbane’s eastern water dragons. Jackson et al. recommend that this experiment be replicated with different species in Brisbane and other cities. While some species are displaced by urban infrastructure, others thrive and adapt. Understanding how organisms respond to urbanization is critical in ensuring their survival as our cities continue to expand.
Read the study
Check out more eastern water dragon research by the Dragon Lab at the University of Queensland!
Featured image: © Andrew Allen, some rights reserved (CC-BY)
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