Microevolution in Invasive Species

Ida Sidik + Julian Balaguer Velazquez

Mentor: Mark Eastburn

Study Background:

The Italian wall lizard, Podarcis sicula, has been a versatile immigrant to various regions of the United States over the past fifty years. Self-sustaining populations have been established in a range of environments, from a compost heap outside of Fenway Park in Boston, MA to a church in Topeka, KS and a police station in Mt. Laurel, NJ. Given that there is at least one documented instance of rapid evolution within this species, this series of observations within the lizards’ natural habitat, which include measurements that were made without any disturbance of individuals or their environment, investigates the hypothesis that microevolution has occurred among the different populations as a consequence of adaptation to harsher winters across their introduced range. Those lizards living in climates with the lowest average temperatures in December and January are predicted to be smaller than their more temperate counterparts, as was supported by data gathered from the Topeka population. Though a variety of metrics were attempted, the methods used were only able to obtain very preliminary results. No genetic or behavioral data was collected due to ethical restrictions, but the data do warrant more intensive investigation by an institution with the appropriate approvals.


Lizards are among the animal groups that have shown rapid adaptation to new environmental pressures within a small number of generations. This includes parallel evolution of lizards in the genus Anolis across several Caribbean islands (Losos, 2011) the development of larger adhesive toepads in response to hurricanes (Donihue et al., 2020), and immune responses among fence lizards (Sceloporus undulatus) that have faced an invasion of fire ants (Tylan et al., 2020). Among European populations, the Italian wall lizard, Podarcis sicula, was shown to undergo rapid evolutionary changes in gut morphology, behavior, and head size within 36 years upon introduction to a new island off the coast of Croatia (Herrel et al., 2008). A related species, the common lizard, Podarcis muralis, has demonstrated extreme adaptability when introduced to new sites across the European continent, after they had presumably been rendered extinct from a variety of locations during the last Ice Age (Schulte et al., 2012).

While P. muralis may also be found in North America, primarily in and around the city of

Cincinnati on the border between Ohio and Kentucky, and evidence shows evolutionary trends in thermal tolerance (Litmer & Murray, 2019), it is the Italian wall lizard, Podarcis sicula, that has the widest range. Populations have been reported as far north as Boston, MA, Vancouver, BC, Seattle, WA, and Garden City and West Hempstead, NY. Southerly locations where P. sicula may be found are Orange County, CA, Houston, TX, and Leesburg, VA. Between these extremes are populations in Mt. Laurel, NJ and Topeka, KS, though this latter population potentially faces the greatest temperature variations throughout the year. Add to these known and documented populations a recently-discovered group in Princeton, NJ, which served as the nexus for generating my interest to learn more about this species.

In the locations where this lizard has been reported, they appear to thrive best in environments that have been modified by humans. Urban settings are where the largest populations may be found, particularly where there are buildings and concrete structures that provide ample locations to hide. For example, the Topeka population for this study was primarily observed at the First Christian Church on SW Gage Blvd, while the Mt. Laurel population was largely seen outside of the Mt. Laurel Community Center and the nearby Mt. Laurel Police Station. It is thought that, due to the far more temperate climates where these lizards originated, these introduced populations (aside from those at the lowest latitudes) can only survive in areas where they can enter cracks in heated buildings where they can escape from freezing through the winter cold. Areas where the Italian wall lizard are known to exist in their native, European range have wintertime temperatures that rarely drop below freezing – Sicily has average low temperatures between 8oC and 11oC in December and January, while Naples experiences temperatures between 4oC and 6oC during that same time. In northern Italy, such as the city of Milan, temperatures dip slightly lower, often reaching an average of -1oC. Still, this is relatively higher than the lowest average temperature of -5oC in Boston during the months of December and January, and the even lower -7oC in Topeka. For reference, all of these data were gathered from The Weather Channel’s website at https://www.weather.com.

On occasion, temperatures may drop even lower in these new habitats, and self-sustaining populations must find ways to survive. Long-term exposure to freezing temperatures is lethal to these little reptiles (Burke et al., 2002), therefore they must find ways to escape. For this reason, it is hypothesized that lizards that must endure the lowest winter temperatures, such as the population in Topeka KS, will tend to have the smallest maximum sizes, while those from more

temperate regions will be larger because there is less of a selective pressure to squeeze as tightly into heated buildings where temperatures in the walls or floors may remain above the freezing point of water and bodily fluids.


Photographic journeys were taken to four of the locations where Podarcis sicula is known to have established new habitats, though all of these are very close to human habitations. These included Riverside Elementary School in Princeton, New Jersey, the Mount Laurel Community Center (and nearby police station) in Mt. Laurel, New Jersey, the Long Island Railroad Station in Old Westbury, New York, and the First Christian Church in Topeka, Kansas. All of these locations were public property except for the last location, where verbal permission was given. For all observations, a Nikon DSLR 3300 camera with a telephoto lens was used.

Lizards were photographed in each of these four sites over a period of several days, both during the spring and summer of 2021 and 2022, and were documented by the spot where they were located. During observations it was quickly noted that lizards would not remain in the same basking spot for long periods of time–particularly in the heat of summer–so it was easy to photograph a lizard, mark the spot where the photograph was taken, and then return to this same point with a 30-cm (one foot) ruler that was then photographed at the vacated basking site to compare the size of the lizard with the ruler and infer the snout-vent length (SVL). After this second photograph was taken, the ruler was removed, and the habitats were not altered or modified in any way. In total, more than 400 lizards were photographed–154 in Princeton, 72 in Mt. Laurel, 38 in Old Westbury, and 141 in Topeka.

After an initial round of photographic analysis, it became clear that I had gathered images from a wide range of sizes that included many juveniles. This would be a confounding variable–especially in the case of photo surveys that were taken later in the summer months, when hatchlings also emerged. For this reason, I only subjected the five largest males and five largest females from each location to statistical analysis, because it was hypothesized that these would represent the maximum size that a lizard could grow in each habitat, which was the focus of this study. These snout-vent lengths (SVLs) were then subject to statistical analysis by a one-way analysis of variance (ANOVA).

Mean values for male lizard measurements are as follows: Princeton 72.2 mm, Mt. Laurel 77.6 mm, Old Westbury 77.2 mm, and Topeka 66.4 mm. The standard deviation for Princeton is 2.86 mm, for Mt. Laurel it is 3.98 mm, for Old Westbury it is 1.92 mm, and for Topeka it is 2.88 mm.

Mean values for female lizard measurements are as follows: Princeton 74 mm, Mt. Laurel 73.2 mm, Old Westbury 70.4 mm, and Topeka 65.6 mm. The standard deviation for Princeton is 7.04 mm, for Mt. Laurel 0.447 mm, for Old Westbury 4.88 mm, and for Topeka 3.05 mm.

When analyzed through one-way ANOVA, only the size of the male Topeka lizards was found to be significantly smaller than the others, with a p value of 0.001 when comparing Mt. Laurel and Old Westbury lizards with the Topeka population, while the p value of the difference between the Princeton population and the Topeka population was 0.034.

Comparing female lizards, all size differences were found to be insignificant between locations except for the Princeton population vs. the Topeka population, which was found to be 0.445.



The preliminary data gathered confirms the hypothesis that the location with the coldest average winter low temperatures, which is Topeka, has male lizards that are the smallest in size. The female sizes are not necessarily significant, particularly because the difference between the Princeton and Topeka populations are likely due to one female of extremely large size. When viewed in the context of winter low temperatures, this makes sense. Average low temperatures in most locations are not much below the freezing point of water, and this might have a buffering

effect on the way that these lizards are able to survive. Because the freezing point of salt water is much lower than fresh water, and because solutes dissolved in water cause freezing point depression, it might be possible that male lizards that are exposed to the lowest average temperatures in locations like Old Westbury (-3oC in January) and Mt. Laurel (also -3oC) are able to avoid freezing while Topeka at -7oC will be too cold for a lizard that is unable to bury itself deep within the cracks of cement between buildings or in the sidewalk and therefore only the smallest adult male individuals are able to survive. Interestingly, Princeton has an average low temperature of -5oC in January, which is intermediate between the two warmer locations and Topeka, and this is the population where the statistical difference in sizes was not quite as significant. It is easy to imagine that a variety of lizards live in each of these locations, and when it gets very cold the individuals who are able to tunnel deepest are the least likely to freeze while larger individuals might be killed off and be unable to pass their genes to the next generation. This would be the easiest way to explain the differences that I observed. The fact that female lizards, which tend to be smaller, do not show this trend is also supportive of the hypothesis, because their average sizes fall within the same range as the smaller males and would therefore have a greater chance of surviving.

Another possibility is that the difference in sizes between these populations is due to different origins of populations across there range in Italy, where these lizards may have adapted to different sizes because of predators or prey. The variation in colorations between populations across North America does suggest that all of these lizards may not be closely related. The only way to resolve this possibility would be to conduct genetic analysis on individuals from these different populations, which is beyond the scope of this research because lizards would need to be captured and their DNA extracted by obtaining a tissue sample or possibly a dropped tail–which is a common defense mechanism in these lizards. There will also be the need for more data collection of other body features like leg length and head width, as these features have been shown to vary in previous studies (Herrell et al., 2008). Further research should also attempt to track these lizards and determine their preferred wintertime hibernating spots.


Observational data of Podarcis sicula in its natural habitat among introduced populations in New Jersey, New York, and Topeka support the hypothesis that male lizards in locations where average winter temperatures are colder tend to have a smaller average body size. This may

be an adaptation to colder climates because lizards in these habitats will need to burrow deeper into cement crevices and walls where they live in urban settings in order to keep from freezing.

About the Scientists

Ida Sidik

Julian Balaguer Velazquez


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