Evolution in response to climate change in the seasonal polyphenism of Colias eurytheme butterflies
Matthew Nielsen
Summary: At the University of North Carolina-Chapel Hill, Matthew Nielsen (NSF-PDRG 1609023) is digitizing images of specimens of a single species (Colias eurytheme) from multiple collections across the US and developing software in python to analyze wing pattern variation among specimens. He will use this data to test for evolutionary changes in C. eurytheme’s seasonal polyphenism in response to the mismatch between photoperiod and temperature caused by climate change.
Description: Seasonality is a major source of environmental variation for many organisms, and has led to the evolution of many types of seasonal plasticity. Seasonal polyphenism is a particularly common form of seasonal plasticity in butterflies in which adult butterflies have different morphs if they develop at different times of year. Determining which seasonal morph to adopt requires the use of an environmental cue to indicate the current season and predict future conditions. Although some organisms use current weather, such as temperature or humidity, as cues, short term variation can make these unreliable predictors of future conditions. As an alternative, many organisms use photoperiod as a reliable indicator of time and year and future temperatures which is fully independent from short term weather variation.
Unfortunately, anthropogenic climate change poses an extra challenge for organisms which use photoperiod as a cue. Photoperiod can be used as a cue for seasonal conditions because of a consistent historical relationship between time of year and temperature. Although humanity is changing many aspects of the environment quite rapidly, including temperature, photoperiod is the one feature of the environment we can’t alter. Thus, contemporary photoperiods no longer predict the same temperatures that they once did, creating a mismatch between the cue (photoperiod) and selective environment (temperature). This would lead organisms to produce the wrong seasonal morph for at least some of the year.
Ventral and dorsal views of the darker spring form and the lighter summer form
My research seeks to determine whether evolution can correct for the mismatch caused by anthropogenic climate change by using Colias eurytheme (the alfalfa butterfly). This widespread, polyvoltine species has a seasonal polyphenism with a known thermoregulatory function. Among other changes, these butterflies have darker ventral wings in cool seasons to allow them to bask more effectively (see pictures). This polyphenism, however, specifically responds to photoperiod, rather than temperature, so climate change should be creating a cue-environment mismatch in this species. Because the cue for this plasticity, photoperiod, hasn’t changed, any change in the time of year when the different morphs are found would reflect an evolutionary change in their response to photoperiod. To test for this change, I am photographing C. eurytheme specimens from natural history collections from the last 60+ years, and will compare their wing patterns to the date they were collected. Specifically, I predict that if C. eurytheme has evolved in response to climate change, the summer form should occur earlier in the spring and later in the fall in years closer to the present, while we would see little or no change if evolution has not been able to keep up with climate change. Given that both photoperiod and temperature also show considerable natural spatial variation, I’m planning to look for this response in multiple parts of the species range, emphasizing variation in latitude.
So far, I have visited the Smithsonian National Museum of Natural History and the McGuire Collection at the Florida Museum of Natural History to photograph specimens. In addition to photographs of about 2,0000 butterflies, this has given me a much better idea of when and where specimens are most available from. Based on this, I’ve decided to focus, at least initially, on three broad latitudinal transects, one for the west coast, one for the east coast, and one for the great plains (Texas to the Dakotas), emphasizing the southern portions of these regions where C. eurytheme is more abundant and found during a greater portion of the year. Temporally, most of these areas are relatively well sampled from the 1940s or 50s through the 1980s or 90, with a few gaps I’m working to fill in. More recent samples, however, are scarce. Moving forward with this project, I’m particularly keen to find samples from the Southeast (North Carolina, South Carolina, and Georgia in particular; specimens from these states are surprisingly rare so far), and anywhere during the last two decades. Any insight into collections where these might be available would be greatly appreciated (my email is maerniel@live.unc.edu).