Female coloration is highly variable within many animal species. What maintains this diversity, and how can it inform our general understanding of evolution? While theory suggests that both sexual and natural selection might diversify female color, the role(s) of these mechanisms in female evolution have not been jointly tested within a single study system. Invasive populations of a damselfly, Ischnura ramburii provide unique opportunities to close this gap. Frequencies of I. ramburii female color morphs have changed dramatically since the colonization of Hawaii. Today, the frequencies of ?andromorph? females, which visually resemble males, are correlated with both invasive population densities and rainfall. Using experimental mesocosms constructed within invaded sites, the investigators will examine how male-female interactions and color morph performance (survival and fecundity) are influenced by morph frequency, by population density, and by the abiotic environment (rainfall). Outcomes will be used to determine how social and physical environments drive female color evolution, and to identify conditions that promote female diversity.
In addition to identifying factors that maintain female variation in nature, the study will shed new light on how invasive populations evolve, suggesting novel strategies for mitigating invasions. Proposed work will provide unique research opportunities for several undergraduates, and findings will be shared with the public via the media, and through a series of highly successful outreach talks that the investigators co-founded in 2008.
What maintains female polymorphism in nature? Theory suggests that both male-female interaction and natural selection could promote female color variation. Our work takes a new approach to understanding how female variability evolves, by investigating the factors that influence the frequencies of genetically determined color morphs among invasive populations of a damselfly. Hawaiian populations of Ramburâ€™s forktail exhibit pronounced spatiotemporal variation in the frequencies of two alternative female color morphs. We tested whether harassment from males (sexual conflict) and/or environmental heterogeneity (natural selection) could explain observed morph frequency dynamics. Previous work indicated that costly harassment from mate-searching males might promote the coexistence of female morphs. Specifically, the morph with male-like coloration ("andromorph") is commonly hypothesized to mimic males, but mimicry may be ineffective when andromorphs become frequent, and/or in low-density populations (in which male-female interactions are comparatively infrequent). If mimicry is costly, the alternative morph ("gynomorph") may therefor have an advantage over andromorphs at low density. Thus, density and/or frequency-dependent harassment might explain color morph persistence throughout the native and invasive ranges of Ramburâ€™s forktail. We established experimental populations in mesocosms, and assessed how male interactions with each female morph were affected by morph frequency and population density. We also tested if/how these factors influenced morph survival and fecundity. We discovered that both harassment and the relative performances of female morphs are both frequency- and density-dependent. The interaction between these factors can help explain both the historical trajectory, and contemporary distribution of color morphs within Hawaiian populations. Thus, we have shown how frequency and density-dependent processes can promote female variation in nature. Our work also reveals how conflict between males and females can drive evolutionary change within the context of biotic invasions, a result that will be of interest to evolutionary biologists, as well as managers seeking to mitigate invasive spread. A diverse group of undergraduate researchers (two female, three male, one African-American) took part in project-related work; two were coauthors on resulting manuscripts (Huckabee, Etheredge), and three continue to work in evolutionary ecology research. The work was presented at national and international meetings (Society for the Study of Evolution, International Society of Behavioral Ecology), and is reported in multiple scientific manuscripts (two are published, with Gering as coauthor; three are in review, with Gering as lead author; Cummings is senior author on two).