Causes and consequences of color polymorphism in Rambur’s forktail (Ischnura ramburii)

Abstract

Variation in male and female forms occurs in countless animal taxa, and has fascinated evolutionary biologists since Darwin and Wallace. The underpinnings of male variation have been elucidated in diverse groups; less is known about the selective forces that diversify female forms in nature. Female-polymorphic damselflies provide ideal systems in which to study how female variation evolves. Color polymorphic damselflies typically contain one female morph that resembles the male (“andromorph”) and one or more alternative morphs with distinctive coloration (“gynomorphs” or “heteromorphs”). My thesis draws upon the unique context of a biological invasion to elucidate factors that promote and maintain this variation in female color. Empirical work in my dissertation is focused upon Rambur’s Forktail (Ischnura ramburii), a species native to the Americas that invaded Hawaii in the 1970s. I first examine whether female color morphs diverge in mating rates or other reproductive traits within the native and invasive range, to see whether such traits might affect morph frequency dynamics in the invasion context (Chapter 2). Next, I test whether variation in selective regimes, both across female development and among populations, predicts variation in andromorph coloration (Chapter 3). Upon finding andromorphs to follow predictions of mimicry theory, I ask whether andromorph presence might result in increased male-male interaction rates, due to sex recognition errors (Chapter 4). Finally, I document recent, rapid evolution of andromorphy within Hawaii populations, and conduct mesocosm experiments to test the potential for density- and frequency-dependent selection to promote and maintain color polymorphism. Results indicate 1) andromorphs may benefit from reduced mating, but male-like morphology may also incur reproductive constraints; 2) andromorph color variation accords with mimicry theory: andromorphs resemble syntopic males, and resemblance is maximized after reproductive onset; 3) male-male interactions increase in the presence of andromorphs, to male detriment; 4) gynomorphs are subject to negative-frequencydependence in high-density populations, which may have driven the rapid evolution of andromorphy in Hawaii following introduction to the islands. These findings offer new insights into multiple mechanisms by which color polymorphism can arise and be maintained within native and invasive contexts.