My research questions are centered on three themes: 1) adaptive evolution, or how environmental variation produces diversity among species or between sexes, 2) comparative methods for the study of adaptation using interspecific data and a phylogeny, and 3) understanding the mechanistic basis for adaptation through functional morphology. Students wishing to work in my lab are free to work on research questions closely allied to mine or not. In either case, students are expected to take the lead in developing projects independently, consulting with me as needed.
Comparing species which differ in ecologically interesting ways is fundamental to studying adaptive evolution. Modern biologists do such comparisons while taking the species pedigree, or phylogeny into account. Now we can explicitly model the adaptive scenario that we suspect to be operative. Check out our project page:
(*Ornstein-Uhlenbeck for Comparative Hypotheses)
Anolis lineatopus, a high size-dimorphism species. Female(l).
Anolis pulchellus, an intermediate size-dimorphism species.
Adaptive patterns affect sexual divergence
Habitat variation has strong and evolutionarily repeated effects on Anolis life history, determining such traits as foraging mode, locomotor strategy, and display behavior. I demonstrated that sexual dimorphism in both body size and body shape are also strongly correlated with habitat type. This is the first demonstration of an organizing role for habitat on body shape dimorphism. I found that males and females have a qualitatively different relationship between habitat and morphology. This is particularly significant in light of the functional comparative studies conducted on males (only), relating structural habitat, body shape (particularly in limb proportions), and locomotor performance. Thus, there is some fundamental difference between the way that male and female morphology evolves in response to habitat. Moreover, particular habitats appear to promote greater divergence between the sexes than others. I found correlative support for the role of sexual selection as an explanatory factor. I am currently working on the role of reproductive constraints acting on females which may impose a tradeoff with locomotor adaptation.
Sexual dimorphism increases adaptive divergence
Patterns of interspecific divergence are almost always described in terms of one sex only, despite the fact that sexual dimorphism is known from such classic examples of adaptive radiation such as East African cichlid fishes and Hawaiian honey creepers. My study was the first to explicitly investigate the contribution of sexual dimorphism to adaptive divergence. The pattern of morphological diversity is more complex when females are included in the analysis. Some of
the habitat types are equally well defined by either males or females. However, some habitat types form different clusters for males and females in multivariate morphospace. These females are more similar to females of other species sharing the same habitat type than they are to their corresponding males (and similarly for males). This is extremely strong convergent evolution, as members of the same habitat type are not necessarily closely related. Moreover, I demonstrated that including females into the study increased significantly the diversity within the clade both in terms of species packing and by increasing the overall extent of morphological disparity. Ignoring sexual variation would thus have greatly underestimated the diversity exhibited by this group as well as missing novel morphological types.
A question that arose from
my dissertation work is: do females adapt in a different way? And if so, what
does this reveal about biological constraints? Lizards are commonly used as
models for locomotor adaptation. However, there is very little known about the
physiological and biomechanical changes to locomotion that occur when females
are pregnant (or gravid). Many species of terrestrial vertebrates have very
large reproductive loads, and seem obviously encumbered, but we do not know
which of the several potential effects are most limiting. I am currently using
Iguana iguana to assign the changes to one of several potential causes:
1) reduced lung volume limiting ventilation and ultimately respiration (since
eggs compete with lungs for space), 2) increased load which alters locomotion
(kinetics, biomechanics and energetics), and 3) impaired axial muscle function
(since the body wall musculature may be significantly stretched and no longer
able to perform normal roles in locomotion and ventilation). Because many aspects
of the lizard body plan are thought to be primitive for terrestrial vertebrates,
this study may reveal important biological constraints which have influenced
the design of the vertebrate body plan.
|Click on image to view a quicktime movie of a fully gravid iguana running on a racetrack. She was carrying 53 eggs weighing approximately 910 g or 35% of her gravid body weight (50% of her post-gravid weight!). Use your browser's "back" button to return after viewing.|
|The same iguana approximately four weeks after laying eggs. Click on image to view movie, and use your browser's "back" button to return.|
I am studying color vision evolution in an adaptive radiation of Megalagrion damselflies. Megalagrion species are closely related but have extensive interspecific color variation and striking sexual dichromatism. Additionally, this small genus is as ecologically disparate as the damselfly suborder considered as a whole, yet may have evolved in as little as 5 million years. In collaboration with the Arikawa lab at Yokohama City University, we are employing techniques in visual physiology (electroretinography and intracellular recordings) and molecular cloning of the visual pigment genes (opsins). I will use this data to relate structural variation to function; investigate whether shifts in habitat and body coloration have occurred simultaneously with visual system evolution; and whether the opsin classes differ in the degree of evolutionary specialization. This study is currently in progress.
Intraspecific variation in body color of M. hawaiiense: typical male color, typical female color, windward Haleakala population male color, south east Koolau population male color.
M. nigrohamatum nigrohamatum male, M. n. nigrolineatum male, female. Note bi-colored eyes. All photos from Hawaiian Damselflies (Polhemus and Asquith 1996).
Department of Zoology
University of Hawaii at Manoa
2538 McCarthy Mall, Edmondson 451
Honolulu, HI 96822
Email: mbutler <at> hawaii <dot> edu