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PHYLOGENETIC ANALYSIS OF SEXUAL DIMORPHISM AND EYE-SPAN ALLOMETRY IN STALK-EYED FLIES (DIOPSIDAE) 1373
2001 The Society for the Study of Evolution. All rights reserved.
Evolution, 55(7), 2001, pp. 13731385
PHYLOGENETIC ANALYSIS OF SEXUAL DIMORPHISM AND EYE-SPAN
ALLOMETRY IN STALK-EYED FLIES (DIOPSIDAE)
R
ICHARD
H. B
AKER
1,2,3
AND
G
ERALD
S. W
ILKINSON
4
1
Department of Entomology, American Museum of Natural History, 79th Street at Central Park West, New York, New York 10024
2
Department of Biology, Yale University, New Haven, Connecticut 06520
4
Department of Biology, University of Maryland, College Park, Maryland 21742
E-mail: gw10@umail.umd.edu
Abstract.
Eye stalks and their scaling relationship with body size are important features in the mating system of
many diopsid species, and sexual selection is a critical force inuencing the evolution of this exaggerated morphology.
Interspecic variation in eye span suggests there has been signicant evolutionary change in this trait, but a robust
phylogenetic hypothesis is required to determine its rate and direction of change. In this study, the pattern of mor-
phological evolution of eye span is assessed in a phylogenetic framework with respect to its function in the sexual
system of these ies. Specically, we examine within the family Diopsidae the pattern of increase and decrease in
sexual dimorphism, the morphological coevolution of eye span between males and females, and the evolutionary
exibility of eye-span allometry. Based on several different methods for reconstructing morphological change, results
suggest a general pattern of evolutionary exibility, particularly for eye-span allometry. Sexual dimorphism in eye
span has evolved independently at least four times in the family and this trait also has undergone several reductions
within the genus Diasemopsis. Despite most species being dimorphic, there is a strong phylogenetic correlation between
males and females for mean eye span. The coevolution between the sexes for eye-span allometry, however, is sig-
nicantly weaker. Overall, eye-span allometry exhibits signicantly more change on the phylogeny than the other
morphological traits. The evolutionary pattern in eye-span allometry is caused primarily by changes in eye-span
variance. Therefore, this pattern is consistent with recent models that predict a strong relationship between sexual
selection and the variance of ornamental traits and highlights the signicance of eye-span allometry in intersexual
and intrasexual signaling.
Key words.
Evolutionary trends, exaggerated trait variance, independent contrasts, scaling relationships, sexual di-
morphism, sexual selection, static allometry.
Received October 14, 1999.
Accepted March 8, 2001.
Exaggerated morphologies resulting from sexual selection
are a conspicuous male characteristic in many insect species.
These structures often increase male reproductive success by
improving their competitive ability against other males or
their attractiveness to females (Andersson 1994). They also
generally exhibit considerable variation, both within and be-
tween species (Emlen and Nijhout 2000). Given this exten-
sive variation, an understanding of the selective forces shap-
ing exaggerated morphologies requires an accurate descrip-
tion of their pattern of evolutionary change. In addition, it
is critical to examine the degree of exaggeration in these male
morphologies relative to the amount of variation in other
morphological traits. The most commonly used traits for
comparison are the homologous character in females and
male body size. Comparison with females provides a measure
of the sexual dimorphism in the character, and comparison
with body size assesses the allometry or scaling relationships
for the exaggerated character. Comparative analyses need to
examine the interspecic relationship between these traits, as
well as the pattern of evolutionary change in their intraspe-
cic relationships. Here, we conduct this type of analysis,
for stalk-eyed ies in the family Diopsidae, by examining
the pattern of evolutionary change in the sexual dimorphism
and static allometry of eye stalks.
Stalk-eyed ies provide an excellent model system for ex-
3
Present address: The Galton Laboratory, Department of Biol-
ogy, 4 Stephenson Way, University College London, London NW1
2HE, United Kingdom; E-mail: richard.baker@ucl.ac.uk.
amining the evolution of exaggerated morphologies and their
scaling relationships. Diopsid ies are characterized by the
elongation of the head into long stalks, and sexual selection
has been a major force shaping the evolution of these eye
stalks. For at least some species, it has been documented that
eye-stalk evolution is mediated by a breeding system that
involves both male-male competition (Burkhardt and de la
Motte 1983, 1987; Panhuis and Wilkinson 1999) and female
choice (Burkhardt and de la Motte 1988; Wilkinson et al.
1998a; Hingle et al. 2001). In many species that are sexually
dimorphic with respect to eye stalks, males ght for and
defend aggregation sites, such as root hairs or leaf surfaces,
where mating occurs. Both eld and laboratory experiments
have demonstrated that the size of a males eye span affects
his ability to control these mating sites and that females tend
to prefer sites controlled by males with larger eye stalks
(Burkhardt and de la Motte 1988; Burkhardt et al. 1994;
Wilkinson and Reillo 1994; Wilkinson et al. 1998a; Panhuis
and Wilkinson 1999; Hingle et al. 2001). There is consid-
erable interspecic variation within the family in terms of
the size, sexual dimorphism, and allometry of eye stalks
(Burkhardt and de la Motte 1985; Wilkinson and Dodson
1997). Results of a systematic treatment of several diopsid
species (Baker et al. 2001) provides a robust hypothesis of
phylogenetic relationships that can serve as a framework for
examining various comparative questions concerning the
evolution of this exaggerated sexual character. Specically,
we will focus on three questions: (1) What is the pattern of
gains and losses of sexual dimorphism and male eye-span 1374
R. H. BAKER AND G. S. WILKINSON
allometry within the family? (2) What is the pattern of mor-
phological coevolution between males and females? (3) What
is the evolutionary exibility of static allometry?
Pattern of Gains and Losses of Sexual Dimorphism and
Eye-Span Allometry
Because in several diopsid species eye span is sexually
monomorphic, the rst step for any comparative analysis is
to establish the plesiomorphic state for the family and then
determine how often and in what direction that character has
changed. Shillito (1971) suggested that increasing eye-stalk
size was the unifying evolutionary trend within the family
but used a weakly corroborated phylogeny in which eye span
was averaged across each genus. Sexual selection can main-
tain a high degree of additive genetic variance for exagger-
ated characters (Pomiankowski and Møller 1995; Rowe and
Houle 1996), suggesting that these traits have the capacity
to respond continually to directional selection. Therefore, a
trend toward increasing eye span may occur if diopsid mating
systems consistently favor males with more exaggerated eye
span. Alternatively, theoretical analysis of runaway sexual
selection (Iwasa and Pomiankowski 1995) has predicted cy-
clical evolution of exaggerated male characters, a phenom-
enon that would not produce a consistent phylogenetic trend.
In addition, it is important to determine whether change in
sexual dimorphism has resulted primarily from change in
male traits or female traits. The interpretation of the selective
forces operating in the lineage would certainly be altered if
sexual dimorphism was caused as often by a reduction in
female eye span as an increase in male eye span. In a study
on the evolution of plumage dichromatism in blackbirds, Ir-
win (1994) showed that, contrary to expectations, change in
dichromatism resulted more often from a change in female
plumage than male plumage.
Coevolution of Males and Females
The Diopsidae is the only group of hypercephalic ies in
which females also possess prominent head projections. This
pattern suggests that males and females share some of the
genetic mechanisms inuencing eye-stalk morphology and
that pleiotropic effects may limit the extent to which male
and female eye span can evolve independently. Experiments
on Cyrtodiopsis dalmanni have demonstrated a genetic cor-
relation between the sexes, with females exhibiting a cor-
related response to selection on male eye span (Wilkinson
1993). The presence of sexual dimorphism in eye span for
many species, however, also suggests males and females have
different adaptive optima for eye-stalk size. Natural selection
on female eye span may oppose a correlated respon