Butterfly Wing Pattern Morphology.
E. J. G.
May 27, 2004
Biology 454; Term Paper
The study of butterfly wing pattern morphology has been critical to our understanding of the
developmental mechanisms involved in morphological evolution. The modes by which this
morphological variation takes place are often initiated by seasonal, environmental, and
geographical features of the individual’s environment. In particular we will discuss how
variation of color patterns can affect the fitness and survivability of species, the effects of
ecdysteroids on the maturing pupae, and the process of mimicry.
Introduction
Butterflies are amongst the most visually stunning and unique organisms known to man, with
approximately 17,000 known species each of which can be distinguished by its intricate wing
color patterns (Caroll et al., 1994). These wing patterns are different from the patterns found on
other animals (such as zebras, leopards and lionfish) in that they lack the randomness.
While stripes and patterns of other animals are unique to each individual like the fingerprint of a
human, the individual elements of the butterfly wing patterns can be found in the same place in
all individuals of a species and even across genera (Nijhout, 2001a).
Butterfly wings are comprised of a forewing and aft wing with ventral and dorsal surfaces often
displaying different patterns. These patterns are based on several systems of symmetry and in
most species consist of serial repetition of similar pattern elements within individual wing cells
(Carol et al., 1994). The systems of symmetry can be broken down into three distinct systems
known as the basal symmetry system, the central symmetry system and the border symmetry
system (Nijhout, 2001a). Each system is characterized by a pair of parallel pigmented bands that
run from the anterior to the posterior of each wing surface. The bands are interrupted by
crossing wing veins giving a segmented appearance of individual wing cells. Wing veination
patterns are remarkably constant within taxon resulting in an equal number of wing cells for all
species of butterflies (Nijhout, 2001a). Small pigmented marks are often found within the border
and central symmetry systems that add another dimension to the possible pattern variation.
Change in wing patterns is determined by hormonal control the initiation of which occurs
through several different modes of selection including seasonal polyphenism, mimicry and
sexual selection. Each of the pattern elements that comprise the wings distinctive color patterns
has evolved the ability to change independent of the other elements. The diverse pattern
variation resulting from this independent change provides scientists with an opportunity to study
the morphological evolution as well as its developmental mechanisms of an incredibly diverse
taxon of which essentially all species are known (Nijhout, 1991).
H. Frederick Nijhout, a Zoology professor at Duke University, is the world’s foremost expert on
the evolution and development of butterfly wing pattern morphology which he has been studying
for over thirty years. Building on the findings of Schwanwitsch, Suffert and Kuhn, he has made
great advances in our understanding of the evolutionary patterning mechanisms and what is
commonly refers to as the nymphalid ground plan. Through his research with the genetics of
mimicry and pattern formation, symmetry system hierarchy, and the evolution of serial
homology, we have been able to identify the specific modes of evolutionary pattern development
that produces the many species-specific patterns found in butterflies.
Results
The individuated color pattern elements of butterfly wings have evolved from developmentally
autonomous, compartmentalized cells within the wings leading to the emergence of serial
homology as well as the release from developmental correlation of individual pattern elements
(Nijhout, 2001b). It is this loss of developmental correlation that allows the individual butterflies
to change one pattern element without affecting another, in turn leading to the great diversity of
available patterns we see today.
Just as the appearance of the environment can change with the seasons, so to can the appearance
of butterfly wing patterns, a phenomenon known as seasonal polyphenism. Many species of
butterfly exhibit this form of variation through varying levels of wing melanization in different
seasons (Brakefield, 1999). Kingsolver has shown that variation of color can lead to increased
relative fitness within Pontia occidentalis, a North American species. In his experiments,
Kingsolver (1995a, b, 1996) found that those individuals with darker wing patterns exhibited a
higher level of fitness under cooler conditions, where as the opposite was true as the temperature
increased.
In separate experiments with Precis coenia, Nijhout showed that day length can also lead to
selection for specific color patterns. Animals reared under long-day conditions developed a
much lighter tan pigmentation (known as the linea form) when compared with the dark reddish-
brown coloration (the rosa form) of those reared under short-day conditions. The cause of this
variation was determined to be the presence of the pigments xanthommatin in the linea form and
dihydro-xanthommatin and ommatin-D in the rosa form. Concentrations of each pigment were
then extracted from the wings using an acid methanol solution. The pigments were then
analyzed using Thin Layer Chromotography (TLC) and spectroscopy. The presence of each
pigment was found to be under control of a developmental biochemical switch that is
programmed by the timed release of ecdysteroids during the onset of adult development
(Nijhout, 1997). Rountree and Nijhout (1995) discovered that the presence of the ecdysteroids
between 24 and 48 hours following pupation would lead to conditions favoring the development
of the light tan or linea form while an absence of ecdysteroids during this period produces
individuals of the reddish-brown or rosa form (Nijhout, 1997).
Aside from thermoregulation, seasonal polyphenism plays a critical role in the effectiveness of
the tropical satyrine butterflies ability to avoid predation. Since most of these species live close
to the ground, it is imperative that they be able to blend in with the seasonal vegetation and leaf
liter to avoid would-be predators. During the dry season when the butterflies experience long
periods of inactivity, it has been observed that individuals lose many of the conspicuous ventral
wing markings such as the marginal eyespots and medial bands that are prominent in the wet
season forms (Brakefield, 1999). These medial bands and large eyespots are thought to serve as
camouflage by helping to disrupt the outline of the wing when the wing is closed and the
butterfly is perched among the leaves and foliage. In the dry season however these traits are
thought to attract predatory birds that may be searching for food on the ground and so selection
works against them (Brakefield, 1997). To further demonstrate the importance of this variation,
Brakefield (1999) released butterflies of the wet season form and dry season forms with paintedon eye spots during the dry season and recorded markedly lower rates of survival for both groups
demonstrating the effects of seasonal pattern composition on an individual’s fitness.
Mimicry is another form of adaptation undertaken by butterflies and a direct result of their wing
pattern morphology. Turner (1984) stated that Batesian and Mullerian mimicry both begin with
a single mutation of large effect resulting in convergence on an intermediate pattern by a lessprotected species with a better-protected one (Nijhout, 1991). Many species such as the Papilio
dardanus found in sub-Saharan Africa, are polymorphic and have evolved the ability to mimic
between four and six different models of unpalatable species. Control of this polymorphism has
been identified as a single genetic locus containing 10 alleles of which four are involved in
mimicry (Ford, 1936, Clark and Sheppard, 1959, 1960). The significance of the remaining 6
alleles not utilized in mimicry has not yet been identified (Nijhout, 1991).
Conclusion
The evolution of butterfly wing patterns has been extensively studied and has contributed a great
deal to our understanding of morphological evolution and development. I believe that the
evolutionary development of butterfly wing patterns is a testament to the effects and importance
of evolutionary change in maintaining species survival. Had butterflies not evolved the ability to
independently vary their pattern elements they probably would not exist today. The ability of
each pattern element to vary independently of the others is the key factor in the development of
the wide range of patterns and adaptations on which the fitness and survival of butterflies
depends.
Butterflies are ideal test subjects in the area of morphological evolution because the are a large
group of more than 17,000 known species the majority of which are morphologically similar
(Caroll et al., 1994). By studying monophyletic groups of morphologically similar species, it is
possible to deduce the path of evolutionary development through the discovery of model
parameters representative of specific pattern development (Nijhout, 1991). Once the parameters
have been determined, we could experimentally make small changes in the parameters to see if
the forms of related species are produced. Groups of parameters can then be used to identify
each species and to determine the morphology of their ancestor.
Based on the literature I reviewed, I believe that our understanding of butterfly wing pattern
morphology is extensive albeit incomplete. Due to the importance of understanding evolutionary
processes, I feel that research in this area should remain a primary focus of evolutionary
biologists as well as entomologists. It would seems that due to the popularity of butterflies
within the general public, it would be easier to gain the backing and financial support required to
complete such research. This popularity should be exploited for the good of the scientific
community and to further our understanding of evolutionary development.
References Cited
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