Why Sensory Ecology Needs to Become More Evolutionary
23
that could freely vary all its traits in all directions, and its history seemed to be of
minor importance. Some physiologists, in fact, dismissed the possibility of evolutionary constraints entirely: " .... there is also the possibility that the evolutionary
history of a species might be important in determining its visual pigments ....
but taking a broader view . . . . the evolutionary explanation is not helpful."
(Lythgoe 1972). Quite often, this view has led workers to detect biological
adaptations where, in fact, there were none (see Chittka and Dornhaus 1999 for
examples).
To test if flower signals drove the evolution of bee color vision, it must be
shown that the ancestors of bees possessed different sets of color receptors prior
to the advent of the flowering plants. To this end, we have to evaluate members of
arthropod taxa whose evolutionary lineages diverged from those of bees before
there were flowers. A phylogenetic analysis reveals that the values of peak
sensitivity in the Crustacea and Insecta fall into three distinct clusters around 350,
440 and 520 nm (Chittka 1996b). The few insect species in which one of the three
types is absent (Periplaneta and Myrmecia) represent secondary losses. Red
receptors show up irregularly in both the Crustacea and Insecta; they have evolved
several times convergently. The photoreceptor wavelength positions of UV, blue,
and green receptors are surprisingly conserved in the Mandibulata. We conclude
that the Cambrian ancestors of extant insects and crustaceans possessed UV, blue,
and green receptors. Insects were well preadapted for flower color coding more
than 500 million years ago, about 400 million years before the extensive
radiation of the flowering plants that started in the mid-Cretaceous (I 00 million
years ago).
To be sure, there are differences of 20-30 nm between the measured peak
sensitivities within each cluster of insect color receptors. This means that our
analysis does leave open the possibility of fine-tuning of receptors to particular
visual tasks in each species. For example, microspectrophotometry reveals that the
long wave pigment of fireflies differs by 12 nm in "-max between nocturnal and
crepuscular species, a difference which can be explained by the specific
requirements posed by detecting and identifying conspecific flashes under
different visual conditions (Cronin et al. 2000). Unfortunately, however, the data
in many other studies were collected by electrophysiological measurements and
are therefore noisy, so we do not yet understand whether the differences between
many species can be attributed to measurement error, different methods, or actual
variation between species.
At first sight, however, considering the large variety of light habitats and
feeding habits of insects, it is surprising how little variation there is. If traits are
conserved within a taxon, although we have reasonable grounds to predict that
they should differ on the basis of optimality arguments, then we must also
take the possibility of phylogenetic constraint seriously. Similar phylogenetic
studies performed on other sensory traits, or on color vision systems in other
animals, may reveal a different pattern; but it is through phylogenetic studies that
we can decipher patterns of adaptation and constraint, convergence, and
homology.
23
that could freely vary all its traits in all directions, and its history seemed to be of
minor importance. Some physiologists, in fact, dismissed the possibility of evolutionary constraints entirely: " .... there is also the possibility that the evolutionary
history of a species might be important in determining its visual pigments ....
but taking a broader view . . . . the evolutionary explanation is not helpful."
(Lythgoe 1972). Quite often, this view has led workers to detect biological
adaptations where, in fact, there were none (see Chittka and Dornhaus 1999 for
examples).
To test if flower signals drove the evolution of bee color vision, it must be
shown that the ancestors of bees possessed different sets of color receptors prior
to the advent of the flowering plants. To this end, we have to evaluate members of
arthropod taxa whose evolutionary lineages diverged from those of bees before
there were flowers. A phylogenetic analysis reveals that the values of peak
sensitivity in the Crustacea and Insecta fall into three distinct clusters around 350,
440 and 520 nm (Chittka 1996b). The few insect species in which one of the three
types is absent (Periplaneta and Myrmecia) represent secondary losses. Red
receptors show up irregularly in both the Crustacea and Insecta; they have evolved
several times convergently. The photoreceptor wavelength positions of UV, blue,
and green receptors are surprisingly conserved in the Mandibulata. We conclude
that the Cambrian ancestors of extant insects and crustaceans possessed UV, blue,
and green receptors. Insects were well preadapted for flower color coding more
than 500 million years ago, about 400 million years before the extensive
radiation of the flowering plants that started in the mid-Cretaceous (I 00 million
years ago).
To be sure, there are differences of 20-30 nm between the measured peak
sensitivities within each cluster of insect color receptors. This means that our
analysis does leave open the possibility of fine-tuning of receptors to particular
visual tasks in each species. For example, microspectrophotometry reveals that the
long wave pigment of fireflies differs by 12 nm in "-max between nocturnal and
crepuscular species, a difference which can be explained by the specific
requirements posed by detecting and identifying conspecific flashes under
different visual conditions (Cronin et al. 2000). Unfortunately, however, the data
in many other studies were collected by electrophysiological measurements and
are therefore noisy, so we do not yet understand whether the differences between
many species can be attributed to measurement error, different methods, or actual
variation between species.
At first sight, however, considering the large variety of light habitats and
feeding habits of insects, it is surprising how little variation there is. If traits are
conserved within a taxon, although we have reasonable grounds to predict that
they should differ on the basis of optimality arguments, then we must also
take the possibility of phylogenetic constraint seriously. Similar phylogenetic
studies performed on other sensory traits, or on color vision systems in other
animals, may reveal a different pattern; but it is through phylogenetic studies that
we can decipher patterns of adaptation and constraint, convergence, and
homology.