Why Sensory Ecology Needs to Become More Evolutionary
29
pseudoobscura (Zuker et al. 1985; Carulli and Hartl 1992). All other insects that
have been surveyed for opsins have been undersampled, so that we cannot be sure
how constrained individual species are in terms of photoreceptor number.
So far we have emphasized changes in the opsin protein as a mechanism for the
evolution of color vision. Arthropod chromophores come in five forms that are all
derived from retinal. When extracted in ethanol, their absorption maxima differ by
several nanometers: RALl (383 run), RAL2 (400 nm), RAL3 (379 run) and RAL4
(377 nm). Visual pigments reconstituted with the same opsin but different
chromophores have slightly different absorption maxima (Seki and Vogt 1998).
In nature, most species use only one chromophore but there are exceptions. In the
firefly squid, for instance, there are two kinds of chromophore and one kind of
opsin giving rise to three kinds of visual pigment (Seidou et al. 1990). Clearly, the
particular chromophore used has an effect on spectral tuning, and in some cases,
the opsins and chromophores may be coevolving.
5 Interindividual Variance
Variance between individuals is the raw material for evolution. This does not
mean that the lack of such variance in extant species indicates the traits are not
adaptive - in fact, if a trait is strongly favored by selection, it is likely that it
becomes fixed in a population, and all variance might be eliminated (Endler 1986;
Reeve and Sherman 1993). The results of many evolutionary "experiments" may
no longer exist in our time, but variance is important, both for animals and for
scientists studying adaptation. It allows populations to respond to ongoing
changes in environmental pressures, and to colonize new habitats. Where there is
lack of heritable variation, such changes cannot occur (Chittka 1997; Goldsmith
1990).
For us, interindividual variance offers the possibility to study predictions of
adaptation. One phenotype may be favored in one photic environment and another
phenotype in a different one. Many physiologists, however, treated such variance
as noise, which needed to be eliminated by averaging large numbers of
measurements from different animals. Sometimes this may be legitimate.
Physiological measurements are often so noisy that extracting any information at
all is not possible without averaging, and strong deviations from expected
observation will in fact often mean that the measurement is imperfect, for
example in electrophysiology: but we may have lost much valuable information
through such averaging! Could it be that the reason for much of the conservatism
in arthropod color receptors exists because there simply is no variance between
individuals of some of the species in question? A large number of scientists have
worked on, e.g., the color receptors of honeybees, and the results differed within
studies as well as across studies, but the debate about these differences mostly
focused on the possible contributions of artifacts or different electrophysiological
methods (Menzel et al. 1986). To be sure, both of these may add noise to the
measurements, but unfortunately, the possibility that interindividual variance may
also contribute has not been considered.
29
pseudoobscura (Zuker et al. 1985; Carulli and Hartl 1992). All other insects that
have been surveyed for opsins have been undersampled, so that we cannot be sure
how constrained individual species are in terms of photoreceptor number.
So far we have emphasized changes in the opsin protein as a mechanism for the
evolution of color vision. Arthropod chromophores come in five forms that are all
derived from retinal. When extracted in ethanol, their absorption maxima differ by
several nanometers: RALl (383 run), RAL2 (400 nm), RAL3 (379 run) and RAL4
(377 nm). Visual pigments reconstituted with the same opsin but different
chromophores have slightly different absorption maxima (Seki and Vogt 1998).
In nature, most species use only one chromophore but there are exceptions. In the
firefly squid, for instance, there are two kinds of chromophore and one kind of
opsin giving rise to three kinds of visual pigment (Seidou et al. 1990). Clearly, the
particular chromophore used has an effect on spectral tuning, and in some cases,
the opsins and chromophores may be coevolving.
5 Interindividual Variance
Variance between individuals is the raw material for evolution. This does not
mean that the lack of such variance in extant species indicates the traits are not
adaptive - in fact, if a trait is strongly favored by selection, it is likely that it
becomes fixed in a population, and all variance might be eliminated (Endler 1986;
Reeve and Sherman 1993). The results of many evolutionary "experiments" may
no longer exist in our time, but variance is important, both for animals and for
scientists studying adaptation. It allows populations to respond to ongoing
changes in environmental pressures, and to colonize new habitats. Where there is
lack of heritable variation, such changes cannot occur (Chittka 1997; Goldsmith
1990).
For us, interindividual variance offers the possibility to study predictions of
adaptation. One phenotype may be favored in one photic environment and another
phenotype in a different one. Many physiologists, however, treated such variance
as noise, which needed to be eliminated by averaging large numbers of
measurements from different animals. Sometimes this may be legitimate.
Physiological measurements are often so noisy that extracting any information at
all is not possible without averaging, and strong deviations from expected
observation will in fact often mean that the measurement is imperfect, for
example in electrophysiology: but we may have lost much valuable information
through such averaging! Could it be that the reason for much of the conservatism
in arthropod color receptors exists because there simply is no variance between
individuals of some of the species in question? A large number of scientists have
worked on, e.g., the color receptors of honeybees, and the results differed within
studies as well as across studies, but the debate about these differences mostly
focused on the possible contributions of artifacts or different electrophysiological
methods (Menzel et al. 1986). To be sure, both of these may add noise to the
measurements, but unfortunately, the possibility that interindividual variance may
also contribute has not been considered.