30
Lars Chittka and Adriana Briscoe
In some vertebrates, conversely, such variance exists and has been well
quantified, for example in guppies (Archer et al. 1987) and primates including
humans (Deeb and Motulsky 1996; Shyue et al. 1995). In invertebrates, to our
knowledge, only a single published study reported intraspecific variance between
the sequences of visual pigments. Ayala et al. (1993) sampled five Rh3 alleles
from each of four species in the Drosophila melanogaster subgroup and three
alleles from D. pseudoobscura, and found a single amino acid polymorphism in
one of the five surveyed species. One of us (A.B.) also found intraspecific amino
acid variation in some of the opsin loci from Papilio glaucus. We cannot be sure,
however, if any of these naturally occurring variants differ in their spectral
sensitivities. Clearly, we need more data.
6 Pleiotropy - Selection Through Correlated
Characters
A possibility rarely considered by sensory ecologists is that a sensory trait under
scrutiny may be favored indirectly, through correlated characters, that do not
necessarily have anything to do with the selective pressures associated with the
perception of sensory stimuli. For example, Hope et al. (1997) tested the
possibility that the pigments of abyssal fish might be adaptations to resist
denaturation by the elevated pressure in deep waters, rather than adaptations to the
photic environment. They rejected this possibility, but it is an approach well worth
considering. Crandall and Hillis ( 1997) suggested that rhodopsin might have a
previously unrecognized function, possibly in the control of circadian rhythms.
This conclusion was based on lack of apparent differences in the rate of molecular
sequence evolution between the opsin homologues of blind subterranean crayfish
and their "sighted" relatives. In another example of possible nonvisual function of
opsin, Alvarez et al. (1996) found that Drosophila Rh2 opsin is expressed in the
testes of male flies, in addition to the ocelli (Pollock and Benzer 1988). If opsins
have such dual functions, then spectral sensitivity may sometimes be an effect of
pleiotropy, i.e., selection on correlated characters that we have not recognized.
7 Adaptation, Genetic Drift, and Biogeography
It is a widespread misconception of non-evolutionary biologists that beneficial
mutations will rapidly spread through a population and eventually wipe out the
pre-mutation genotype. However, new mutations are frequently lost immediately
upon their introduction because of stochastic processes commonly designated
genetic drift (Gould and Lewontin 1979). This may cause considerable
evolutionary inertia particularly in large populations, or if the adaptive value of
the new mutation is relatively small. There are many cases where local
populations are kept considerably below their adaptive peaks because of
continued gene flow with a large parental population (Stanton and Galen 1997).
Lars Chittka and Adriana Briscoe
In some vertebrates, conversely, such variance exists and has been well
quantified, for example in guppies (Archer et al. 1987) and primates including
humans (Deeb and Motulsky 1996; Shyue et al. 1995). In invertebrates, to our
knowledge, only a single published study reported intraspecific variance between
the sequences of visual pigments. Ayala et al. (1993) sampled five Rh3 alleles
from each of four species in the Drosophila melanogaster subgroup and three
alleles from D. pseudoobscura, and found a single amino acid polymorphism in
one of the five surveyed species. One of us (A.B.) also found intraspecific amino
acid variation in some of the opsin loci from Papilio glaucus. We cannot be sure,
however, if any of these naturally occurring variants differ in their spectral
sensitivities. Clearly, we need more data.
6 Pleiotropy - Selection Through Correlated
Characters
A possibility rarely considered by sensory ecologists is that a sensory trait under
scrutiny may be favored indirectly, through correlated characters, that do not
necessarily have anything to do with the selective pressures associated with the
perception of sensory stimuli. For example, Hope et al. (1997) tested the
possibility that the pigments of abyssal fish might be adaptations to resist
denaturation by the elevated pressure in deep waters, rather than adaptations to the
photic environment. They rejected this possibility, but it is an approach well worth
considering. Crandall and Hillis ( 1997) suggested that rhodopsin might have a
previously unrecognized function, possibly in the control of circadian rhythms.
This conclusion was based on lack of apparent differences in the rate of molecular
sequence evolution between the opsin homologues of blind subterranean crayfish
and their "sighted" relatives. In another example of possible nonvisual function of
opsin, Alvarez et al. (1996) found that Drosophila Rh2 opsin is expressed in the
testes of male flies, in addition to the ocelli (Pollock and Benzer 1988). If opsins
have such dual functions, then spectral sensitivity may sometimes be an effect of
pleiotropy, i.e., selection on correlated characters that we have not recognized.
7 Adaptation, Genetic Drift, and Biogeography
It is a widespread misconception of non-evolutionary biologists that beneficial
mutations will rapidly spread through a population and eventually wipe out the
pre-mutation genotype. However, new mutations are frequently lost immediately
upon their introduction because of stochastic processes commonly designated
genetic drift (Gould and Lewontin 1979). This may cause considerable
evolutionary inertia particularly in large populations, or if the adaptive value of
the new mutation is relatively small. There are many cases where local
populations are kept considerably below their adaptive peaks because of
continued gene flow with a large parental population (Stanton and Galen 1997).