Why Sensory Ecology Needs to Become More Evolutionary
31
Island populations may adapt more readily to local conditions because gene flow
with the parental population is disrupted (Emlen 1978). On the other hand,
deleterious mutations may also be common on islands, especially where the
populations started out from very few animals, or experienced occasional strong
reductions in population size (founder and bottleneck effects) (Endler 1986). It is
important to keep these points in mind when considering sensory adaptations.
Of course, a particularly strong case for adaptation could be made if we found
biogeographical differences in color vision systems within species, and if we
could link these differences to particular environmental pressures exerted on
different populations of the species. Unfortunately, no studies in insects are
available to date, but a few observations on humans are worth considering.
Humans have a single amino acid polymorphism at position 180 in the opsin
protein. 62% of white Caucasians have serine at this position, and their red
receptor absorbs maximally at 557 nm. The remaining 38% have alanine, and their
red receptors are maximally sensitive at 552 nm. This difference has been shown
to be relevant in color discrimination, and males with Ser at residue 180 have
higher sensitivity to red light (Deeb and Motulsky 1996). This is just the variation
we need for selection to act on! Even more interestingly, this polymorphism is
different in different human populations: it is 80% Ser; 20% Ala in African
Americans and 84/16% in Japanese (Deeb and Motulsky 1996). Are these
differences adaptive, however? We favor the notion that these differences are due
to genetic drift. Likewise, Ayala eta!. (1993), who discovered a single amino acid
polymorphism in Drosophila, concluded that the alleles are evolving by
selectively neutral processes.
Another finding from human color vision is worth consideration. On the tiny
South Sea island Pingelap, 75 of the 700 inhabitants are totally colorblind: they
have only rods for scotopic vision. This is the result of a classic bottleneck effect.
In 1775 this island had almost 1000 inhabitants, when it was struck by a typhoon,
which reduced the population of the island to 20 survivors, one of whom was the
king. After a few generations, the population was almost back to its pretyphoon
level. Unfortunately, the king was carrier of the gene responsible for color
blindness, so that today one third of the population carries the recessive gene that
is responsible for this defect, and more than 10% of the population is
phenotypically color blind (Sacks 1997). In other human populations, the
frequency of this defect is about 1 in 30 000.
We do not rule out the possibility that adaptive differences in sensory systems
exist between populations. In sticklebacks, for example, McDonald and
Hawryshyn (1995) were able to link between-population differences with the light
environment; but we wish to caution that not all differences between populations
may be adaptive, and most likely random evolutionary processes can explain some
of the differences between species as well. The most important message is that the
data base is slim, and that we need more studies.
31
Island populations may adapt more readily to local conditions because gene flow
with the parental population is disrupted (Emlen 1978). On the other hand,
deleterious mutations may also be common on islands, especially where the
populations started out from very few animals, or experienced occasional strong
reductions in population size (founder and bottleneck effects) (Endler 1986). It is
important to keep these points in mind when considering sensory adaptations.
Of course, a particularly strong case for adaptation could be made if we found
biogeographical differences in color vision systems within species, and if we
could link these differences to particular environmental pressures exerted on
different populations of the species. Unfortunately, no studies in insects are
available to date, but a few observations on humans are worth considering.
Humans have a single amino acid polymorphism at position 180 in the opsin
protein. 62% of white Caucasians have serine at this position, and their red
receptor absorbs maximally at 557 nm. The remaining 38% have alanine, and their
red receptors are maximally sensitive at 552 nm. This difference has been shown
to be relevant in color discrimination, and males with Ser at residue 180 have
higher sensitivity to red light (Deeb and Motulsky 1996). This is just the variation
we need for selection to act on! Even more interestingly, this polymorphism is
different in different human populations: it is 80% Ser; 20% Ala in African
Americans and 84/16% in Japanese (Deeb and Motulsky 1996). Are these
differences adaptive, however? We favor the notion that these differences are due
to genetic drift. Likewise, Ayala eta!. (1993), who discovered a single amino acid
polymorphism in Drosophila, concluded that the alleles are evolving by
selectively neutral processes.
Another finding from human color vision is worth consideration. On the tiny
South Sea island Pingelap, 75 of the 700 inhabitants are totally colorblind: they
have only rods for scotopic vision. This is the result of a classic bottleneck effect.
In 1775 this island had almost 1000 inhabitants, when it was struck by a typhoon,
which reduced the population of the island to 20 survivors, one of whom was the
king. After a few generations, the population was almost back to its pretyphoon
level. Unfortunately, the king was carrier of the gene responsible for color
blindness, so that today one third of the population carries the recessive gene that
is responsible for this defect, and more than 10% of the population is
phenotypically color blind (Sacks 1997). In other human populations, the
frequency of this defect is about 1 in 30 000.
We do not rule out the possibility that adaptive differences in sensory systems
exist between populations. In sticklebacks, for example, McDonald and
Hawryshyn (1995) were able to link between-population differences with the light
environment; but we wish to caution that not all differences between populations
may be adaptive, and most likely random evolutionary processes can explain some
of the differences between species as well. The most important message is that the
data base is slim, and that we need more studies.