Rg. Gillespie et Gs. Oxford, SELECTION ON THE COLOR POLYMORPHISM IN HAWAIIAN HAPPY-FACE SPIDERS - EVIDENCE FROM GENETIC-STRUCTURE AND TEMPORAL FLUCTUATIONS, Evolution, 52(3), 1998, pp. 775-783
Throughout this century genetic polymorphisms for color have been wide
ly used as a research tool to allow insights into key evolutionary pro
cesses. Although color variants can often be diverse within population
s, frequencies of different morphs may be similar across populations,
either as a result of balancing selection or gene how. Under these cir
cumstances selection can be extremely difficult to demonstrate. Here w
e test for balancing selection on the naturally occurring color forms
of the Hawaiian happy-face spider, Theridion grallator with two approa
ches. First, allozyme loci are used to generate a null model against w
hich to test selection. Frequencies of alleles involved in the color p
olymorphism of T. grallator are used to generate another estimate for
comparison. The results suggest that statistically similar frequencies
of color morphs among populations of T. grallator may be maintained b
y some form of balancing selection. Second, we make use of an unusual
event in which the normally stable frequencies of unpatterned and patt
erned morphs within a population were found to have shifted toward an
excess of unpatterned morphs. We scored offspring of all fertilized, u
npatterned (bottom-recessive) females found during this period of skew
ed morph frequencies and also in a year when morph frequencies were no
rmal to deduce paternal color phenotypes. Mating was found to be rando
m in the normal year, but in the perturbed year females had mated with
rare (patterned) males twice as frequently as expected on the basis o
f the frequency of this morph type in the population. Both of these re
sults are consistent with selection operating on the color polymorphis
m, and we speculate that apostatic selection, perhaps mediated by bird
predators, may provide the mechanism.