Rarer need not be better if commoner is worse: Frequency-dependent selection for developmental time at the alcohol dehydrogenase locus of the olive fruit fly, Bactrocera oleae
N. Cosmidis et al., Rarer need not be better if commoner is worse: Frequency-dependent selection for developmental time at the alcohol dehydrogenase locus of the olive fruit fly, Bactrocera oleae, EVOLUTION, 53(2), 1999, pp. 518-526
Whereas the importance of frequency-dependent selection in Life-history tra
its, behavioral characters and source allocation patterns is widely accepte
d, its role in governing biochemical and molecular polymorphisms remains po
orly understood. Here we demonstrate a case of allozyme frequency-dependent
selection. When olive fruit flies (Bactrocera oleae) are reared on an arti
ficial larval medium, an allele at the alcohol dehydrogenase locus that is
present in very low frequency in natural populations increases to about one
-third in less than five generations. We show here that the time from the h
atching of the egg to the eclosion of the adult is affected by the genotype
composition of the larval population that grows in the same cup of food. C
ultures consisting of one genotype only have the longest developmental time
, and two-allele cultures in which the two homozygotes and the heterozygote
occur in a 1:1:2 ratio show the shortest developmental time. Cultures with
intermediate genotypic compositions show intermediate levels of developmen
tal time. The results can be explained by assuming that the developmental t
ime of a genotype depends an the frequency array of all genotypes in the la
rval population and is not merely a function of its own frequency. It is ev
en possible that the developmental time of a genotype becomes longer as the
genotype becomes rarer, yet the genotype will be favored because the devel
opmental times of the competing genotypes become even longer owing to the a
ssociated increase of their frequencies. Given that developmental time is i
nversely related to fitness, this generates a frequency-dependent selection
, with developmental times changing progressively until the population arri
ves at an equilibrium. One optimum population composition that provides a s
atisfactory fit to allele frequency changes in our experimental populations
is when the two alleles occur in equal frequencies and genotypes are in Ha
rdy-Weinberg proportions. We argue that this type of selection is consisten
t with the role of alcohol dehydrogenase as a detoxifying enzyme in a mediu
m that undergoes continuous chemical changes during its use by the feeding
larvae.