We measured the size of eggs produced by populations of Drosophila mel
anogaster, that had been collected along latitudinal gradients in diff
erent continents or that had undergone several years of culture at dif
ferent temperatures in the laboratory. Australian and South American p
opulations from higher latitudes produced larger eggs when ail were co
mpared at a standard temperature. Laboratory populations that had been
evolving at 16.5 degrees C produced larger eggs than populations that
had evolved at 25 degrees C or 29 degrees C, suggesting that temperat
ure may be an important selective agent in producing the latitudinal d
ines. Flies from laboratory populations produced larger eggs at an exp
erimental temperature of 16.5 degrees C than at 25 degrees C, and ther
e was no indication of genotype-environment interaction for egg size.
Evolution of egg size in response to temperature cannot be accounted f
or by differences in adult body size between populations. It is not cl
ear which life-history traits are direct targets of thermal selection
and which are showing correlated responses, and disentangling these is
a task for the future.