Rapid evolution of a geographic cline in size in an introduced fly

The introduction and rapid spread of Drosophila subobscura in the New-World two decades ago provide an opportunity to determine the predictability and rate of evolution of a geographic cline. In ancestral Old World population s, wing length increases clinally with latitude, in North American populati ons, no wing length cline was detected one decade after the introduction. A fter two decades, however, a cline has evolved and largely converged on the ancestral cline. The rate of morphological evolution on a continental scal e is very fast, relative even to rates measured within local populations. N evertheless, different wing sections dominate the New versus Old World clin es. Thus, the evolution of geographic variation in wing length has been pre dictable, but the means by which the cline is achieved is contingent.