Changes in the genetic structure of the Katsunuma natural population of Dro
sophila melanogaster have been examined during the past 35 years. The frequ
ency of recessive lethal genes on the second chromosome once increased from
15% to 30% in the early 1970s, then decreased to about 24% in the late 197
0s, and thereafter showed no significant changes. Sterility genes, the freq
uency of which is always less than the lethals, showed a similar tendency.
The SD (segregation distorter) mutant gene disappeared but some others such
as rbl (reduced bristle) and bw (brown) persisted in the population. The f
requency of inversion-carrying chromosomes gradually decreased in the perio
d, such that the standard chromosome frequency in the second and third chro
mosomes increased from about 40% to more than 80%. Coincident with these fr
equency changes is the invasion of a transposable element P into the Katsun
uma population. The P element should have invaded into Katsunuma in the lat
e 1960s. It spread over the population apparently inducing deleterious muta
tions, causing the decrease in the allelism rate, and hence increasing the
effective population size. Soon, however, most flies became resistant to th
e P element-mediated transposition as they began to harbor defective P elem
ents. During the course of spreading, the P element must also have induced
deleterious mutations on the polymorphic inversions, breaking up the hetero
tic gene complexes along the chromosomes, which probably caused the reducti
on in the frequency of inversion chromosomes. Temporal invasion of D. simul
ans, a sibling species of D. melanogaster, into Katsunuma occurred several
times after 1978, and the species seems to have been settled since 1990. Th
is, however, did not have any effect on the genetic structure of D. melanog
aster population.