The evolutionarily stable (ESS) dispersal range for annual plants is s
tudied in a stable environment when there is a trade-off between seed
survivability and dispersal range via seed size. Larger seed size is m
ore beneficial in the competition for safe sites, but likely to be dis
persed a shorter distance and to suffer competition among siblings. Pr
eviously Hamilton & May found that dispersal can be adaptive in a stab
le environment to reduce competition among sibs, but they assumed that
dispersers were likely to enter all the patches equally-this is not s
uitable for many terrestrial plants with limited dispersal range. In t
his article I discuss the evolution of dispersal range for wind disper
sed seeds when dispersal range is tightly coupled with seed size. I as
sume that the density of dispersed seed follows a two-dimensional norm
al distribution function, with variance decreasing with seed size. Due
to the trade-off between the seed number and the survivability of a s
eedling offspring, there is a seed size (w) over tilde that maximizes
the product of the two quantities. This is the optimal seed size when
size-dependent dispersal is neglected. The ESS seed size considering t
he size-dependent dispersal w is also calculated by neglecting the ef
fect of spatial clumping of relatives. Under the environment unfavorab
le for seed dispersal, the ESS seed size w can be much smaller than t
he optimal seed size (w) over tilde, but there is a lower limit for th
e ESS dispersal range even in the extremely sticky environment. Even i
f the dependency of seed survivability on the seed size is so weak tha
t the cost of long-range dispersal is small, the ESS seed dispersal ra
nge cannot become very large. These results are confirmed by individua
l-based computer simulations with more realistic assumptions consideri
ng spatial clumping of non-sib relatives. (C) 1998 Academic Press Limi
ted.