While the thrill of mastering a 
spiny Alpine road in the BMW built 
for it remains as potent as ever, 
we recognize that these are value- 
conscious times. 
We are therefore pleased to an- 
nounce that a BMW bought in 
Europe is now substantially less ex- 
pensive than one bought in the 
United States. 
Further, if you imagine that the 
joys of European Tourist Delivery are 
clouded by 42-page forms, rest 
assured. Because your BMW dealer 
here at home handles everything. 
c 1981 BMW of North America. Inc The BMW trademark and logo are registered trademarks of 
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THE ULTIMATE DRIVING MACHINE. 
American 
Museum of 
Natural 
History 
GALAPAGOS ISLANDS CRUISE 
June 13 to 25, 1981 
Discovery Tours 
Share the educational holiday of a lifetime with 
American Museum scholars and 65 compatible 
travelers aboard the comfortable M.V. Bucca- 
neer. Enjoy superb climate and sandy beaches, 
while exploring volcanic landscapes, home to 
exotic and tame birds and reptiles. For further 
information write to DISCOVERY TOURS, 
American Museum of Natural History, Central 
Park West at 79 St., NY NY 1 0024, or call 
the broad range of tolerance among 
the progeny compared with the par- 
ents. For this new generation to display 
as adults the same range as their par- 
ents, the two extremes — low tolerance 
and high tolerance — would have to be 
eliminated. This elimination of ex- 
tremes among offspring is, we believe, 
the measure of natural selection that 
maintains the tolerant adaptation 
among adults. 
What about the tendency of gene 
flow from the pasture to dilute the 
effects of selection for tolerance? Here 
all is revealed by the mine offspring, 
germinated from seeds obtained from 
the adult plants still growing on the 
mine. The average tolerance of these 
seedlings is between four and five, well 
below that of the parents. The range 
of tolerance is broad, with three seed- 
lings displaying the highest index of 
seven and one showing no tolerance 
at all. Remember, these are the in- 
dividuals that under natural conditions 
would disperse across the mine to de- 
velop into the adults of the next gen- 
eration. For the mine population to 
continue to have an average tolerance 
of six or seven, most plants with a 
tolerance below five would have to 
die. Three-quarters of this offspring 
population will indeed succumb to 
toxic concentrations of copper — a very 
strong selection. 
The average tolerance of the mine 
seedlings is four or five, well below 
that of greenhouse seedlings at six or 
seven. This difference must be due 
to the migration of nontolerant genes: 
pasture pollen blows onto the mine, 
pollinates adults, and lowers the aver- 
age tolerance of their progeny. Adult 
plants are capable of producing highly 
tolerant offspring when they inter- 
breed in isolation, but when they grow 
on the mine in the presence of foreign 
pasture pollen, the highest tolerance 
of their offspring is seven. Thus, the 
population can never achieve total tol- 
erance as long as mine individuals con- 
tinue to cross with pasture plants. The 
force of natural selection is tempered 
by the flow of genes. 
The polarity of the pollen-bearing 
wind in the valley where McNeilly 
worked permitted a field test of his 
findings. He marked off two transects 
across the boundary from the mine 
soils into the surrounding pasture: one 
was set across the prevailing wind, 
and the other ran downwind from the 
mine into the pasture. At selected lo- 
cations McNeilly sampled plants and 
their seed, tested them for tolerance, 
34 
