In Burgers' Zoo (Arnhem, the Netherlands) a large enclosure was build to mi
mic a tropical rain forest. In this so called 'Bush' several ponds exist in
which guppies (Poecilia reticulata) can be found. One of these ponds conta
ins caymen and is therefore in a secluded area of the Bush were other anima
ls can not enter. Among these other animals are snakebirds (Anhinga melanog
aster) which in the rest of the Bush predate on guppies devouring up to a 1
000 guppies a day. After several years it became noticeable that the guppie
s which were initially from the same stock, differed between the Cayman pon
d (without predation) and the Seacow pond (with predation). The two populat
ions showed marked differences in length and colouration. Young from both s
tock were then raised under identical conditions and were still found to di
ffer. This leads to the conclusion that some of the differences are genetic
al and therefore the result of evolution within this semi-natural environme
nt. The speed of these changes are compatible to what has been published ab
out wild guppies. Also differences in behaviour between the two populations
were ascertained.