Endler's Discovery
When evolutionary biologist John Endler began studying Trinidad's
wild guppies in the 1970s, he was struck by the wide variation among guppies from
different streams, even among guppies living in different parts of the same stream.
Males from one pool sported vivid blue and orange splotches along their sides, while
those further downstream carried only modest dots of color near their tails. Endler
also observed differences in the distribution of guppy predators, and in the color
and size of gravel in different stream locations.
Endler photographed hundreds of guppies and carefully measured
their size, color, and the size and placement of their spots. He began to see a strong
correlation between where guppies lived in a particular stream and whether the fish
were bright or drab. But what was responsible for these trends in coloration? And if
bright colors made guppies more conspicuous to predators, why should males be colorful
at all? To find out, Endler formed a hypothesis based on his observations, then set
out to test it. His results would prove to be one of evolutionary biology's most
important discoveries.
Make your own observations of
a guppy stream.
Investigate the different pools in this stream more closely.
Look for color trends in the guppies. These may vary relative to the presence or
absence of specific predator species in the pools.
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