Stabilizing
selection on English Sparrows
(Bumpus, 1898)
During a severe winter storm in New England in 1898, 136 English
Sparrows (Passer domesticus) succumbed to cold
shock and literally fell out of trees. HC Bumpus
collected the birds and took them to his lab, where 72
recovered and 64 died. Bumpus made a series of measurements
on both groups [left]. His data tended to demonstrate that
the standard deviation of each measurement (grey
bars on either side of the mean) and hence the variance
was greater in the birds that died, which
suggested truncation of either tail of their distributions.
This is summarized in the bar graph [right] for body size of
females (measured as Length and weight). Stabilizing Selection
therefore operated to maintain population variation closer
to the mean. Bumpus published his raw data measurements, and
the "Bumpus data set" has been subjected to
many subsequent re-analyses.