Description of Research Project on Striped Tree Leaves
A certain type of tree has stripes on its leaves. The number of stripes on a leaf varies, but in a sample of 100 leaves taken
from one particular cluster of trees, the average number of stripes was 10.2 stripes per leaf, with a margin of error of 4.1
stripes. The time that this sample was taken is considered time 0 years. Each of the 100 leaves measured at this time had
between 7 and 14 stripes.
Seeds from this species of tree were then planted in two different environments. The first environment had sparse
vegetation and low annual rainfalls. The second environment had more ground-level vegetation and higher annual
After 4 years, 50 leaves from the trees grown from the newly planted seeds in each environment were selected. The
number of stripes on each leaf was recorded.
After 8 years, 50 more leaves were selected from trees in each environment. By this time, the number of stripes
tended to cluster into two distinct groups. This pattern continued through a time measurement 12 years after the
initial seeding. In both the 8 year and 12 year measurements, the cluster of lower stripe counts was observed from
the environment with low rainfall.
The average number of stripes recorded during years 4, 8 and 12 are as follows:
Year 4 8 12 Low Rainfall Environment (mean) 13.6 11.2 7.5 Margin of error for mean of low rainfall environment 2.2 1.6 3.4 High Rainfall Environment (mean) 14.1 19.3 23.6 Margin of error for mean of high rainfall environment 2.4 1.5 0.8
Consider each of the following statements. Does the information in the three sources support the inference as stated?
The mean number of stripes in the high rainfall environment is conclusively higher than the mean number of stripes in the low rainfall environment after 4 years.
Trees in the high rainfall environment seem to have decreasing genetic variability in the number of stripes over time.
The divergence in the number of stripes between the original environment at 0 years and the specialized environment at 12 years is greater for trees that ended up in the high rainfall environment than those that ended up in the low rainfall environment.