Starting Population Count
Spine Density
Spine Density
3. Cacti with very few spines tend to be eaten by animals, and cacti
with many spines tend to be preferred by parasitic insects. This
causes cacti with an average number of spines to be preferred. How
would you explain this?
A. This is an example of a stabilizing selection in which the
intermediate form of a trait is advantageous compared to the
extreme forms of the trait.
B. This is an example of bidirectional selection in which
evolutionary pressures from multiple sources encourage an
► organism to adapt.
C. This is an example of a disruptive selection in which the
population of cacti is divided and well defined.
D. This is an example of directional selection in which one end of
the bell curve is better adapted to its environment than the other
end.

increase soil erosion                                             reduce soil erosion

cattle grazing                                                         hydroseeding

deforestation                                                         contour plowing

                                                                                planting vegetation

plant roots basically hold the top soil

together and stop soil erosion because

in such situation top soil is not removed

during flood but cattle grazing causes

soil erosion because cattle consume the

grasses and herbs and top soil is exposed.

same situation occurs because of

deforestation, the trees and plants are cut

down to expose top soil which causes soil

erosion