Measuring Populations Answers

More boys are born than girls, but males generally have higher mortality and women tend to live longer. The higher number of male births partly compensates for this difference, so the sex ratio becomes more balanced as people age and women make up a larger share of older populations.

While stabilizing selection selects for the most common, normal traits in a population, disruptive selection selects for extremes. Disruptive selection eliminates the very intermediate traits that stabilizing selection preserves.

Natural selection has favored traits that help sharks and alligators survive and reproduce in their environments. Stabilizing selection tends to preserve successful, well-adapted traits and act against less advantageous variations. As a result, their general forms have changed relatively little over thousands of years, though they have not stopped evolving.

Random distribution is rare because individuals are seldom placed independently of one another. Resources are often unevenly distributed, causing organisms to cluster, while competition and territorial behavior can space them apart. These ecological factors usually produce clustered or regular patterns rather than random ones.

Disruptive selection favors both extreme forms of a trait over the intermediate form. Directional selection favors just one extreme, shifting the population's average trait in that direction.

Disruptive selection favors individuals with either of two extreme forms of a trait and selects against the intermediate form. Stabilizing selection favors the intermediate form and selects against individuals at both extremes.