Measuring Populations Answers

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.

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.

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.

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.

While directional selection selects for extreme variation of one genetic trait in a population, disruptive selection selects for both extremes. Disruptive selection eliminates intermediate traits leading to selection at either extreme. Directional selection, meanwhile, preferentially selects for individuals expressing the extreme trait.

Stabilizing natural selection keeps gene frequencies constant in a population. The normal, most common traits are selected for and preserved so only a limited amount of evolution occurs and species can remain unchanged even over long periods of time.

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.

This trend means that men have a higher mortality rate than women. While men start out as a larger percentage of the population, their higher mortality rate means that among the elderly, women are a larger percentage of the population than men.

Very few species have no pattern of interaction between individuals. It is rare for members of a species to not be attracted to one another or compete for resources, as is required for random distribution which makes this the rarest distribution pattern.