Measuring Populations

Explain how and why natural selection has allowed species such as sharks and alligators to remain genetically stable for thousands of years.

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.

Natural selection does not always result in an increase in genetic diversity.

true

The rarest distribution pattern in nature is random.

true

Which of the following best describes the relationship between jaguars and panthers?

b. Panthers are jaguars with predominantly dark or black pigmentation.

Natural selection is an evolutionary mechanism by which _______.

b. advantageous traits become more common in a population

Stabilizing selective pressures result in _______.

b. limited evolutionary changes

These two graphs represent _______ and _______ selection.

b. stabilizing; disruptive

Which of the following is the most obvious sign of cluster distribution in termites?

c. They are social insects.

Natural selection is best described as _______.

c. a mechanism of evolution

The most common type of distribution pattern(s) in nature is(are) _______.

c. cluster

Adaptive melanism describes selection for _______.

c. darker pigmentation

Regular distribution is observed for many organisms due to _______.

c. interactions between individuals

Which of the following is not true about the distribution of species?

d. All the organisms in an ecosystem share distribution patterns.

Which of the following species would you most expect to be distributed according to a regular pattern?

d. creosote bushes

Disruptive selection events select for _______.

d. extreme traits

Stationary population pyramids are representative of countries with shorter than average life expectancies.

false

Among humans more boys are born than girls, but women have significantly higher life expectancies. What is the significance of this?

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.

Explain why random distribution patterns for species are the rarest in nature.

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.

What distinguishes disruptive and directional selection pressures when both select for extreme genetic traits?

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.

Contrast disruptive and stabilizing selections.

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.