GRE Subject Test: Biology : Evolutionary Factors

Example Questions

Example Question #1 : Evolutionary Factors

In the Hardy-Weinberg equations, what quantities are represented by the variables  and ?

Allele frequencies

Genotype frequencies

Observed phenotype frequencies

Expected phenotype frequencies

Allele frequencies

Explanation:

The variables  and  are specifically referring to the allele frequencies of the dominant and the recessive allele in a population, respectively.

Expected genotype frequencies can be seen in the equation:

In this equation, represents the expected genotype frequency of homozygous dominant organisms,  represents the expected frequency of heterozygous organisms, and represents the frequency of homozygous recessive organisms. These values are the expected frequencies in the population, based on the Hardy-Weinberg conditions and allele frequencies; they may not be the values actually observed. To get observed genotype and phenotype frequencies, more information about the size and makeup of the population would be needed.

Example Question #1 : Understanding Hardy Weinberg Equilibrium

Which of the following variables would not be observed in a population at Hardy-Weinberg equilibrium?

No new mutations are appearing in the population

Females mate with males of the same color to avoid a mixing of colors in the population

The population has a very large number of organisms

No immigrating organisms are allowed to enter the population

Females mate with males of the same color to avoid a mixing of colors in the population

Explanation:

Hardy Weinberg equilibrium has requirements that must be met by a population in order to confirm that evolution is not taking place:

1. The population must be large in number.

2. There can be no new mutations entering the population.

3. Immigration and emigration cannot change the allelic frequencies of the population.

4. Mating must be random.

5. Natural selection cannot be taking place.

Since it was said that females are selectively choosing which males they mate with, Hardy Weinberg equilibrium is being violated.

Example Question #1 : Evolutionary Factors

A population at Hardy-Weinberg equilibrium has two alleles for fur color: red and black. Assume black is dominant to red fur color. Of the animals in the population, 16 percent of the animals have red fur.

What percentage of the alleles in the population code for black fur?

Explanation:

Since we know that the population is in Hardy-Weinberg equilibrium and that there are only two alleles, we can use the Hardy-Weinberg equation to solve this problem:

Lets say that  represents the black allele, and  represents the red allele. Since we know that red is recessive to black, only animals with two red alleles will be red. Fortunately, the  portion of the equation is the only portion that deals with red animals (the other two variables are black: both homozygous dominant as well as heterozygous). This means that  is equal to the frequency of red animals in the population:

Since we now know the frequency of the red allele in the population, we simply subtract it from one in order to find the frequency of the black allele, which turns out to be 0.6.

Example Question #4 : Understanding Hardy Weinberg Equilibrium

In a population that is in Hardy-Weinberg equilibrium, the frequency of homozygous dominant individuals is 0.36. What is the percentage of homozygous recessive individuals in the population?

Explanation:

The two equations pertaining to Hardy-Weinberg equilibrium are:

In this second equation, each term refers to the frequency of a given genotype.  is the homozygous dominant frequency,  is the heterozygous frequency, and  is the homozygous recessive frequency.

From the question, we know that:

We now know the dominant allele frequency. Using the other Hardy-Weinberg equation, we can find the recessive allele frequency:

Returning to our genotype frequency terms, we can use this recessive allele frequency to find the homozygous recessive frequency:

Example Question #1 : Evolutionary Factors

Which is not a necessary condition for the Hardy-Weinberg equation to be true?

No net migration of individuals into or out of the population

No natural selection

Random mating

No mutations in the gene pool

Small population

Small population

Explanation:

For the Hardy-Weinberg equation to be true, the population in question must be very large. This ensures that coincidental occurrences do not drastically alter allelic frequencies.

Example Question #11 : Evolution And Mutations

A population is in Hardy-Weinberg equilibrium. The gene of interest has two alleles, with 16% of the population portraying the features of the recessive phenotype. What percentage of the population is heterozygous?

Explanation:

Using the Hardy-Weinberg equilibrium equations, you can determine the answer.

The value of  gives us the frequency of the dominant allele, while the value of  gives us the frequency of the recessive allele. The second equation corresponds to genotypes.  is the homozygous dominant frequency,  is the heterozygous frequency, and  is the homozygous recessive frequency.

16% of the population shows the recessive phenotype, and therefore must carry the homozygous recessive genotype. We can use this information to solve for the recessive allele frequency.

We can use the value of  and the first Hardy-Weinberg equation to solve for .

Knowing both  and , you can use the second equation to find the percent of heterozygous organisms in the population.

Example Question #1 : Understanding Genetic Drift

Genetic drift is a phenomenon by which __________ in a population change.

natural selection

random mating

mutation frequencies

allele frequencies

allele frequencies

Explanation:

Genetic drift specifically refers to the change of allele frequencies because of random sampling of gametes. Essentially, this induces genetic bias for particular alleles and can lead to speciation events simply by the chance event of certain gametes producing offspring rather than others.

Changes in mutation frequencies, random mating, and natural selection may lead to changes in allele frequencies, but they are not necessarily the cause of genetic drift.

Example Question #1 : Evolutionary Factors

Which of the following factors plays the biggest role in the impact of genetic drift on a population?

Random chance

Immigration of animals

Mating preferences

Natural selection

Random chance

Explanation:

Genetic drift occurs when a gene's frequency is changed in a population due to pure chance. Consider a rock rolling down a hill and crushing all flowers that have white petals. The population will now have only red petals because the white ones were destroyed. Were the red petal flowers more suited to their environment? No, it just so happened that all of the white were removed from the gene pool. This shows that evolution can occur due to random chance as well as natural selection, and both forces can have an impact on a population.

Example Question #1 : Evolution

Which of the following is an example of natural selection?

I.  Horses are bred for strength and endurance, and over time, the population of horses is more robust.

II.  A late spring storm kills all the young plants in a region, but they are spared outside the storm zone.

III.  Ancient ancestors of giraffes instinctively wanted to have longer necks to reach food higher in the trees, leading to the present appearance of giraffes.

IV.  A flower that happens to be more attractive to pollinators is more likely to have reproductive success.

V.  A mutation of a bacterium caused by exposure to ultraviolet light causes the originally red colonies to be yellow instead.

IV

I

V

III

II

IV

Explanation:

It is always difficult to rephrase "survival of the fittest" in some new, clever way. The flowers which BY CHANCE have developed a different color, pattern, or odor that better attracts pollinators are indeed more likely to experience reproductive success and pass on these genes to their offspring. Competing plants might do well for a while, but they are already disfavored, and further environmental changes may put them even more at risk (or have no effect, or again favor them over the presently more attractive plants).

The horse choice is an example of intentional breeding—artificial selection.

The storm option does not imply any condition in any of the plants which conferred an advantage against freezing to death, or even any difference between species of plants; it is more akin to a question about mass extinction than to one about evolution.

The giraffe choice relates to the Lamarckian fallacy of being able to pass on acquired characteristics; species that are more successful just plain "luck out" relative to environmental stresses.

The bacterial response discusses a mutation without likely survival implications for the bacterium.

Example Question #1 : Evolutionary Factors

Vertebrates are evolutionarily adapted to terrestial life. Which one of the following adaptations is LEAST likely to contribute to this land-based predominance?

Internal fertilization

Internal lungs

Short loops of Henle

Development of legs

Impermeable outer skin