Biology 212 Chapter 23; 1-6 Reviews & Questions

In a population of 2500, how many babies would you expect to have cystic fibrosis, a homozygous recessive condition, if the frequency of the dominant allele is 0.9 and the population is at Hardy-Weinberg equilibrium?

0.1 × 0.1 × 2500 = 25

Toucans can eat fruits with large seeds because their large bills can open very wide. Most other birds in the same forest can only eat small seeds. Ecologist Mauro Galetti and his colleagues measured the seed sizes of palms in forest fragments with and without toucans. The graphs show two of the forest populations they studied. What is the take-home message of the data?

Average seeds size is lower in the forest fragment with toucans absent (about 9.5 mm) than in the forest with toucans present (about 11 mm).

Balancing selection occurs when no single phenotype is favored; there is a balance among alleles in terms of fitness and frequency

Balancing selection preserves genetic variation

Do these data illustrate directional, stabilizing, disruptive, or balancing selection? Justify your answer in terms of fitness.

Directional selection. The average seed size of palms has declined in the absence of toucans, presumably because smaller seeds have higher fitness in the absence of toucans

Disruptive selection favors extreme phenotypes and thus maintains genetic variation in populations

Disruptive selection sometimes leads to the formation of new species

Suppose you were studying several species of monkeys. In one, males never helped females raise offspring. In another, males provided just as much parental care as females except for actually carrying the fetus during pregnancy. How does the fundamental asymmetry of sex compare in the two species? How would you expect sexual dimorphism to compare between the two species?

For the species in which the males never help raise offspring, the fundamental asymmetry of sex is pronounced and sexual dimorphism should be high. If males invest a great deal in raising offspring, then the fundamental asymmetry of sex is small and sexual dimorphism should be low.

The data in the graphs are from two of the 22 forest fragments studied by the researchers: 7 with toucans present, 15 with toucans absent. Why do you think the researchers bothered to study so many forest fragments?

Forest fragments may vary in slope, moisture, species composition, and other factors. If the researchers compare just two fragments, they may happen to get a result that is not typical of the other fragments, by chance. By comparing numerous fragments, the researchers can be more confident that the trend in their samples represents a real trend in the rest of the forest.

Large seeds carry more resources than small seeds and tend to have a higher rate of survival, especially after being dispersed by a bird. Predict how the local extinction of toucans will affect the palm population over time.

If larger palm seeds have higher fitness than smaller palm seeds (in the presence of toucans), then the fitness of palms will decrease over time in the absence of toucans because the large seeds will no longer be dispersed and will have a lower chance of survival.

In humans, albinism is caused by loss-of-function mutations in genes involved in the synthesis of melanin, the dark pigment in skin. Only people homozygous for a loss-of-function allele (genotype aa) have the albino phenotype. In Americans of northern European ancestry, albino individuals are present at a frequency of about 1 in 10,000 (or 0.0001). Assuming that genotypes are in Hardy-Weinberg equilibrium, what is the predicted frequency of Caucasians in the United States who carry a single allele for albinism?

If we let p stand for the frequency of the loss-of-function allele, we know that p2 = 0.0001; therefore p = 0.0001 = 0.01 . By subtraction, the frequency of normal alleles is 0.99. Under the Hardy-Weinberg principle, the frequency of heterozygotes is 2pq, or 2 × 0.01 × 0.99, which is 0.0198.

Why is genetic drift aptly named?

It causes allele frequencies to drift up or down randomly.

Stabilizing selection eliminates phenotypes with extreme characteristics

It decreases the diversity of alleles in populations

In what sense is the Hardy-Weinberg principle a null hypothesis?

It defines what genotype and allele frequencies are expected if evolutionary processes and nonrandom mating are not occurring

Why isn't inbreeding considered an evolutionary process?

It does not change allele frequencies.

Directional selection favors phenotypes at one end of a distribution, causing the average phenotype to change

It tends to decrease the diversity of alleles in populations

Sexual selection is a type of natural selection that leads to the evolution of traits that enable individuals to attract mates

It usually has a stronger effect on males than on females

In the 1700s and 1800s, royalty in Europe often married their close relatives; furthermore, recessive genetic diseases such as hemophilia showed up much more often among royals than in the general population. Explain the likely connection.

Marrying close relatives is a form of inbreeding that would have increased the homozygosity of recessive alleles (as inbreeding tends to shift alleles from heterozygotes, where they don't cause disease, to homozygotes, where they do). As a result, the royal families were plagued by genetic diseases

Mutation is the only evolutionary process that creates new alleles

They may be beneficial, neutral, or deleterious. Sometimes a loss-of-function allele can be adaptive

Determine what is incorrect in the following statement: Red aphids mutated their genes so that they could be green and avoid predation by ladybird beetles.

This statement is false. Mutations do not occur because an organism wants or needs them. They just happen by accident and can be beneficial, neutral, or deleterious.

If you were a journalist covering this story, how could you use data from this study to respond to the following web post? "Evolution is a slow process. Humans do not cause evolution in other organisms."

This statement is false. You could point out that the data show clearly that evolution in palm seed size has occurred over the few decades since humans have caused forest fragmentation and local extinction of toucans. Evolution, a change in allele frequencies in a population, can occur as quickly as in a single generation. In this case, human fragmentation of the forest has already caused measurable evolution in the forest—and this effect is likely to increase over time as the fitness of the palms declines, causing a cascade of effects on other species.

Draw a small concept map showing how selection, genetic drift, gene flow, and mutation relate to genetic variation.

Your concept map should have linking verbs that relate the following information: Selection may decrease, maintain, or increase genetic variation. Genetic drift tends to reduce it by causing random loss or fixation of alleles. Gene flow may increase or decrease it (depending on whether immigrants bring new alleles or emigrants remove alleles). Mutation increases it.

Inbreeding can

accelerate natural selection and can cause inbreeding depression

Mutation occurs too infrequently to be a major cause of change in allele frequency alone

but it is important when combined with natural selection, genetic drift, and gene flow

Toucans disperse seeds of key forest species such as juçara palms by eating the fruit and defecating the seeds in new locations, sometimes more than a kilometer away. If there are no toucans, is the genetic diversity of palms likely to increase or decrease within forest fragments (and why)?

decrease (due to decreased gene flow)

Gene flow tends to

equalize allele frequencies among populations.

The most common form of intersexual selection occurs when

females choose to mate with males who exhibit "good alleles" or the ability to provide resources to females

The introduced alleles may

have a beneficial, neutral, or deleterious effect

Gene flow can

introduce alleles from one population to another when individuals move among populations

Inbreeding—mating among relatives (or, in some species, self-fertilization)

is a form of nonrandom mating. It leads to an increase in homozygosity and a decrease in heterozygosity

The most common form of intrasexual selection occurs when

males compete with each other to gain access to female mates

Mutation occurs by chance

not because an organism wants or needs a new allele

Natural selection is the

only evolutionary process that produces adaptation

Nonrandom mating changes

only genotype frequencies, not allele frequencies, so is not an evolutionary process itself

The Hardy-Weinberg principle can serve as a null hypothesis in evolutionary studies because it

predicts what genotype and allele frequencies are expected if mating is random with respect to the gene in question and if none of the four evolutionary processes is operating on that gene

Genetic drift causes

random changes in allele frequencies

Genetic drift can result from

random fusion of gametes at fertilization, founder events, and population bottlenecks

Genetic drift is particularly important in

small populations, and it tends to reduce overall genetic diversity

The Hardy-Weinberg principle played an important role in

the synthesis of Mendelian genetics and Darwinian evolution

True or false? Gene flow can either increase or decrease the average fitness of a population. Explain.

true. If gene flow into a population introduces a beneficial allele, fitness will go up; but if gene flow introduces a deleterious allele, fitness will go down.