Chapter 23 review

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Natural selection changes allele frequencies because some _______ survive and reproduce more successfully than others.


No two people are genetically identical, except for identical twins. The main source of genetic variation among human individuals is

the reshuffling of alleles in sexual reproduction.

A fruit fly population has a gene with two alleles, A1 and A2. Tests show that 70% of the gametes produced in the population contain the A1 allele. If the population is in Hardy-Weinberg equilibrium, what proportion of the flies carry both A1 and A2?


You read about soapberry bugs and select the correct statement describing relative fitness in these individuals.

A soapberry bug with high relative fitness has a relatively high number of offspring that survive to reproductive age.

The fruit fly Drosophila melanogaster has an allele that confers resistance to DDT and similar insecticides. Laboratory strains of D. melanogaster have been established from flies collected in the wild in the 1930s (before the widespread use of insecticides) and the 1960s (after 20 years of DDT use). Lab strains established in the 1930s have no alleles for DDT resistance. In lab strains established in the 1960s, the frequency of the DDT-resistance allele is 37%. Which statement is correct?

The evolutionary fitness associated with the heritable trait of DDT resistance changed once DDT use became widespread.

One out of 10,000 babies born in North America is affected by cystic fibrosis, a recessive condition. Assuming that the North American human population is in Hardy-Weinberg equilibrium for this trait, what percentage of the population is heterozygous for this trait? (Remember the equation for a population in Hardy-Weinberg equilibrium: p2 + 2pq + q2 = 1.)


Allele frequencies in a gene pool may shift randomly and by chance. What is this random shift called?

genetic drift

Which statement correctly describes the role of chance in evolution?

An allele that increases evolutionary fitness cannot be lost from a population by chance events.
The ultimate source of new alleles is mutation, random changes in the nucleotide sequences of an organism’s DNA.
Evolution by natural selection proceeds by an accumulation of changes that occur by chance.

The ultimate source of new alleles is mutation, random changes in the nucleotide sequences of an organism’s DNA

If the nucleotide variability of a locus equals 0%, what is the gene variability and number of alleles at that locus?

gene variability = 0%; number of alleles = 2

There are 40 individuals in population 1, all with genotype A1A1, and there are 25 individuals in population 2, all with genotype A2A2. Assume that these populations are located far from each other and that their environmental conditions are very similar. Based on the information given here, the observed genetic variation is most likely an example of

genetic drift

What is true of microevolution?

It is the kind of evolution that can change the relative abundance of a particular allele in a gene pool over the course of generations.

The original source of all genetic variation is _____.


How does diploidy help to preserve genetic variation?

It allows recessive alleles that may not be favored in the current environment to be preserved in the gene pool by propagation in heterozygotes.

According to the Hardy-Weinberg theorem, the frequencies of alleles in a population will remain constant if _____ is the only process that affects the gene pool.

sexual reproduction

The evolutionary effects of genetic drift are greatest when _____.

the population size is small

What situation most likely explains the occasional high frequency of certain inherited disorders among human populations established by a small population?

founder effect

In evolutionary terms, an organism’s fitness is measured by its _____.

contribution to the gene pool of the next generation

Blue poppies native to China were grown at a plant-breeding center in California. The plants with the thickest leaves were most likely to survive and reproduce in the drier climate. After several generations, the percentage of thick-leaved plants had increased by 42%. This adaptation of the poppies to their new environment is due to _____.

directional selection

What is the only evolutionary mechanism that consistently leads to adaptive evolution?

natural selection

Which of the following is the best modern definition of evolution?

descent with modification

Cystic fibrosis is a genetic disorder in homozygous recessives that causes death during the teenage years. If 9 in 10,000 newborn babies have the disease, what are the expected frequencies of the dominant (A1) and recessive (A2) alleles according to the Hardy-Weinberg model?

f(A1) = 0.9700, f(A2) = 0.0300

Suppose 64% of a remote mountain village can taste phenylthiocarbamide (PTC) and must, therefore, have at least one copy of the dominant PTC taster allele. If this population conforms to Hardy-Weinberg expectations for this gene, what percentage of the population must be heterozygous for this trait?


For biologists studying a large flatworm population in the lab, which Hardy-Weinberg condition is most difficult to meet?

no mutation

The inability of organisms to evolve anything that could be an advantage reflects _____.

the limits of historical constraints

Use the following description to answer the question(s) below.

On the Bahamian island of Andros, mosquitofish populations live in various, now-isolated, freshwater ponds that were once united. Currently, some predator-rich ponds have mosquitofish that can swim in short, fast bursts; other predator-poor ponds have mosquitofish that can swim continuously for a long time. When placed together in the same body of water, the two kinds of female mosquitofish exhibit exclusive breeding preferences.

If one builds a canal linking a predator-rich pond to a predator-poor pond, then what type(s) of selection should subsequently be most expected among the mosquitofish in the original predator-rich pond, and what type(s) should be most expected among the mosquitofish in the formerly predator-poor pond?

less-intense directional selection; more-intense directional selection

A biologist doing a long-term study on a wild spider population observes increased variation in silk thickness. Which of the following could the spider population be experiencing?

disruptive selection

Which statement about the beak size of finches on the island of Daphne Major during prolonged drought is true

Each bird’s survival was strongly influenced by the depth and strength of its beak as the drought persisted.

Which statement about variation is true?

All new alleles are the result of nucleotide variability

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