in-text art, ch. 15, p. 289 (1)

In-Text Art, Ch. 15, p. 289 (1)

In-Text Art, Ch. 15, p. 289 (2)

Figure 15.1 The Voyage of the Beagle

Figure 15.2 Milestones in the Development of Evolutionary Theory

Concept 15.1 Evolution Is Both Factual and the Basis of Broader Theory

Discuss the validity of the following statement:

When scientists speak of evolutionary theory, the word “theory” has the same meaning as it does in everyday language, referring to the fact that evolutionary theory is just an idea that is not proven and backed by scientific evidence.

Concept 15.1 Evolution Is Both Factual and the Basis of Broader Theory

Consider the validity of the following statement and then select a correct answer from the options given:

When scientists speak of evolutionary theory, the word “theory” has the same meaning as it does in everyday language, referring to the fact that evolutionary theory is just an idea that is not proven and backed by scientific evidence.

a. True

b. False

c. I don’t understand the question.

Concept 15.2 Mutation, Selection, Gene Flow,Genetic Drift, and Nonrandom Mating Result in Evolution

Biological evolution refers to changes in the genetic makeup of populations over time.

Population—a group of individuals of a single species that live and interbreed in a particular geographic area at the same time.

Individuals do not evolve; populations do.


Because of mutation, different forms of a gene, or alleles, may exist at a locus.

Gene pool—sum of all copies of all alleles at all loci in a population.

Allele frequency—proportion of each allele in the gene pool.

Genotype frequency—proportion of each genotype among individuals in the population.

Figure 15.3 A Gene Pool

Figure 15.4 Many Vegetables from One Species

Figure 15.5 Artificial Selection

Figure 15.6 Artificial Selection Reveals Genetic Variation


Genetic drift—random changes in allele frequencies from one generation to the next.

In small populations, it can change allele frequencies. Harmful alleles may increase in frequency, or rare advantageous alleles may be lost.


A population bottleneck—an environmental event results in survival of only a few individuals.

Genetic drift can change allele frequencies.

Populations that go through bottlenecks loose much of their genetic variation.


Founder effect—genetic drift changes allele frequencies when a few individuals colonize a new area.

Figure 15.7 A Population Bottleneck


Sexual selection—mates are chosen based on phenotype, e.g., bright-colored feathers of male birds.

There may be a trade-off between attracting mates (more likely to reproduce) and attracting predators (less likely to survive).


Or, phenotype may indicate a successful genotype, e.g., female frogs are attracted to males with low-frequency calls, which are larger and older (hence successful).

Studies of African long-tailed widowbirds showed that females preferred males with longer tails, which may indicate greater health and vigor.

Figure 15.8 What Is the Advantage?

Figure 15.9 Sexual Selection in Action

Figure 15.9 Sexual Selection in Action (Part 1)

Figure 15.9 Sexual Selection in Action (Part 2)

Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Random Mating Result in Evolution

Discuss the following scenarios with reference to whether or not they correctly describe examples of the process we attribute to having been first described by Charles Darwin - evolution by “natural selection”:

• The development of a curved back over the period of your lifetime

• Giraffes’ necks lengthening during their lifetime as they reach up to high branches to eat the leaves of trees

• A drought affects an island where a population of a particular finch species lives. The species naturally has a small amount of variability in bill (beak) size. The drought results in finches with larger bills surviving at a greater rate than those with smaller bills, since the larger billed birds can crack open and eat very tough seeds that the small billed individuals cannot.

• A mutation in an insect results in increased digestive efficiency that allows females to obtain more energy from their food, and convert that energy into larger eggs that are more likely to survive, resulting in these females producing more surviving offspring

Concept 15.2 Mutation, Selection, Gene Flow, Genetic Drift, and Random Mating Result in Evolution

Which of the following scenarios correctly describe examples of the process we attribute to having been first described by Charles Darwin - evolution by “natural selection”:

a. The development of a curved back over the period of your lifetime

b. Giraffes’ necks lengthening during their lifetime as they reach up to high branches to eat the leaves of trees

c. A drought affects an island where a population of a particular finch species lives. The species naturally has a small amount of variability in bill (beak) size. The drought results in finches with larger bills surviving at a greater rate than those with smaller bills, since the larger billed birds can crack open and eat very tough seeds that the small billed individuals cannot.

d. A mutation in an insect results in increased digestive efficiency that allows females to obtain more energy from their food, and convert that energy into larger eggs that are more likely to survive, resulting in these females producing more surviving offspring

e. Both c and d

Concept 15.3 Evolution Can Be Measured by Changes in Allele Frequencies

Evolution can be measured by change in allele frequencies.

Allele frequency =

population in alleles all of copies of number total

population in allele of copies of number


For two alleles at a locus, A and a, three genotypes are possible: AA, Aa, and aa.

p = frequency of A; q = frequency of a

N

NNp AaAA

2

2

N

NNq Aaaa

2

2

Figure 15.10 Calculating Allele and Genotype Frequencies


For each population, p + q = 1, and q = 1 – p.

Monomorphic: only one allele at a locus, frequency = 1. The allele is fixed.

Polymorphic: more than one allele at a locus.

Genetic structure—frequency of alleles and genotypes of a population.


Hardy–Weinberg equilibrium—allele frequencies do not change across generations; genotype frequencies can be calculated from allele frequencies.

If a population is at Hardy-Weinberg equilibrium, there must be no mutation, no gene flow, no selection of genotypes, infinite population size, and random mating.


At Hardy-Weinberg equilibrium, allele frequencies don’t change.

Genotypes frequencies:

Genotype AA Aa aaFrequency p2 2pq q2

Figure 15.11 One Generation of Random Mating Restores Hardy–Weinberg Equilibrium

Figure 15.11 One Generation of Random Mating Restores Hardy–Weinberg Equilibrium (Part 1)

Figure 15.11 One Generation of Random Mating Restores Hardy–Weinberg Equilibrium (Part 2)


Probability of 2 A-gametes coming together:

Probability of 2 a-gametes coming together:

Overall probability of obtaining a heterozygote:

3025.0)55.0( 22 ppp

0.2025qqq 22 )45.0(

0.495pq 2

Apply the Concept page 299

Evolution can be measured by changes in allele frequencies

Imagine you have discovered a new population of curly-tailed lizards established on an island after immigrants have arrived from several different source populations during a hurricane. You collect and tabulate genotype data for the lactate dehydrogenase gene (ldh) for each of the individual lizards

Use the table to answer the following questions.

1. Calculate the allele and genotype frequencies of ldh in this newly founded population.

2. Is the population in Hardy-Weinberg equilibrium? If not, which genotypes are over- or underrepresented? Given the population’s history, what is a likely explanation for your answer?

3. Under Hardy-Weinberg assumptions, what allele and genotype frequencies do you predict for the next generation?

4. Imagine that you are able to continue studying this population and determine the next generation’s actual allele and genotype frequencies. What are some of the principal reasons you might expect the observed allele frequency to differ from the Hardy-Weinberg expectations you calculated in question 3?

Apply the Concept, Ch. 15, p. 299


Consider and discuss the following scenarios in relation to Hardy–Weinberg equilibrium, and determine whether or not allele frequencies are likely to change, leading to evolution:

• An isolated and highly endangered population of 50 woodland caribou

• A large population of lizards whose males have red, blue, or green tails; females preferentially mate with red-tailed males

• A large population of fish in an isolated lake; every 5 years a flood results in some fish from a population in an adjacent lake mixing with this population


In which of the following scenarios are allele frequencies are likely to change, leading to evolution, according to Hardy–Weinberg assumptions:

a. An isolated and highly endangered population of 50 woodland caribou

b. A large population of lizards whose males have red, blue, or green tails; females preferentially mate with red-tailed males

c. A large population of fish in an isolated lake; every 5 years a flood results in some fish from a population in an adjacent lake mixing with this population

d. All of the above

e. None of the above

Concept 15.4 Selection Can Be Stabilizing, Directional, or Disruptive

Natural selection can act on quantitative traits in three ways:

• Stabilizing selection favors average individuals.

• Directional selection favors individuals that vary in one direction from the mean.

• Disruptive selection favors individuals that vary in both directions from the mean.


Stabilizing selection reduces variation in populations, but does not change the mean.

It is often called purifying selection—selection against any deleterious mutations to the usual gene sequence.

Figure 15.12 Natural Selection Can Operate in Several Ways

Figure 15.12 Natural Selection Can Operate in Several Ways (Part 1)

Figure 15.13 Human Birth Weight Is Influenced by Stabilizing Selection

Figure 15.14 Long Horns Are the Result of Directional Selection

Figure 15.15 Disruptive Selection Results in a Bimodal Character Distribution


A flock of 150 tiny orange and brown sparrows is blown off course and ends up on a huge island where there is a lot of open shrubby land adjacent to low hills with trees. There are mammals, many plants, some insects, lizards, and a few hawks, but there are no other small birds. There are two types of plants with seeds edible for the sparrows: a small-seeded tree and a large-seeded bush.

Discuss what you think might happen to this population of birds over many generations with respect to the three different types of selection discussed in the text:

• Stabilizing

• Directional

• Disruptive


What you think might happen to this population of birds over many generations (refer to graphs below)?

a. Stabilizing selection will operate on population beak size.

b. Directional selection will operate on population beak size.

c. Disruptive selection will operate on population beak size.

d. Population beak size will not change; the birds will maintain their original genetic diversity.

A B C

Concept 15.5 Genomes Reveal Both Neutraland Selective Processes of Evolution

Types of mutations: • Nucleotide substitution—change in one nucleotide in

a DNA sequence (a point mutation).• Synonymous substitution—most don’t affect

phenotype because most amino acids are specified by more than one codon.

• Nonsynonymous substitution—deleterious or selectively neutral.


Substitution rates are highest at positions that do not change the amino acid being expressed.

Substitution is even higher in pseudogenes, copies of genes that are no longer functional.

Figure 15.16 When One Nucleotide Changes

Figure 15.16 When One Nucleotide Changes (Part 1)

Figure 15.16 When One Nucleotide Changes (Part 2)

Figure 15.17 Rates of Substitution Differ


Neutral theory—at the molecular level, the majority of variants in most populations are selectively neutral.

Neutral variants must accumulate through genetic drift rather than positive selection.


Rate of fixation of neutral mutations by genetic drift is independent of population size.

N = population size

μ = neutral mutation rate

N

N2

12


Relative rates of substitution types differ as a function of selection:

• If similar, the corresponding amino acid is likely drifting neutrally among states.

• If nonsynonymous substitution exceeds synonymous, positive selection results in change in the corresponding amino acid.

• If synonymous substitution exceeds nonsynonymous, purifying selection resists change in the corresponding amino acid.

Concept 15.5 Genomes Reveal Both Neutral and Selective Processes of Evolution

In a population of size N of a diploid organism, the rate of fixation of neutral mutations () in this population is given by the equation:

From this equation, discuss what can we deduce about the influence of population size (N) on the rate of fixation of neutral mutations in a population.


In a population of size N of a diploid organism, the rate of fixation of neutral mutations () in this population is given by the equation:

From this equation, we can deduce that population size (N) has _______ on the rate of fixation of neutral mutations?

a. an important effect

b. no effect

c. I don’t understand the question.


Evolution of lysozyme:

Lysozyme digests bacteria cell walls; found in almost all animals as a defense mechanism.

Some mammals are foregut fermenters, which has evolved twice—in ruminants and leaf-eating monkeys (langurs). Lysozyme in these lineages has been modified to rupture some bacteria in the foregut to release nutrients.


Lysozyme-coding sequences were compared in foregut fermenters and their non-fermenting relatives, and rates of substitutions were determined.

The rate of synonymous substitution in the lysozyme gene was much higher than nonsynonymous, indicating that many of the amino acids are evolving under purifying selection.


Replacements in lysozyme happened at a much higher rate in langur lineage.

Lysozyme went through a period of rapid change in adapting to the stomachs of langurs.

Lysozymes of langurs and cattle share five convergent amino acid replacements, which make the protein more resistant to degradation by the stomach enzyme pepsin.

Figure 15.18 Convergent Molecular Evolution of Lysozyme

Figure 15.18 Convergent Molecular Evolution of Lysozyme (Part 1)


Lysozyme in the crop of the hoatzin, a foregut-fermenting bird, has similar adaptations as those of langurs and cattle.

Figure 15.18 Convergent Molecular Evolution of Lysozyme (Part 2)

Apply the concept page 305

Genomes reveal both neutral and selective processes of evolution

Analysis of synonymous and nonsynonymous substitutions in protein-coding genes can be used to detect neutral evolution, positive selection, and purifying selection. An investigator compared many gene sequences that encode the protein hemagglutinin (a surface protein of influenza virus) sampled over time, and collected this data.

Use the table to answer the questions.

1. Which codon positions encode amino acids that have probably changed as a result of positive selection? Why?

2. Which codon position is most likely to encode an amino acid that drifts neutrally among states?

3. Which codon positions encode amino acids that have probably changed as a result of purifying selection?

Apply the Concept, Ch. 15, p. 305

Figure 15.19 A Heterozygote Mating Advantage

Figure 15.19 A Heterozygote Mating Advantage (Part 1)

Figure 15.19 A Heterozygote Mating Advantage (Part 2)

Concept 15.6 Recombination, Lateral Gene Transfer,and Gene Duplication Can Result in New Features

Sexual reproduction results in new combinations of genes and produces genetic variety that increases evolutionary potential.

But in the short term, it has disadvantages:• Recombination can break up adaptive combinations of

genes• Reduces rate at which females pass genes to

offspring• Dividing offspring into genders reduces the overall

reproductive rate


Why did sexual reproduction evolve? Possible advantages:

• It facilitates repair of damaged DNA. Damage on one chromosome can be repaired by copying intact sequences on the other chromosome.

• Elimination of deleterious mutations through recombination followed by selection.


• In asexually reproducing species, deleterious mutations can accumulate; only death of the lineage can eliminate them

Muller called this the genetic ratchet—mutations accumulate or “ratchet up” at each replication; Muller’s ratchet.


• The variety of genetic combinations in each generation can be advantageous (e.g., as defense against pathogens and parasites).

Sexual recombination does not directly influence the frequencies of alleles. Rather, it generates new combinations of alleles on which natural selection can act.

Concept 15.6 Recombination, Lateral Gene Transfer, and Gene Duplication Can Result in New Features

Why sex is good

Scientists have long puzzled over why sex has evolved, given the disadvantages of sex:

1. Gene mixing tends to break up favorable combinations, and why break up a good thing?

2. Asexual reproduction is twice as efficient as sexual reproduction at passing on genes to the next generation. Every time a sexual mother produces a child, only one-half of the child’s genes come from the mother; the other half are from the father. Reproducing parthenogenetically, an asexual mother passes on to her child a complete copy of her genes. It stands to reason that such populations should rapidly out-reproduce a sexual population, since every individual is a female that can reproduce offspring.


Why sex is good (continued)

For these two reasons, it seems clearly disadvantageous for individuals to reproduce sexually! Yet sex has evolved and some kind of genetic recombination (sex) occurs and retained in most organisms.

German biologist August Weismann proposed one possible explanation for this conundrum, suggesting that sex increases advantageous genetic variation.

When two different individuals mate by joining their gametes together, they produce a brand new genetic mixture and this promotes evolutionary adaptation. In other words, sex is good because it allows you to evolve more quickly when conditions change.



A team of scientists at the Imperial College London investigated the hypothesis that the genetic recombination that results from sexual reproduction is advantageous. They published their results in Nature magazine in March 2005.

They performed an experiment on yeasts, which are single-celled fungi. Yeasts can reproduce both sexually and asexually, are easy to keep in the lab, and reproduce rapidly.

Yeasts normally reproduce asexually, but will reproduce sexually when they are stressed (starved, high temperatures, etc.). The team of scientists did not want this sexual/asexual switching to occur so they genetically manipulated one asexual strain. They deleted the two genes required for normal meiosis, so that sexual reproduction was impossible. Now they had two pure strains of yeast—an asexual strain and a sexual strain.



The team compared the reproductive rate of the asexual vs. the sexual yeasts in two different environments: one benign and one harsh.

• The benign environment had plenty of nutrients although glucose was limited so that growth was not uncontrolled.

• The harsh environment had the same glucose concentration but was at a higher temperature and had more demanding osmotic conditions (e.g., the water was more salty).

Evolutionary “fitness” was measured by comparing the growth rate of the asexual and sexual strains of yeast.


On the graph below, plot the results you would expect if Weismann’s hypothesis were correct. Plot the changes in fitness values over time in the populations of sexual yeasts in benign conditions, asexual yeasts in benign conditions, asexual yeasts in harsh conditions, and sexual yeasts in harsh conditions.


From this graph showing the experimental results of growing genetically manipulated sexual and asexual yeast strains in harsh versus benign conditions, we can interpret that:

a. Sexual reproduction is advantageous in harsh environments.

b. Asexual reproduction is always equally advantageous to sexual reproduction.

c. Asexual reproduction is always advantageous to sexual reproduction.

d. There is no difference in fitness between sexual and asexually reproducing yeasts.

e. Sexual reproduction is always advantageous to asexual reproduction.


The amount of nonconding DNA may be related to population size.

Noncoding sequences that are only slightly deleterious are likely to be purged by selection most efficiently in species with large population sizes.

In small populations genetic drift may overwhelm selection against these sequences.

Figure 15.20 Genome Size Varies Widely

Figure 15.21 A Large Proportion of DNA Is Noncoding


Lateral gene transfer—individual genes, organelles, or genome fragments move horizontally from one lineage to another.

• Species may pick up DNA fragments directly from the environment.

• Genes may be transferred to a new host in a viral genome.

• Hybridization results in the transfer of many genes.


Lateral gene transfer can be advantageous to a species that incorporates novel genes.

Genes that confer antibiotic resistance are often transferred among bacteria species.


Gene duplication—genomes can gain new functions.

Gene copies may have different fates:

1. Both copies retain original function (may increase amount of gene product).

2. Gene expression may diverge in different tissues or at different times in development.


3. One copy may accumulate deleterious mutations and become a functionless pseudogene.

4. One copy retains original function, the other changes and evolves a new function.


Sometimes entire genomes may be duplicated, providing massive opportunities for new functions to evolve.

In vertebrate evolution, genomes of the jawed vertebrates have 4 diploid sets of many genes.

Two genome-wide duplication events occurred in the ancestor of these species. This allowed specialization of individual vertebrate genes.

Figure 15.22 A Globin Family Gene Tree

Figure 15.23 In Vitro Evolution

Figure 15.23 In Vitro Evolution (Part 1)

Figure 15.23 In Vitro Evolution (Part 2)

Concept 15.7 Evolutionary Theory Has Practical Applications

Discuss the validity of following statements given what you have learned about evolution:

• Pesticide resistance exhibited by insects in agricultural settings provides direct evidence that evolution is occurring.

• Antibiotic resistance is an example of evolution that helps to keep pharmaceutical companies in business.

• In vitro evolution at the molecular level, as described in Figure 15.23 of the textbook, is analogous to the artificial selection of pigeons and dogs that Darwin was interested in more than two centuries ago.

Concept 15.7 Evolutionary Theory Has Practical Applications

Given what you have learned about evolution, which of the following statements is true?

a. Pesticide resistance exhibited by insects in agricultural settings provides direct evidence that evolution is occurring.

b. Antibiotic resistance is an example of evolution that helps to keep pharmaceutical companies in business.

c. In vitro evolution at the molecular level, as described in Figure 15.23 of the textbook, is analogous to the artificial selection of pigeons and dogs that Darwin was interested in more than two centuries ago.

d. All of the above

e. None of the above

Figure 15.24 Evolutionary Analysis of Surface Proteins Leads to Improved Flu Vaccines

in-text art, ch. 15, p. 289 (1)

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