17.1 Rise of the Super Rats

Rats that carry pathogens and parasites associated with infectious diseases thrive wherever people do

Fighting rats with poisons such as warfarin usually doesnt exterminate rat populations instead, it selects for rats that are genetically resistant to the poisons

Rats as Pests

Rats infesting rice fields in the Philippine Islands ruin more than 20% of the crop

17.2 Individuals Dont Evolve, Populations Do

Evolution starts with mutations in individuals, which introduces new alleles into a population

Sexual reproduction can quickly spread a mutation through a population

population A group of organisms of the same species who live in a specific location and breed with one another more often than they breed with members of other populations

Variation in Populations

Individuals of a population share morphological, physiological, and behavioral traits with a heritable basis

Variations within a population arise from different alleles of shared genes: A trait with only two forms is dimorphic; traits with more than two distinct forms are polymorphic

Traits that vary continuously often arise by interactions among alleles of several genes, and may be influenced by environmental factors

Phenotypic Variation in Humans

Sources of Variation in Traits

Genetic Event EffectMutation >Source of new allelesCrossing over >Introduces new combinations ofalleles into chromosomesIndependent >Mixes maternal and paternalassortmentchromosomesFertilization >Combines alleles from two parentsChanges in >Transposition, duplication, orchromosome loss of chromosomesnumber or structure

An Evolutionary View of Mutations

Mutations are the original source of new alleles; many are lethal or neutral mutations

lethal mutation Mutation that drastically alters phenotypeCauses death

neutral mutation A mutation that has no effect on survival or reproduction

Adaptive Mutations

Occasionally, a change in the environment favors a mutation that had previously been neutral or even somewhat harmful

Through natural selection, a beneficial mutation tends to increase in frequency in a population over generations

Mutations are the source of Earths staggering biodiversity

Allele Frequencies

All alleles in a population form a gene pool

Microevolution (changes in the allele frequencies of a population) occurs constantly by processes of mutation, natural selection, genetic drift, and gene flow

Key Terms

gene pool All of the alleles of all of the genes in a population; a pool of genetic resources

microevolution Change in allele frequencies in a population or species

allele frequency Abundance of a particular allele among members of a population

Genetic Equilibrium

A theoretical reference point, genetic equilibrium, occurs when the allele frequencies of a population do not change

It requires five conditions that are never met in nature, so natural populations are never in genetic equilibrium

genetic equilibrium Theoretical state in which a population is not evolving

Conditions of Genetic Equilibrium

Five theoretical conditions of genetic equilibrium:(1) Mutations never occur(2) Population is infinitely large(3) Population is isolated from all other populations of the species (no gene flow)(4) Mating is random(5) All individuals survive and produce the same number of offspring

Key Concepts

MicroevolutionIndividuals of a population inherit different alleles, and so they differ in phenotypeOver generations, any allele may increase or decrease in frequency in a populationSuch change is called microevolution

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17.3 A Closer Look at Genetic Equilibrium

Researchers know whether a population is evolving by tracking deviations from a baseline of genetic equilibrium

We use deviations from genetic equilibrium to study how a population is evolving

The HardyWeinberg Formula

Gene pools can remain stable only when the five theoretical conditions of genetic equilibrium are being met

Hardy and Weinberg developed a simple formula that can be used to track whether a population of any sexually reproducing species is in a state of genetic equilibrium

The following example illustrates how the Hardy-Weinberg formula is used

Allele Frequencies in Butterflies (1)

Consider a hypothetical gene that encodes a blue pigment in butterflies:Two alleles of this gene, B and b, are codominantA butterfly homozygous for the B allele (BB) has dark-blue wingsA butterfly homozygous for the b allele (bb) has white wingsA heterozygous butterfly (Bb) has medium-blue wings

Allele Frequencies in Butterflies (2)

At genetic equilibrium, the proportions of the wing-color genotypes are:

p2(BB) + 2pq(Bb) + q2(bb) = 1.0

where p and q are the frequencies of alleles B and b

This is the HardyWeinberg equilibrium equation; it defines the frequency of a dominant allele (B) and a recessive allele (b) for a gene that controls a particular trait in a population

Allele Frequencies in Butterflies (3)

The frequencies of B and b must add up to 1.0

Example: If B occupies 90% of the loci, then b must occupy the remaining 10 percent (0.9 + 0.1 = 1.0)

No matter what the proportions:

p + q = 1.0

Allele Frequencies in Butterflies (4)The Punnett square below shows the genotypes possible in the next generation (BB, Bb, and bb)

The frequencies of the three genotypes add up to 1.0:p2 + 2pq + q2 = 1.0

BB (p2 )p. 260ppqqBb (pq)BBbbbb (q2)Bb (pq)Allele Frequencies in Butterflies (4)

Allele Frequencies in Butterflies (5)

If 1,000 individuals each produces two gametes:490 BB individuals make 980 B gametes420 Bb individuals make 420 B and 420 b gametes90 bb individuals make 180 b gametes

The frequency of alleles B and b among 2,000 gametes is:

B = (980 + 420) 2,000 alleles = 1,400 2,000 = 0.7 = p

b = (180 + 420) 2,000 alleles = 600 2,000 = 0.3 = q

Allele Frequencies in Butterflies (6)

At fertilization, gametes combine at random and start a new generation

If the population size stays constant at 1,000, there will be 490 BB, 420 Bb, and 90 bb individuals

Allele frequencies for dark-blue, medium-blue, and white wings are the same as they were in the original gametes the population is not evolving

Frequencies of Wing-Color Alleles

Fig. 17.3, p. 260490 BB butterflies dark-blue wings3rd Generation90 bb butterflies white wings2nd Generation90 bb butterflieswhite wingsStarting Population490 BB butterflies dark-blue wings420 Bb butterflies medium-blue wings490 BB butterflies dark-blue wings420 Bb butterflies medium-blue wings90 bb butterflies white wings420 Bb butterflies medium-blue wingsFrequencies of Wing-Color Alleles

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Applying the Rule

In the real world, researchers can use the HardyWeinberg formula to estimate the frequency of carriers of alleles that cause genetic traits and disorders

Example: Hereditary hemochromatosis (HH) in IrelandIf the frequency of the autosomal recessive allele that causes HH is q = 0.14, then p = 0.86The carrier frequency (2pq) is calculated to be about 0.24 Such information is useful to doctors and to public health professionals

17.4 Patterns of Natural Selection

Natural selection occurs in three different patterns, depending on the organisms involved and their environment

natural selectionProcess in which environmental pressures result in differential survival and reproduction of individuals of a population who vary in details of shared, heritable traits

Three Patterns of Natural Selection

Directional selection shifts the range of variation in traits in one direction

Stabilizing selection favors intermediate forms of a trait

Disruptive selection favors forms at the extremes of a range of variation

Fig. 17.4, p. 261disruptive selectionstabilizing selectiondirectional selectionpopulation before selectionThree Patterns of Natural Selection

17.5 Directional Selection

Directional selection shifts an alleles frequency in a consistent direction, so forms at one end of a range of phenotypic variation become more common over time

directional selection Mode of natural selection in which phenotypes at one end of a range of variation are favored

Directional Selection

Bell-shaped curves indicate continuous variation in a butterfly wing-color trait

Red arrows show which forms are being selected against; green, forms that are being favored

Fig. 17.5a, p. 262Directional Selection

Fig. 17.5a, p. 262Range of values for the traitNumber of individuals in populationTime 1Directional Selection

Fig. 17.5b, p. 262Directional Selection

Fig. 17.5b, p. 262Time 2Directional Selection

Fig. 17.5c, p. 262Directional Selection

Fig. 17.5c, p. 262Time 3Directional Selection

Fig. 17.5, p. 262Stepped ArtDirectional Selection

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The Peppered Moth

The peppered moths coloration camouflages it from predatory birds

When the air was clean, trees were light-colored, and so were most peppered moths

When smoke from coal-burning factories changed the environment, predatory birds ate more white moths selection pressure favored darker moths

Directional Selection: Peppered Moth

Fig. 17.6a, p. 262Directional Selection: Peppered Moth

Fig. 17.6a, p. 262Directional Selection: Peppered Moth

Fig. 17.6b, p. 262Directional Selection: Peppered Moth

Fig. 17.6b, p. 262Directional Selection: Peppered Moth

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Rock Pocket Mice

Directional selection also affects the color of rock pocket mice in Arizonas Sonoran Desert

Mice with light fur are more common in areas with light-colored granite; mice with dark fur are more common in areas with dark basalt

Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations

Directional Selection: Rock Pocket Mice

Fig. 17.7a, p. 263Directional Selection: Rock Pocket Mice

Fig. 17.7b, p. 263Directional Selection: Rock Pocket Mice

Fig. 17.7c.1, p. 263Directional Selection: Rock Pocket Mice

Fig. 17.7c.2, p. 263Directional Selection: Rock Pocket Mice

Fig. 17.7d.1, p. 263Directional Selection: Rock Pocket Mice

Fig. 17.7d.2, p. 263Directional Selection: Rock Pocket Mice

Antibiotic Resistance

Antibiotics have been used in humans since the 1940s, but they are also fed daily to cattle, pigs, chickens, fish, and other animals raised on factory farms

Bacteria that survive this selection pressure are antibiotic-resistant an increasing problem in hospitals and schools

This trend is bad news for millions of people each year who contract cholera, tuberculosis, or another dangerous bacterial disease

****Figure 17.1 Rats as pests. Above, rats infesting rice fields in the Philippine Islands ruin more than 20 percent of the crop. Opposite, rats thrive wherever people do. Dousing buildings and soil with poisons does not usually exterminate rat populations, which recover quickly. Rather, the practice selects for rats that are resistant to the poisons.

***Figure 17.2 Sampling phenotypic variation in A (opposite) a type of snail found on islands in the Caribbean, and B humans. The variation in shared traits among individuals is mainly an outcome of variations in alleles that influence those traits.

*******************Figure 17.3 Finding out whether a population is evolving. The frequencies of wing-color alleles among all of the individuals in this hypothetical population of butterflies are not changing; thus, the population is not evolving.

****Figure 17.4 Overview of three modes of natural selection.

***Figure 17.5 Directional selection. The bell-shaped curves indicate continuous variation in a butterfly wing-color trait. Red arrows show which forms are being selected against; green, forms that are being favored.

*Figure 17.5 Directional selection. The bell-shaped curves indicate continuous variation in a butterfly wing-color trait. Red arrows show which forms are being selected against; green, forms that are being favored.

*Figure 17.5 Directional selection. The bell-shaped curves indicate continuous variation in a butterfly wing-color trait. Red arrows show which forms are being selected against; green, forms that are being favored.

*Figure 17.5 Directional selection. The bell-shaped curves indicate continuous variation in a butterfly wing-color trait. Red arrows show which forms are being selected against; green, forms that are being favored.

*Figure 17.5 Directional selection. The bell-shaped curves indicate continuous variation in a butterfly wing-color trait. Red arrows show which forms are being selected against; green, forms that are being favored.

*Figure 17.5 Directional selection. The bell-shaped curves indicate continuous variation in a butterfly wing-color trait. Red arrows show which forms are being selected against; green, forms that are being favored.

*Figure 17.5 Directional selection. The bell-shaped curves indicate continuous variation in a butterfly wing-color trait. Red arrows show which forms are being selected against; green, forms that are being favored.

***Figure 17.6 Directional selection in the peppered moth. A Light peppered moths on a nonsooty tree trunk are hidden from predators. B Dark ones stand out. In places where soot darkens tree trunks, the dark color C is more adaptive than D the light color.

*Figure 17.6 Directional selection in the peppered moth. A Light peppered moths on a nonsooty tree trunk are hidden from predators. B Dark ones stand out. In places where soot darkens tree trunks, the dark color C is more adaptive than D the light color.

*Figure 17.6 Directional selection in the peppered moth. A Light peppered moths on a nonsooty tree trunk are hidden from predators. B Dark ones stand out. In places where soot darkens tree trunks, the dark color C is more adaptive than D the light color.

*Figure 17.6 Directional selection in the peppered moth. A Light peppered moths on a nonsooty tree trunk are hidden from predators. B Dark ones stand out. In places where soot darkens tree trunks, the dark color C is more adaptive than D the light color.

***Figure 17.7 Directional selection in populations of rock pocket mice. A Mice with light fur are more common in areas with light-colored granite. B Mice with dark fur are more common in areas with dark basalt. C,D Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations.

*Figure 17.7 Directional selection in populations of rock pocket mice. A Mice with light fur are more common in areas with light-colored granite. B Mice with dark fur are more common in areas with dark basalt. C,D Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations.

*Figure 17.7 Directional selection in populations of rock pocket mice. A Mice with light fur are more common in areas with light-colored granite. B Mice with dark fur are more common in areas with dark basalt. C,D Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations.

*Figure 17.7 Directional selection in populations of rock pocket mice. A Mice with light fur are more common in areas with light-colored granite. B Mice with dark fur are more common in areas with dark basalt. C,D Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations.

*Figure 17.7 Directional selection in populations of rock pocket mice. A Mice with light fur are more common in areas with light-colored granite. B Mice with dark fur are more common in areas with dark basalt. C,D Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations.

*Figure 17.7 Directional selection in populations of rock pocket mice. A Mice with light fur are more common in areas with light-colored granite. B Mice with dark fur are more common in areas with dark basalt. C,D Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations.

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