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Chapter 17 Processes of Evolution Sections 1-6. 17.1 Rise of the Super Rats. When warfarin was used to control rats, natural selection favored individuals with a mutation in the VKORC1 gene which resulted in warfarin resistance

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17 1 rise of the super rats
17.1 Rise of the Super Rats
  • When warfarin was used to control rats, natural selection favored individuals with a mutation in the VKORC1 gene which resulted in warfarin resistance
  • When warfarin resistance increased in rat populations, people stopped using warfarin to kill rats
  • The frequency of the warfarin-resistance allele in rat populations declined, probably because rats that carry the allele are not as healthy as ones that do not
variation in populations
Variation In Populations
  • All individuals of a species share certain morphological, physiological, and behavioral traits
  • Apopulationis a group of interbreeding individuals of the same species in a specified area
  • Individuals of a population with different alleles of shared genes vary in the details of their shared traits
an evolutionary view of mutations
An Evolutionary View of Mutations
  • Mutations are the source of new alleles that give rise to differences in details of shared traits
  • Lethal mutations result in death
  • Neutral mutationshave no effect on survival or reproduction
  • Beneficial mutationsconvey an advantage
the gene pool
The Gene Pool
  • Gene pool
    • All genes found in one population
  • Alleles
    • Different forms of the same gene
    • Determine genotype and phenotype
    • Dimorphism and polymorphism
allele frequencies and microevolution
Allele Frequencies and Microevolution
  • Allele frequencyrefers to the relative abundance of a particular allele of a given gene in a population
  • Changes in allele frequency of a population (or a species) is called microevolution
  • Microevolutionary processes include mutation, natural selection, genetic drift, and gene flow
genetic equilibrium
Genetic Equilibrium
  • Under certain ideal conditions, the frequency of an allele in a sexually reproducing population’s gene pool should remain stable from one generation to the next
  • Five conditions required for a genetic equilibrium:
    • Mutations do not occur
    • Population is infinitely large
    • Population is isolated (no gene flow)
    • Mating is random
    • All individuals survive and reproduce equally
the hardy weinberg formula
The Hardy-Weinberg Formula
  • The Hardy-Weinberg formula can be used to determine if a population is in genetic equilibrium

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

  • The frequency of the dominant allele (B) plus the recessive allele (b) equals 1.0

p + q = 1.0

applying the rule
Applying the Rule
  • A population consists of 1,000 plants: 490 homozygous (BB), 420 heterozygous(Bb), and 90 homozygous (bb)
  • Each plant makes two gametes:
    • All gametes made by BB individuals have the B allele
    • All gametes made by bb individuals have the ballele
    • Bb individuals have half B gametes half b gametes
applying the rule1
Applying the Rule
  • Frequency of the B allele:

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

  • Frequency of the b allele:

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

natural selection
Natural Selection
  • Natural selectionresults from the differential survival and reproduction among individuals of a population that vary in details of their shared traits
  • Natural selection occurs in three recognizable patterns depending on the organisms and their environment:
    • Directional selection
    • Stabilizing selection
    • Disruptive selection
17 5 directional selection
17.5 Directional Selection
  • Changing environmental conditions can result in a directional shift in allele frequencies
  • Directional selection
    • Changing environmental conditions can shift allele frequencies in a consistent direction
    • Forms of traits at one end of a range of phenotypic variation become more common
slide16

Number of individuals

in population

Time 1

Range of values for the trait

Time 2

Time 3

Stepped Art

Figure 17-5 p276

directional selection in peppered moths
Directional Selection in Peppered Moths
  • In preindustrial England, most moths were light-colored, and a dominant allele that resulted in darker coloration was rare
  • In post-industrial England, pollution from coal-burning factories changed the colors of the forests
  • Predatory birds ate more light-colored moths in soot-darkened forests, and more dark-colored moths in clean forests
  • Light color is adaptive in areas of low pollution; dark color is adaptive in areas of high pollution
directional selection in rock pocket mice
Directional Selection in Rock-Pocket Mice
  • In rock-pocket mice, two alleles of a single gene control coat color
  • Night-flying owls are the selective pressure that directionally shifts the allele frequency
  • Most of the mice in populations that inhabit dark rock have dark gray coats
  • Most of the mice in populations that inhabit light brown rock have light brown coats
directional selection in antibiotic resistant bacteria
Directional Selection in Antibiotic Resistant Bacteria
  • A typical two-week course of antibiotics can exert selection pressure on over a thousand generations of bacteria
  • Antibiotics are used preventively in humans, cattle, pigs, chickens, fish, and other animals raised on factory farms
  • Bacteria with alleles that allow them to survive antibiotic treatment (antibiotic resistant strains) are now common in hospitals and schools
take home message what is the effect of directional selection
Take-Home Message: What is the effect of directional selection?
  • Directional selection causes allele frequencies underlying a range of variation to shift in a consistent direction
17 6 stabilizing and disruptive selection
17.6 Stabilizing and Disruptive Selection
  • Stabilizing selection
    • Natural selection that favors an intermediate phenotype and eliminates extreme forms
  • Disruptive selection
    • Natural selection that favors extreme forms of a trait and eliminates the intermediate forms
slide25

Number of individuals

in population

Time 1

Range of values for the trait

Time 2

Time 3

Stepped Art

Figure 17-8 p278

slide26

Number of individuals

in population

Time 1

Range of values for the trait

Time 2

Time 3

Stepped Art

Figure 17-10 p279

take home message natural selection can favor intermediate or extreme forms of traits
Take-Home Message: Natural selection can favor intermediate or extreme forms of traits
  • With stabilizing selection, an intermediate phenotype is favored, and extreme forms are selected against
  • With disruptive selection, an intermediate form of a trait is selected against, and extreme phenotypes are favored
balanced polymorphism
Balanced Polymorphism
  • Balanced polymorphism
    • A state in which natural selection maintains two or more alleles at relatively high frequencies
    • Occurs when environmental conditions favor heterozygotes
  • Example: Sickle cell anemia and malaria
    • Mosquitoes transmit the parasitic protist that causes malaria, Plasmodium, to human hosts
    • HbA/HbS heterozygotes survive malaria more often than people who make only normal hemoglobin
take home message how does natural selection maintain diversity
Take-Home Message: How does natural selection maintain diversity?
  • With sexual selection, a trait is adaptive if it gives an individual an advantage in securing mates
  • Sexual selection reinforces phenotypical differences between males and females, and sometimes gives rise to exaggerated traits
  • Environmental pressures that favor heterozygotes can lead to a balanced polymorphism
genetic drift
Genetic Drift
  • Genetic drift
    • A random change in allele frequencies over time
    • Can lead to a loss of genetic diversity, especially in small populations
  • When all individuals of a population are homozygous for an allele, that allele is fixed
bottlenecks
Bottlenecks
  • Bottleneck
    • A drastic reduction in population size brought about by severe pressure
    • After a bottleneck, genetic drift is pronounced when a few individuals rebuild a population
    • Example: Northern elephant seals
the founder effect
The Founder Effect
  • Founder effect
    • Genetic drift is pronounced when a few individuals start a new population
  • Inbreeding
    • Breeding or mating between close relatives who share a large number of alleles
    • Example: Old Order Amish in Lancaster County, Pennsylvania (Ellis-van Creveld syndrome)
gene flow
Gene Flow
  • Gene flow
    • Physical movement of alleles caused by individuals moving into and away from populations
    • Tends to counter the evolutionary effects of mutation, natural selection, and genetic drift on a population
  • Example: Movement of acorns by blue jays allows gene flow between oak populations
take home message how does a population s genetic diversity become reduced
Take-Home Message: How does a population’s genetic diversity become reduced?
  • Genetic drift, or random change in allele frequencies, can reduce a population’s genetic diversity; its effect is greatest in small populations, such as one that endures a bottleneck
  • Gene flow is the physical movement of alleles into and out of a population; it tends to counter the evolutionary effects of mutation, natural selection, and genetic drift
17 9 reproductive isolation
17.9 Reproductive Isolation
  • Speciation differs in its details, but reproductive isolating mechanisms are always part of the process
  • Speciation
    • Evolutionary process by which new species form
    • Reproductive isolating mechanisms are always part of the process
  • Reproductive isolation
    • The end of gene exchange between populations
    • Beginning of speciation
reproductive isolating mechanisms
Reproductive Isolating Mechanisms
  • Reproductive isolating mechanisms prevent interbreeding among species
  • Heritable aspects of body form, function, or behavior that arise as populations diverge
  • Prezygotic isolating mechanisms prevent pollination or mating
  • Postzygotic isolating mechanisms result in weak or infertile hybrids
prezygotic isolating mechanisms
Prezygotic Isolating Mechanisms
  • With temporal isolation populations can’t interbreed because the timing of their reproduction differs
  • With mechanical isolation, the size or shape of an individual’s reproductive parts prevent it from mating with members of another population
prezygotic isolating mechanisms cont
Prezygotic Isolating Mechanisms (cont.)
  • Populations adapted to different microenvironments in the same region may be ecologically isolated
  • In animals, behavioral differences can stop gene flow between related species (behavioral isolation)
  • In gamete incompatibility, gametes of different species meet but have molecular incompatibilities that prevent a zygote from forming
postzygotic isolation mechanisms
Postzygotic Isolation Mechanisms
  • Hybrid inviability
    • Extra or missing genes, or incompatible gene products
    • Offspring may be inviable, or have reduced fitness (ligers, tigons)
  • Hybrid sterility
    • Some interspecies crosses produce robust but sterile offspring (e.g. mules)
    • Fertile offspring may have lower fitness with successive generations
slide45

Different species form and . . .

Prezygotic reproductive isolation

Individuals reproduce at different times (temporal isolation).

Physical incompatibilities prevent individuals from interbreeding (mechanical isolation).

Individuals live in different places so they never meet up for sex (ecological isolation).

Individuals ignore or do not get the required cues for sex (behavioral isolation).

Mating occurs and . . .

No fertilization occurs (gamete incompatibility).

Zygotes form and . . .

Postzygotic reproductive isolation

Hybrid embryos die early, or new individuals die before they can reproduce (hybrid inviability).

Hybrid individuals or their offspring do not make functional gametes (hybrid sterility).

Interbreeding

is successful

Stepped Art

Figure 17-16 p284

take home message how do species attain and maintain separate identities
Take-Home Message: How do species attain and maintain separate identities?
  • Speciation is an evolutionary process by which new species form; it varies in its details and duration
  • Reproductive isolation, which occurs by one of several mechanisms, is always a part of speciation
17 10 allopatric speciation
17.10 Allopatric Speciation
  • In allopatric speciation a physical barrier arises and ends gene flow between populations
  • Genetic divergence results in speciation
  • Example: Geographic isolation of Atlantic and Pacific species caused by the formation of the Isthmus of Panama
sympatric speciation
Sympatric Speciation
  • In sympatric speciation, new species form within a home range of an existing species, in the absence of a physical barrier
  • Sympatric speciation can occur in a single generation when the chromosome number multiplies (polyploidy)
  • Example: Common bread wheat originated after related species hybridized, then the chromosome number of the hybrid offspring doubled
sympatric speciation in wheat
Sympatric Speciation in Wheat

Triticum

turgidum

(emmer)

Aegilops

tauschii

(goatgrass)

Aegilops

(wild goatgrass, unknown species)

Triticum

(hybrid)

Triticum

urartu (wild einkorn)

Triticum

aestivum

(bread wheat)

14 AA

X

14 BB

14 AB

28 AABB

X

14 DD

42 AABBDD

sympatric speciation in cichlids
Sympatric Speciation in Cichlids
  • Sympatric speciation can also occur with no change in chromosome number
  • Example: More than 500 species of cichlid speciated in the shallow waters of Lake Victoria – they vary in color and in patterning depending on differences in light color and water clarity in different parts of the lake (reproductive isolation)
parapatric speciation
Parapatric Speciation
  • In parapatric speciation, populations in contact along a common border evolve into distinct species
  • Hybrids in the contact zone are less fit than individuals on either side
  • Example: Two species of velvet walking worm in Tasmania can interbreed, but they only do so in a tiny area where their habitats overlap – hybrid offspring are sterile
17 12 macroevolution
17.12 Macroevolution
  • Macroevolutionincludes large-scale patterns of change such as one species giving rise to multiple species, the origin of major groups, and major extinction events
  • Examples:
    • Flowering plants evolved from seed plants,
    • Animals with four legs (tetrapods) evolved from fish
    • Birds evolved from dinosaurs
exaptation and stasis
Exaptation and Stasis
  • Exaptation (preadaptation)
    • Some complex traits in modern species held different adaptive value in ancestral lineages (e.g. feathers in birds and dinosaurs)
  • Stasis
    • A lineage exists for millions of years with little or no change (e.g. coelacanth)
mass extinctions
Mass Extinctions
  • Extinction
    • The irrevocable loss of a species from Earth
  • Mass extinctions
    • Extinctions of many lineages, followed by adaptive radiations
    • Five catastrophic events in which the majority of species on Earth disappeared
adaptive radiation
Adaptive Radiation
  • Adaptive radiation
    • A burst of speciation that occurs when a lineage encounters a new set of niches
  • Key innovation
    • A structural or functional adaptation that allows individuals to exploit their habitat in a new way
coevolution
Coevolution
  • Two species in close ecological contact act as agents of selection on each other (coevolution)
    • Predator and prey
    • Host and parasite
    • Pollinator and flower
  • Over time, the two species may come to depend on each other
coevolved species myrmica sabuleti and maculinea arion
Coevolved Species: Myrmicasabuletiand Maculineaarion.
  • After hatching, the larvae (caterpillars) of the large blue butterfly (Maculinea arion), feed on wild thyme flowers and then drop to the ground
  • An ant (Myrmica sabuleti)strokes the caterpillar, eats the honey the caterpillar exudes, and takes the caterpillar back to the ant nest
  • The caterpillar lives in the nest and feeds on ant larvae until it metamorphoses into a butterfly
evolutionary theory
Evolutionary Theory
  • Evolutionary biologists try to explain how all species are related by descent from common ancestors
  • Genetic change is the basis of evolution, but many biologists disagree about how it occurs
take home message what is macroevolution
Take-Home Message:What is macroevolution?
  • Macroevolution comprises large-scale patterns of evolutionary change such as adaptive radiation, the origin of major groups, and loss through extinction