Processes of Evolution
Chapter 12
12.1 Impacts/Issues
Rise of the Super Rats
 When humans tried to eradicate rats with
warfarin, natural selection favored individuals
with a mutation for warfarin resistance
Video: Rise of the super rats
12.2 Making Waves in the Gene Pool
 Individuals in a population share the same traits
(phenotype) because they share the same
genes (genotype)
 Gene pool
• All of the genes in a population
Alleles and Traits
 Alleles of the same genes are the main source
of variation in a population
• Traits with two distinct forms are dimorphic
• Traits with several distinct forms are polymorphic
• Traits with continuous variation may have
interactions of several genes or be influence by
 Mutation is the source of new alleles
Sources of Variation in Traits
Phenotypic Variation in Humans
Mutation Revisited
 Mutations are the original source of alleles, but
many are lethal or neutral
 Lethal mutation
• Mutation that drastically alters phenotype; usually
causes death
 Neutral mutation
• A mutation that has no effect on survival or
Allele Frequencies
 Microevolution (change in allele frequencies) is
always occurring in natural populations
 Microevolution
• Small-scale change in allele frequencies of a
population or species
 Allele frequency
• Abundance of a particular allele among members
of a population
Genetic Equilibrium
 Genetic equilibrium
• Theoretical state in which a population is not
evolving (allele frequencies do not change)
 Only occurs if five conditions are met:
• Mutations never occur, population is infinitely
large, population is isolated from gene flow,
mating is random, all individuals survive and
reproduce equally
Processes of Microevolution
 Genetic equilibrium does not occur in nature
because processes that drive microevolution are
always in play
Natural selection
Genetic drift
Gene flow
Animation: Adaptation to what?
Animation: How to find out if a
population is evolving
12.3 Natural Selection Revisited
 Natural selection occurs in different patterns
depending on species and selection pressures
• Directional selection
• Stabilizing selection
• Disruptive selection
Directional Selection
 Directional selection
• Mode of natural selection in which phenotypes at
one end of a range of variation are favored
• Allele frequencies shift in a consistent direction in
response to selection pressure
 Examples: peppered moths, rock pocket mice,
antibiotic-resistant bacteria
Directional Selection
Directional Selection
in Peppered Moths
 Predation pressure favors moths that are best
camouflaged when the environment changes
Fig. 12-4a, p. 219
Fig. 12-4b, p. 219
Fig. 12-4c, p. 219
Fig. 12-4d, p. 219
Directional Selection
in Rock Pocket Mice
 Mice with coat colors that do not match their
surroundings are more easily seen by predators
Stabilizing Selection
 Stabilizing selection
• Mode of natural selection in which intermediate
phenotypes are favored and extreme forms are
 Example: sociable weavers
Stabilizing Selection
Stabilizing Selection
in Sociable Weavers
 Body weight in sociable weavers is a trade off
between starvation and predation
Disruptive Selection
 Disruptive selection
• Mode of natural selection that favors extreme
phenotypes in a range of variation
• Intermediate forms are selected against
 Example: African seedcrackers
Disruptive Selection
Disruptive Selection
in African Seedcrackers
 African seedcrackers tend to have either a large
bill or a small one – but no sizes between
12.4 Factors That
Affect Variation in Traits
 Interactions within species, and among genes
and the environment, also influence natural
Sexual selection
Balanced polymorphism
Genetic drift
Gene flow
Sexual Selection
 Sexual selection leads to forms of traits that
enhance reproductive success
 Sexual dimorphism is one outcome
 Sexual selection
• Some individuals of a population outreproduce
others because they are better at securing mates
Examples of Sexual Selection
Balanced Polymorphism
 In balanced polymorphism, nonidentical alleles
for a trait are maintained in a population
 Balanced polymorphism
• Maintenance of two or more alleles for a trait in
some populations
• Occurs when environmental conditions favor
heterozygotes over homozygotes
Balanced Polymorphism
and Sickle Cell Trait
Genetic Drift
 Genetic drift can lead to the loss of genetic
diversity (fixation)
 Genetic drift
• Random change in allele frequencies in a
population over time, due to chance alone
 Fixed
• Refers to an allele for which all members of a
population are homozygous
Genetic Drift in Flour Beetles
Genetic Drift and Bottlenecks
 Genetic drift is pronounced in small or
inbreeding populations, such as those that occur
after an evolutionary bottleneck
 Bottleneck
• Reduction in population size so severe that it
reduces genetic diversity
 Inbreeding
• Nonrandom mating among close relatives
Genetic Drift and the Founder Effect
 A bottleneck can lead to the founder effect
 Founder effect
• Change in allele frequencies that occurs after a
small number of individuals establish a population
The Founder Effect in Old Order Amish
 Populations of Old Order Amish in Pennsylvania
are moderately inbred – 1 in 200 is homozygous
for the recessive allele that causes Ellis vanCreveld syndrome
Gene Flow
 Gene flow counters the effects of mutation,
natural selection, and genetic drift in a
 Gene flow
• The physical movement of alleles into and out of
a population, as by individuals that immigrate or
12.5 Speciation
 Individuals of sexually reproducing species can
interbreed successfully under natural conditions,
produce fertile offspring, and are reproductively
isolated from other species
 Speciation
• Process by which new species arise from existing
Four Butterflies, Two Species
Reproductive Isolation
 Reproductive isolation typically occurs after
gene flow stops
 Divergences then lead to speciation
 Reproductive isolation
• Absence of gene flow between populations
Different species
form and . . .
Prezygotic mechanisms
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 (ecological
Individuals ignore or do not get the
required cues for sex (behavioral
Mating occurs
and . . .
No fertilization occurs
(gamete incompatibility).
Zygotes form
and . . .
Postzygotic mechanisms
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).
is successful
Fig. 12-15, p. 227
Mechanical Isolation
Animation: Albatross courtship
Allopatric Speciation
 In allopatric speciation, a geographic barrier
interrupts gene flow between populations
 Genetic divergences then give rise to new
 Allopatric speciation
• Speciation pattern in which a physical barrier that
separates members of a population ends gene
flow between them
Allopatric Speciation in Snapping Shrimp
Sympatric Speciation
 With sympatric speciation, populations in
physical contact speciate
 Polyploid species of many plants (and a few
animals) originated by chromosome doublings
and hybridizations
 Sympatric speciation
• Pattern in which speciation occurs in the absence
of a physical barrier
Sympatric Speciation in Wheat
Unknown spontaneous
species of chromosome T. turgidum
× 14BB
T. tauschii
T. aestivum
Fig. 12-19, p. 229
Animation: Sympatric speciation in
Sympatric Speciation in Cichlids
Different Speciation Models
Animation: Models of speciation
Animation: Temporal isolation among
 With stasis, a lineage changes very little over
evolutionary time
 Stasis
• Macroevolutionary pattern in which a lineage
persists with little or no change over evolutionary
Stasis: The Coelacanth
 Coevolution occurs when two species act as
agents of selection upon one another
 Coevolution
• The joint evolution of two closely interacting
species; each species is a selective agent that
shifts the range of variation in the other
• Examples: predator and prey, host and parasite,
pollinator and flower
Coevolved Species
 Madagascar orchid and its pollinator
 Permanent loss of a species is extinction
 Extinct
• A species that has been permanently lost
 Mass extinction
• Simultaneous extinction of many lineages
Adaptive Radiation
 An adaptive radiation is a rapid diversification
into new species that occupy novel niches
 Adaptive radiation
• A burst of genetic divergences from a lineage
gives rise to many new species
 Key innovation
• An evolutionary adaptation that gives its bearer
the opportunity to exploit a particular environment
more efficiently or in a new way
Adaptive Radiation
of Hawaiian Honeycreepers
Fig. 12-23 (right), p. 232
Fig. 12-23a, p. 232
Fig. 12-23b, p. 232
Fig. 12-23c, p. 232
Fig. 12-23d, p. 232
Fig. 12-23e, p. 232
Fig. 12-23f, p. 232
Fig. 12-23g, p. 232
Fig. 12-23h, p. 232
Fig. 12-23i, p. 232
Fig. 12-23j, p. 232
Fig. 12-23k, p. 232
Evolutionary Theory
 Many biologists disagree about how
macroevolution occurs
 Dramatic jumps in morphology may result from
mutations in homeotic or other regulatory genes
 Macroevolution may be an accumulation of
many microevolutionary events, or an entirely
different process
Animation: Allopatric speciation on an
12.7 Organizing Information
About Species
 Taxonomy
• Science of naming and classifying species
• In traditional taxonomy, species are organized
into a series of ranks (taxa) based on their traits
• Such systems do not necessarily reflect
evolutionary relationships
 Taxon (taxa)
• A grouping of organisms
Linnaean Classification System
Ranking Versus Grouping
 Cladistics is a set of methods that allow us to
reconstruct evolutionary history (phylogeny)
 Phylogeny
• Evolutionary history of a species or group of
 Cladistics
• Method of determining evolutionary relationships
by grouping species into clades
 Cladistics groups species into clades on the
basis of shared characters
 Character
• Quantifiable, heritable characteristic or trait
 Clade
• A group of species that share a set of characters
Cladistic Analysis
 The result of a cladistic analysis is an
evolutionary tree diagram in which a line
represents a lineage
 Evolutionary tree
• Type of diagram that summarizes evolutionary
relationships among a group of species
 In evolutionary trees called cladograms, a line
(lineage) can branch into two sister groups at a
node, which represents a shared ancestor
 Cladogram
• Evolutionary tree diagram that shows a network
of evolutionary relationships among clades
 Sister groups
• Two lineages that emerge from a node on a
 Every branch of a cladogram ends in a clade
 Ideally, each clade is a monophyletic group
 Monophyletic group
• An ancestor and all of its descendants
animals with a skull
cartilaginous fishes
ray-finned fishes
lobe-finned fishes
birds, and
animals with a
backbone and a skull
animals with a swim
bladder or lungs, a
backbone, and a skull
animals with four limbs,*
a swim bladder or lungs,
a backbone, and a skull
animals with four
membranes around their
eggs, four limbs,* a swim
bladder or lungs, a
backbone, and a skull
*Snakes are included in
these clades because
their ancestors had four
Fig. 12-25b, p. 234
Classification Systems
 Evolutionary trees are revised as new
information is gathered
 Two different ways to organize life’s diversity
• Six-kingdom classification system
• Three-domain classification system
Two Major Classification Systems
Animation: Current evolutionary tree
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