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Processes of Evolution
Chapter 12
Part 2
12.4 Factors That
Affect Variation in Traits
 Interactions within species, and among genes
and the environment, also influence natural
selection
•
•
•
•
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
Fig. 12-12a, p. 225
Fig. 12-12b, p. 225
Animation: Simulation of genetic drift
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
population
 Gene flow
• The physical movement of alleles into and out of
a population, as by individuals that immigrate or
emigrate
Animation: Distribution of sickle-cell trait
Animation: Life cycle of Plasmodium
Animation: Morphological differences
within a species
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
species
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
Reproductive Isolating Mechanisms
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
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 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).
Interbreeding
is successful
Fig. 12-15, p. 227
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
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 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).
Interbreeding
is successful
Stepped Art
Fig. 12-15, p. 227
Animation: Reproductive isolating
mechanisms
Mechanical Isolation
Behavioral Isolation
Animation: Albatross courtship
Allopatric Speciation
 In allopatric speciation, a geographic barrier
interrupts gene flow between populations
 Genetic divergences then give rise to new
species
 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
2
Triticum
monococcum
(einkorn)
14AA
1
×
Unknown spontaneous
species of chromosome T. turgidum
(emmer)
doubling
Triticum
14BB
14AB
28AABB
3
×
T. tauschii
(goatgrass)
14DD
T. aestivum
(common
bread
wheat)
42AABBDD
Fig. 12-19, p. 229
Animation: Sympatric speciation in
wheat
Sympatric Speciation in Cichlids
Different Speciation Models
Animation: Models of speciation
Animation: Temporal isolation among
cicadas
12.6 Macroevolution
 Macroevolution refers to large-scale patterns of
evolutionary change such as adaptive radiations,
the origin of major groups, and loss through
extinction
 Macroevolution
• Patterns of evolution that occur above the
species level
Stasis
 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
time
Stasis: The Coelacanth
Coevolution
 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
Exaptation
 In exaptation, a lineage uses a structure for a
different purpose than its ancestor did
 Exaptation
• Adaptation of an existing structure for a
completely different purpose; a major
evolutionary novelty
Extinction
 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
archipelago
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
DOMAIN
KINGDOM
PHYLUM
CLASS
ORDER
FAMILY
GENUS
SPECIES
COMMON
NAME
Eukarya
Plantae
Magnoliophyta
Magnoliopsida
Apiales
Apiaceae
Daucus
carota
carrot
Eukarya
Plantae
Magnoliophyta
Magnoliopsida
Rosales
Cannabaceae
Cannabis
sativa
marijuana
Eukarya
Plantae
Magnoliophyta
Magnoliopsida
Rosales
Rosaceae
Malus
domesticus
apple
Eukarya
Plantae
Magnoliophyta
Magnoliopsida
Rosales
Rosaceae
Rosa
acicularis
arctic rose
Eukarya
Plantae
Magnoliophyta
Magnoliopsida
Rosales
Rosaceae
Rosa
canina
dog rose
Fig. 12-24, p. 233
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
species
 Cladistics
• Method of determining evolutionary relationships
by grouping species into clades
Cladistics
 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
Cladograms
 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
cladogram
Clades
 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
Cladograms
hagfishes
lampreys
cartilaginous fishes
ray-finned fishes
lobe-finned fishes
lungfishes
amphibians
amniotes
(reptiles,
birds, and
mammals)
Fig. 12-25a, p. 234
hagfishes
animals with a skull
lampreys
cartilaginous fishes
ray-finned fishes
lobe-finned fishes
lungfishes
amphibians
amniotes
(reptiles,
birds, and
mammals)
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
legs.
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
Bacteria
Archaea
Protists
Plants
Fungi
Animals
A. This tree represents all life classified into six
kingdoms. We have discovered that the kingdom of
protists is not monophyletic, so some biologists now
divide it up into a number of new kingdoms.
Fig. 12-26a, p. 235
Bacteria
Archaea
Eukarya
B. This tree represents all life classified
into three domains. The kingdoms protists,
plants, fungi, and animals are subsumed
into domain Eukarya.
Fig. 12-26b, p. 235
Animation: Classification systems
Animation: Constructing a cladogram
Animation: Current evolutionary tree
Animation: Evolutionary tree diagrams
Animation: Interpreting a cladogram
12.8 Impacts/Issues Revisited
 The allele that confers resistance to warfarin in
rats also causes vitamin K deficiency
• The trait is polymorphic – adaptive when warfarin
is present, maladaptive when it is not
Digging Into Data: Resistance to
Rodenticides in Wild Rat Populations
not resistant to warfarin or bromadiolone
21%
58% 21%
resistant to warfarin
resistant to warfarin and bromadiolone
Olfen
5%
8%
Germany
87%
Stadtlohn
5%
5%
56%
100%
44%
Dorsten
Ludwigshafen
90%
Drensteinfurt
Fig. 12-27, p. 237