Processes of Evolution
Chapter 17
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
Rise of the Super Rat
 Rat (Rattus)
•
•
•
•
Most notorious of mammalian pests
City = 1 rat for every 10 people
Reproduce fast
Cost us about $19 billion per year due to damages
(infection or property)
• Warfarin – rodenticide of choice
• Blood thinner  internal bleeding and increase blood
loss from cuts or scrapes
• 10% of rats in urban area in the US are resistant to
warfarin
• Gene mutation (enzyme insensitive to inhibitory
action of warfarin)
Rise of the Super Rat
 Rat (Rattus)
• Mutation = Evolution by natural selection
• Warfarin = pressure on rat population
• Pressure lead to population changes
• Survivors (or mutants) survived while normal rats
died
• Survivors passed warfarin-resistance alleles to their
offspring
• Selective pressure can and do change
• Alternating pressures in the key to success!!
Rise of the Super Rats
17.2 Populations Evolve
 Population
• Individuals of the same species in the same area
• Generally the same number and kinds of genes
for the same traits
 Gene pool
• All the alleles of the genes of a population
• Pool of genetic resources
Variation in Alleles
 Individuals who inherit different combinations of
alleles vary in details of one or more traits
• Polymorphism: Several alleles in a population
 Mutations are the original source of new alleles
• Lethal mutations result in altered phenotype and
death
• Neutral mutations neither help nor hurt
Phenotypic Variation in Populations
Microevolution
 Allele frequency
• Abundance of a particular allele among members
of a population
• Change in an allele’s frequency in a population is
the same thing as change in a line of descent
 Microevolution (evolution) = Changes in allele
frequencies of a population
•
•
•
•
Mutation
Natural selection
Genetic drift
Gene flow
When Is A Population Not Evolving?
 Genetic equilibrium
• A state in which a population is not evolving
• Never occurs in nature
 Deviations from theoretical genetic equilibrium
are used to study how a population is evolving
17.3 Hardy-Weinberg Equilibrium Equation
 Independently applied the rules of probability to
sexually reproducing population
 Discovered that Gene Pools can remain stable
only when 5 conditions are met
1. Mutations do not occur
2. The population is infinitely large
3. The population is isolated from all other
populations of the species
4. Mating is random
5. All individuals survive and produce the same
number of offspring
17.3 Hardy-Weinberg Equilibrium Equation
 If frequency of alleles in a population changes,
the population is evolving
p + q = 1.0
p2 + 2pq + q2 = 1.0
p and q = frequencies of alleles A and a
2 pq = carrier frequency
***Page 260-261 in TEXT***
Is This Population Evolving? NO!!
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
490 AA butterflies
dark-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
medium-blue wings
420 Aa butterflies
medium-blue wings
90 aa butterflies
white wings
90 aa butterflies
white wings
90 aa butterflies
white wings
Starting Population
Next Generation
Next Generation
Fig. 17.3, p.268
Natural Selection
 Natural selection
• Environmental pressure result in the differential
survival and reproduction among individuals of a
population that vary in the details of shared,
heritable traits(alleles)
 Allele frequencies
• Maintained by stabilizing selection
• Shifted by directional or disruptive selection
• Directional selection  mode of natural selection in
which phenotypes at one end of a range of variation
are favored
17.5 Modes of Natural Selection
Directional Selection
 Shifts range of variation in traits in one direction
• Individuals at one end of the range are favored;
those at the other end are not
 Examples:
• Peppered moth, pocket mice (predation)
• Antibiotic resistance
Directional
Selection
Peppered Moth
Pocket Mice
17.6 Selection For or Against
Extreme Phenotypes
 Stabilizing Selection
• Mode of natural selection in which intermediate
forms of a trait are favored over extremes
• Favors intermediate forms
 Disruptive selection
• Mode of natural selection that favors forms of a
trait at the extremes of a range of variation
• Favors forms at extremes of the range
Stabilizing and Disruptive Selection
Stabilizing Selection: Birth Weight
Disruptive Selection: Finch Bill Size
Soft seed eaters
Hard seed eaters
Intermediate-sized bills can not open soft or hard seeds as efficiently.
Decreased survival!
17.7 Maintaining Variation
1. Sexual Selection (sexual dimorphism)
• A female or a male acts as an agent of selection on
its own species
• Some individuals out reproduce others of a population
because they are better at securing mates
• Leads to trait forms that favor reproductive success
2. Balanced polymorphism
• Nonidentical alleles for a trait are maintained at
relatively high frequencies in a population
• Result of natural selection against homozygous
Sexual Selection
Balanced Polymorphism
Malaria
Sickle-cell allele
Increase survival from malaria
17.8 Genetic Drift
 Genetic drift
• Random change in a population’s allele frequencies
over time, due to chance
• Can lead to loss of genetic diversity
 Fixed
• Refer to an allele for which all members of a
population are homozygous
• Frequency of fixed alleles will not change unless
mutation or another process introduces a different
allele
Effect of Population Size on Genetic Drift
Frequency of b+ allele
100%
50%
N = 10
0
0
4
8
12
16
20
generations
N = number of breeding individuals per generation
a The size of these populations of beetles was maintained at 10 breeding
individuals. Allele b+ was lost in one population (one graph line ends at 0).
Drift was greatest in sets of 10 beetles
and least in the sets of 100.
Fig. 17.14a, p.276
Frequency of b+ allele
100%
50%
N = 100
0
0
4
8
12
16
20
generations
N = number of breeding individuals per generation
b The size of these populations was maintained at 100 individuals. Drift
in these populations was less than the small populations in (a).
Fig. 17.14b, p.276
17.8 Genetic Drift
 Most pronounced in small or inbred populations
• Bottleneck:
• Drastic reduction in population so severe that is
reduces genetic diversity
• Can result in a few individuals rebuilding a population
or starting a new one
• Example elephant seals (hunted until 20 remained,
hunted banned and now 170,000 exist)
• Genetic drift after bottleneck  fixed all the alleles in the
population
17.8 Genetic Drift
 Most pronounced in small or inbred populations
• Founder effect:
• change in allele frequencies that occurs when a small
number of individuals establish a new population
• Occurs if the small group is not representative of the
original population in terms of allele frequencies
• New population is not representative of the old
• Genetic diversity of reduced
Founder Effect
17.9 Gene Flow
 Gene flow
• Movement of alleles into or out of a population by
immigration or emigration
• Helps keep populations of same species similar
 Counters processes that cause populations to
diverge (mutation, natural selection, genetic drift)
Gene Flow Between Oak Populations
Key Concepts:
MICROEVOLUTION
 Populations evolve
 Individuals of a population differ in which alleles
they inherit, and thus in phenotypes
 Over generations, any allele may increase or
decrease in frequency in a population
 Such shifts occur by the microevolutionary
processes of mutation, natural selection, genetic
drift, and gene flow
17.10 Reproductive Isolation
 Individuals of a sexually reproducing species can
produce fertile offspring, but are reproductively
isolated
• Sexually reproducing species attain and maintain
their separate identities
• Prevent interbreeding and reinforces differences
between diverging populations
 Reproductive isolating mechanisms evolve when
gene flow between populations stops
17.10 Reproductive Isolation
 Reproductive isolation is always part of speciation
• Speciation  one of several processes by which
new species arise
 Divergences may lead to new species
Reproductive Isolating Mechanisms
 Prezygotic isolating mechanisms – pollination or
mating cannot occur or zygotes can not form
•
•
•
•
•
Temporal isolation
Mechanical isolation
Behavioral isolation
Ecological isolation
Gamete incompatibility
 Postzygotic isolating mechanisms – if hybrids
form but are infertile
• Hybrid sterility or in-viability
Reproductive Isolating Mechanisms
Different
species!
Prezygotic isolating mechanisms
Temporal isolation: Individuals of different species
reproduce at different times.
Mechanical isolation: Individuals cannot mate or
pollinate because of physical incompatibilities.
Behavioral isolation: Individuals of different species ignore
or do not get the required cues for sex. Ex. Different
vocalizations and movements from the male bird.
Ecological isolation: Individuals of different species live in
different places and never do meet up.
They interbreed
anyway.
Gamete incompatibility: Reproductive cells
meet up, but no fertilization occurs.
Zygotes form,
but . . .
Postzygotic isolating mechanisms
Hybrid inviability: Hybrid embryos may die early, OR new
individuals die before they can reproduce due to reduced
fitness, health, and life expectancy. Ex. Ligers and tigons
Hybrid sterility: Hybrid individuals or their
offspring do not make functional gametes. Ex. Mule
(female horse and male donkey) OR offspring have lower
and lower fitness with each successive generation
Mechanical Isolation
Black sage  honeybees
White sage  larger bees
and hawkmoths bc
honeybees are too small
Behavioral Isolation
Species-specific
courtship prior to sex.
17.11 Allopatric Speciation
 Allopatric speciation  One way new species form
• Speciation pattern in which a physical barrier that
separates members of a population ends gene flow
between them.
 A geographic barrier stops gene flow between two or
more populations of a species
• Example: The Great Wall of China and insectpollinated plants, isolated continents or archipelagos
 Genetic divergence and reproductive isolation give
rise to new species
An Isolated Archipelago
17.12 Other Speciation Models
 Sympatric speciation (sym – same)
• Pattern in which speciation occurs in the absence
of a physical barrier
• Populations in physical contact diverge into
separate species
• Polyploid species of many plants originated by
chromosome doublings and hybridizations
Sympatric Speciation: Wheat
Triticum
Unknown
monococcum species of
(einkorn)
wild wheat
a By 11,000 years ago, humans
were cultivating wild wheats.
Einkorn has a diploid
chromosome number of 14
(two sets of 7). It probably
hybridized with another wild
wheat species having the
same number of
chromosomes.
Hybridization
was followed by
spontaneous
chromosome
doubling.
T. turgidum
(wild emmer)
b About 8,000 years ago, the alloploid
wild emmer originated from an AB
hybrid wheat plant in which the
chromosome number doubled. Wild
emmer is tetraploid, or AABB; it has
two sets of 14 chromosomes. There
is recently renewed culinary interest
in emmer, also called farro.
T.
tauschii
(a wild
relative)
T. aestivum
(one of the
common
bread
wheats)
c AABB emmer probably
hybridized with T. tauschii, a
wild relative of wheat. Its diploid
chromosome number is 14
(two sets of 7 DD). Common
bread wheats have a
chromosome number of 42
(six sets of 7 AABBDD).
Fig. 17.23, p.282
Other Speciation Models
 Parapatric speciation
• Different selection pressures lead to divergences
within a single population
• Occur when one population extends across a
broad region encompassing diverse habitats
• Different habitats exert distinct selection pressures
on parts of the population  Divergence 
speciation
• Hybrids form in the contact zone between
habitats
• Hybrids are less fit than individuals outside of the
zone
Parapatric Speciation
Walking worms can
interbreed.
Hybrids are sterile.
T. barretti
hybrid zone
T. anophthalmus
Fig. 17.25c, p.283
Different Speciation Models
Key Concepts:
HOW SPECIES ARISE
 Sexually reproducing species consist of one or
more populations of individuals that interbreed
successfully under natural conditions, produce
fertile offspring, and are reproductively isolated
from other species
 The origin of new species varies in details and
duration
Key Concepts:
HOW SPECIES ARISE (cont.)
 Typically, speciation starts after gene flow ends
between parts of a population
 Microevolutionary events occur independently,
lead to genetic divergence of subpopulations
 Such divergences are reinforced as reproductive
isolation mechanisms evolve
17.13 Macroevolution
 Review: Microevolution is a change in allele
frequencies within a single species or population
 Macroevolution  Large-scale patterns of evolution
• One species giving rise to others
• Dinosaur  Bird
• Fish  4 leg tetrapod animals
• Seed plant  Flowering plant
• Origin of major groups
• Major extinctions
Patterns of Macroevolution
1. Preadaptation OR Exaptation
• A lineage may use a structure for a different purpose
than its ancestor did
• Evolutionary novelty
• ex. Feathers Birds (flight) vs. Dinosaurs (insulation)
2. Stasis
• Lineage persists with little or no change over
evolutionary time
3. Extinct
• Refers to species that have been permanently lost
• Fossils indicate at least 20 mass extinctions, 5 being
catastrophic events
Patterns of Macroevolution
4. Adaptive radiation
• Burst of genetic divergences from a lineage  new
species
• Ex. After individuals colonize a new environment that
has a variety of different habitats with few or no
competitors  honeybee creepers of Hawaiian
• Cause: 1) Key innovation 2) geologic or climatic
events eliminate some species from a habitat
 Key innovation
• Modification or new trait that allows an organism to
exploit its environment in new, more efficient way
• Ex. Evolution of the lung
Patterns of Macroevolution
5. Coevolution
• The joint evolution of two closely interacting
species
• Each species is a selective agent for traits of the
other
• Each adapt to changes in the other
• Species may become interdependent
Key Concepts:
PATTERNS IN LIFE’S HISTORY
 Genetic change above the population level is
called macroevolution
 Recurring patterns of macroevolution include
preadaptation, adaptive radiation, coevolution,
and extinction
Adaptation to What?
 Evolutionary adaptation
• Heritable traits that improve an individual’s
chance of surviving and reproducing (under
conditions that prevailed when genes evolved)
Adaptation to Elevation
Key Concepts:
ADAPTATION AND THE ENVIRONMENT
 An evolutionary adaptation is a heritable aspect
of form, function, behavior, or development that
increases an individual’s capacity to survive and
reproduce in a particular environment
17.14 Phylogeny
 Phylogeny  Evolutionary history of a species or
groups of species
 Evolutionary tree
• Type of diagram that summaries evolutionary
relationships among a group of species
 Cladistics
• Method of determining evolutionary relationships by
grouping species into clades based on shared
characters
• Clade: a species or groups of species that share a
set of characteristics
• Character: quantifiable, heritable characteristic/trait
17.14 Phylogeny
 Cladogram
• Evolutionary tree that shows a network of evolutionary
relationships among clades
 Monophyletic group
• Each clade is a monophyletic group that comprises an
ancestor and all of it descendants
 Sister groups
• Two lineages that emerge from a node on a cladogram
Constructing a Cladogram
Fig. 16.27d, p.260
Animation: Adaptation to what?
Animation: Albatross courtship
Animation: Allopatric speciation on an
archipelago
Animation: Change in moth population
Animation: Directional selection
Animation: Disruptive selection
Animation: Disruptive selection among
African finches
Animation: Distribution of sickle-cell trait
Animation: How to find out if a
population is evolving
Animation: Models of speciation
Animation: Reproductive isolating
mechanisms
Animation: Simulation of genetic drift
Animation: Stabilizing selection
Animation: Sympatric speciation in
wheat
Animation: Temporal isolation among
cicadas