Transcript chapter17_Sections 6

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 17
Processes of Evolution
(Sections 17.6 - 17.10)
Albia Dugger • Miami Dade College
17.6 Stabilizing and
Disruptive Selection
• Stabilizing selection is a form of natural selection that
maintains an intermediate phenotype
• Disruptive selection favors forms of a trait at both ends of a
range of variation
Stabilizing Selection
• Stabilizing selection is also called balancing selection
because it tends to preserve the midrange phenotypes in a
population
• stabilizing selection
• Mode of natural selection in which intermediate forms of a
trait are favored over extremes
Stabilizing Selection
• Extreme forms of a trait
are eliminated, and
intermediates are
favored
• Red arrows indicate
forms selected against;
green, forms that are
being favored
Stabilizing Selection
Fig. 17.8a, p. 264
Number of individuals
in population
Stabilizing Selection
Time 1
Range of values for the trait
Fig. 17.8a, p. 264
Stabilizing Selection
Fig. 17.8b, p. 264
Stabilizing Selection
Time 2
Fig. 17.8b, p. 264
Stabilizing Selection
Fig. 17.8c, p. 264
Stabilizing Selection
Time 3
Fig. 17.8c, p. 264
Time 1
Number of individuals
in population
Stabilizing
Selection
Range of values for the trait
Time 2
Time 3
Stepped Art
Fig. 17.8, p. 264
Animation: Stabilizing Selection
Sociable Weavers
• The body weight of sociable weavers (Philetairus socius) is
subject to stabilizing selection
• Body weight is a trade-off between risks of starvation and
predation: Leaner birds do not store enough fat to avoid
starvation, and predators select against birds of high body
weight
• Birds of intermediate weight have the selective advantage
Stabilizing Selection: Sociable Weavers
Disruptive Selection
• Conditions that favor forms of a trait at both ends of a range of
variation drive disruptive selection
• disruptive selection
• Mode of natural selection that favors forms of a trait at the
extremes of a range of variation
• Intermediate forms are selected against
Disruptive Selection
• Midrange forms are
eliminated; extreme
forms are maintained
• Red arrows indicate
forms selected against;
green, forms that are
being favored
Disruptive Selection
Fig. 17.10a, p. 265
Number of individuals
in population
Disruptive Selection
Time 1
Range of values for the trait
Fig. 17.10a, p. 265
Disruptive Selection
Fig. 17.10b, p. 265
Disruptive Selection
Time 2
Fig. 17.10b, p. 265
Disruptive Selection
Fig. 17.10c, p. 265
Disruptive Selection
Time 3
Fig. 17.10c, p. 265
Time 1
Number of individuals
in population
Disruptive
Selection
Range of values for the trait
Time 2
Time 3
Stepped Art
Fig. 17.10, p. 265
ANIMATION: Disruptive selection
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African Seedcrackers
• In black-bellied seedcrackers, dimorphism in bill size results
from competition for two types of food in the dry season
• Small-billed birds are better at opening soft seeds, but largebilled birds are better at cracking hard seeds
• These conditions favor birds with bills that are either 12
millimeters wide or 15 to 20 millimeters wide.
• Birds with bills of intermediate size are selected against
African Seedcrackers
ANIMATION: Disruptive selection among
African finches
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17.7 Fostering Diversity
• Selection pressures that operate on natural populations are
complex; an allele may be adaptive in one circumstance but
harmful in another
• Any mode of natural selection may maintain two or more
alleles in a population
Nonrandom Mating
• With sexual selection, the most adaptive forms of a trait are
those that help individuals defeat same-sex rivals for mates,
or are the ones most attractive to the opposite sex
• sexual selection
• Mode of natural selection in which some individuals of a
population out-reproduce others because they are better
at securing mates
Examples of Sexual Selection
• Male elephant seals
fight for sexual access
to a cluster of females
Examples of Sexual Selection
• A male bird of paradise
engages in flashy
courtship display to
catch the sexual interest
of a female
• Females are choosy; a
male mates with any
female that accepts him
Examples of Sexual Selection
• Stalk-eyed flies cluster
on aerial roots to mate
• Females prefer males
with longer eyestalks
• A male with very long
eyestalks (top) has
captured the interest of
three females below
Balanced Polymorphism
• In an environment that favors heterozygotes (individuals with
nonidentical alleles), any mode of natural selection may result
in a balanced polymorphism
• balanced polymorphism
• Maintenance of two or more alleles for a trait at high
frequency in a population as a result of natural selection
against homozygotes
Malaria and Sickle-Cell Anemia
• A mutation in the normal beta globin chain of hemoglobin
(HbA) causes sickle-cell anemia; individuals homozygous for
the mutated HbS allele often die young
• The HbS allele persists at high frequencies in tropical regions
of Africa because HbA/HbS heterozygotes are more likely to
survive than HbA/HbA homozygotes
Distributions of
Malaria and Sickle-Cell Anemia
Malaria
• Malaria is caused by
Plasmodium infection
carried by mosquitoes
• A physician searches
for mosquito larvae in
Southeast Asia
17.8 Genetic Drift
• Random change in allele frequencies, or genetic drift, can
lead to loss of genetic diversity by causing alleles to become
fixed, especially in small populations
• genetic drift
• Change in allele frequencies in a population due to chance
alone
• fixed
• Refers to an allele for which all members of a population
are homozygous
Genetic Drift
• The larger the population, the smaller the impact of random
changes in allele frequencies
• Example: Allele X occurs at a 10% frequency
• In a population of 10, only one person carries the allele,
and if that person dies, the allele is lost
• In a population of 100, all 10 people who carry the allele
would have to die for the allele to be lost
Genetic Drift in Flour Beetles
Genetic Drift in Flour Beetles
Fig. 17.14a, p. 268
Genetic Drift in Flour Beetles
Fig. 17.14b, p. 268
ANIMATION: Simulation of genetic drift
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Bottlenecks
• Genetic drift can be dramatic when a few individuals rebuild a
population or start a new one
• Example: Hunting reduced an elephant seal population to 20;
the population is now homozygous at every gene; genetic drift
after the bottleneck fixed all alleles in the population
• bottleneck
• Drastic reduction in population size so severe that it
reduces genetic diversity
Founder Effect
• Bottlenecks also occur when a small group of individuals
founds a new population – the new population’s genetic
diversity may be quite reduced
• founder effect
• Change in allele frequencies that occurs when a small
number of individuals establish a new population
Inbreeding
• Genetic drift is pronounced in inbreeding populations
• Inbreeding lowers a population’s genetic diversity, so more
individuals in the population are homozygous for recessive
alleles with harmful effects
• inbreeding
• Nonrandom mating among close relatives
Founder Effect and Inbreeding
• Ellis-van Creveld
syndrome:
characterized by
dwarfism, polydactyly,
and heart defects
• Caused by a recessive
allele common in the
Old Order Amish of
Lancaster County, PA
17.9 Gene Flow
• Individuals, along with their alleles, move into and out of
populations
• This gene flow stabilizes allele frequencies, so it counters the
effects of mutation, natural selection, and genetic drift that
tend to occur within a population
• gene flow
• The movement of alleles into and out of a population
Gene Flow
• Blue jays move acorns, and their alleles, among populations
of oak trees that would otherwise be genetically isolated
Key Concepts
• Processes of Microevolution
• Natural selection may maintain or shift the range of
variation of a shared heritable trait in a population
• Gene flow counters the evolutionary effects of mutation,
natural selection, and genetic drift
17.10 Reproductive Isolation
• Speciation differs in its details, but reproductive isolating
mechanisms are always part of the process
• speciation
• One of several processes by which new species arise
• reproductive isolation
• Absence of gene flow between populations
• Always part of speciation
Reproductive Isolation
Prevents Interbreeding
• When gene flow does not occur between populations,
different genetic changes accumulate in each
• Reproductive isolation reinforces differences between
diverging populations:
• If pollination or mating cannot occur, or if zygotes cannot
form, the isolation is prezygotic
• If hybrids form but are unfit or infertile, the isolation is
postzygotic
Reproductive Isolation
Prevents Interbreeding
Reproductive
Isolation
Prevents
Interbreeding
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 . . .
Interbreeding
is successful
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).
Fig. 17.17, p. 270
Reproductive
Isolation
Prevents
Interbreeding
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 . . .
Interbreeding
is successful
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).
Stepped Art
Fig. 17.17, p. 270
ANIMATION: Reproductive isolating
mechanisms
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7 Mechanisms of
Reproductive Isolation
• Temporal isolation
• Some populations can’t interbreed because the timing of
their reproduction differs
• Mechanical isolation
• In some cases, the size or shape of an individual’s
reproductive parts prevent it from mating with members of
another population
Mechanical Isolation in Sage
Mechanical
Isolation in
Sage
Fig. 17.18a, p. 271
Mechanical
Isolation in
Sage
A Black sage is pollinated mainly by
honeybees and other small insects.
Fig. 17.18a, p. 271
Mechanical
Isolation in
Sage
Fig. 17.18b, p. 271
Mechanical
Isolation in
Sage
B The flowers of black sage are too delicate to support
larger insects. Big insects access the nectar of small
sage flowers only by piercing from the outside, as this
carpenter bee is doing. When they do so, they avoid
touching the flower’s reproductive parts.
Fig. 17.18b, p. 271
Mechanical
Isolation in
Sage
Fig. 17.18c, p. 271
Mechanical
Isolation in
Sage
anthers
stigma
C The reproductive parts (anthers and stigma) of white
sage flowers are too far away from the petals to be
brushed by honeybees, so honeybees cannot pollinate this
species. White sage is pollinated mainly by larger bees and
hawkmoths, which brush the flower’s stigma and anthers
as they pry apart the petals to access nectar.
Fig. 17.18c, p. 271
7 Mechanisms of
Reproductive Isolation (cont.)
• Ecological isolation
• Populations adapted to different microenvironments in the
same region may be physically separated
• Behavioral isolation
• In animals, behavioral differences can stop gene flow
between related species
• Example: Males and females of some bird species engage
in courtship displays before sex
Behavioral Isolation in Albatrosses
7 Mechanisms of
Reproductive Isolation (cont.)
• Gamete incompatibility
• Even if gametes of different species meet, they often have
molecular incompatibilities that prevent them from fusing
• Primary speciation route of animals that release freeswimming sperm in water
7 Mechanisms of
Reproductive Isolation (cont.)
• Hybrid inviability
• If genetic incompatibilities disrupt development, a hybrid
embryo may die, or hybrid offspring that survive may have
reduced fitness (e.g. ligers)
• Hybrid sterility
• Some interspecies crosses produce robust but sterile
offspring (e.g. mules)
ANIMATION: Temporal Isolation Among
Cicadas
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