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Chapter 17
Processes of
Evolution
Sections 7-12
Albia Dugger • Miami Dade College
17.7 Fostering Diversity
• Individuals may be selective agents for their own species
• Any mode of natural selection may maintain two or more
alleles in a population
• An allele may be adaptive in one circumstance but harmful in
another
Nonrandom Mating
• Competition for mates is a selective pressure
• With sexual selection, some version of a trait gives an
individual an advantage over others in attracting mates
• Distinct male and female phenotypes (sexual dimorphism) is
one outcome of sexual selection
Sexual Selection: Elephant Seals
Sexual Selection: Birds of Paradise
Sexual Selection: Stalk-Eyed Flies
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
Searching for Mosquito Larvae
in Southeast Asia
Sickle Cell Anemia and Malaria
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
18.7 Genetic Drift and Gene Flow
• Especially in small populations, random changes in allele
frequencies can lead to a loss of genetic diversity
• Individuals, along with their alleles, move into and out of
populations
• This flow of alleles counters genetic change that tends to
occur within a population
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
Genetic Drift in a Small Population
Genetic Drift in a Larger Population
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
• 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)
Ellis-van Creveld Syndrome
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
Gene Flow Between Oak Populations
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
ANIMATED FIGURE: Simulation of genetic
drift
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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 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
• 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
Figure 17-17a p285
Figure 17-17b p285
anthers
stigma
Figure 17-17c p285
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
Behavioral Isolation in Jumping Spiders
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
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
Figure 17-16 p284
ANIMATED FIGURE: Reproductive isolating
mechanisms
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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
ANIMATION: Temporal isolation among
cicadas
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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
Allopatric Speciation in Snapping Shrimp
Allopatric Speciation in Archipelagos
• Winds or ocean currents carry a few individuals of mainland
species to remote, isolated islands chains (archipelagos) such
as Hawaii
• Habitats and selection pressures that differ within and
between the islands foster divergences that result in allopatric
speciation
• Example: Hawaiian honeycreepers and thousands of other
species of finches are unique to the Hawaiian archipelago
Allopatric Speciation on an
Archipelago
Take-Home Message: What happens after a
physical barrier arises between populations?
• A physical barrier that intervenes between populations or
subpopulations of a species prevents gene flow among them
•
As gene flow ends, genetic divergences give rise to new
species
• This process is allopatric speciation
ANIMATED FIGURE: Allopatric speciation
on an archipelago
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17.11 Other Speciation Models
• In sympatric speciation and parapatric speciation, populations
speciate even without a physical barrier that blocks gene flow
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
Aegilops
Triticum
(wild goatgrass, unknown
(hybrid)
species)
Triticum
urartu (wild
einkorn)
14 AA
X
14 BB
14 AB
Triticum
turgidum
(emmer)
28 AABB
Aegilops
tauschii
(goatgrass)
X
14 DD
Triticum
aestivum
(bread wheat)
42 AABBDD
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)
Sympatric Speciation in Cichlids
Sympatric Speciation in Warblers
• A chain of populations of greenish warblers encircles the
Tibetan plateau central Asia (a ring species)
• Gene flow occurs continuously all around the chain, but the
two populations at the ends of the chain are different species
• Individuals of these two populations overlap in range, but do
not interbreed because they do not recognize one another’s
songs (behavioral isolation)
Sympatric Speciation in Warblers
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
A Giant velvet walking worm,
Tasmanipatus barretti
Figure 17-23a p289
b Blind velvet walking worm,
T. anophthalmus
Figure 17-23b p289
T. barretti
hybrid zone
T. anophthalmus
C The habitats of
the worms overlap
in a hybrid zone on
the island of
Tasmania.
Figure 17-23c p289
Take-Home Message: Can
speciation occur
without a physical barrier?
• By a sympatric speciation model, new species arise from a
population even in the absence of a physical barrier
• By a parapatric speciation model, populations maintaining
contact along a common border evolve into distinct species
Table 17-2 p292
ANIMATED FIGURE: Sympatric speciation
in wheat
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ANIMATION: Models of speciation
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17.12 Macroevolution
• Macroevolution includes 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 (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)
Fossil Coelacanth
Living Coelacanth
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
• 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
• 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.
• 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
Coevolved Species:
Myrmica sabuleti and Maculinea arion.
Maculinea arion
Myrmica sabuleti
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?
• Macroevolution comprises large-scale patterns of evolutionary
change such as adaptive radiation, the origin of major groups,
and loss through extinction
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