Processes of Evolution
Chapter 18 Part 2
18.6 Maintaining Variation
 Natural selection theory helps explain diverse
aspects of nature, including differences between
males and females, and the relationship
between sickle-cell anemia and malaria
Sexual Selection
 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
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
• HbA/HbS heterozygotes survive malaria more often
than people who make only normal hemoglobin
Sickle Cell Anemia and Malaria
Fig. 18-13a, p. 287
Fig. 18-13b, p. 287
Fig. 18-13c, p. 287
18.7 Genetic Drift—
The Chance Changes
 Genetic drift
• A random change in allele frequencies over time
• Can lead to a loss of genetic diversity, especially
in small populations
 Fixation has occurred when all individuals in a
population are homozygous for one allele
Genetic Drift and Population Size
Genetic Drift and Population Size
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)
18.3-18.6 Key Concepts
Patterns of Natural Selection
 Natural selection is a microevolutionary process
 Depending on the population and its
environment, natural selection can shift or
maintain the range of variation in heritable traits
18.8 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
Gene Flow Between Oak Populations
18.7-18.8 Key Concepts
Other Microevolutionary Processes
 With genetic drift, change can occur in a line of
descent by chance alone
 Gene flow counters the evolutionary effects of
mutation, natural selection, and genetic drift
18.9 Reproductive Isolation
 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
Four Butterflies, Two Species
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
 Temporal isolation
 Mechanical isolation
 Behavioral isolation
 Ecological isolation
 Gamete incompatibility
Mechanical Isolation
Behavioral Isolation
Animation: Albatross courtship
Postzygotic Isolation Mechanisms
 Reduced hybrid viability (ligers, tigons)
• Extra or missing genes
 Reduced hybrid fertility (mules)
• Robust but sterile offspring
 Hybrid breakdown
• Lower fitness with successive generations
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.
Ecological isolation: Individuals of
different species live in different places
They interbreed and never do meet up.
anyway.
Gamete incompatibility: Reproductive
cells meet up, but no fertilization occurs.
Zygotes form,
but . . .
Postzygotic isolating mechanisms
Hybrid inviability: Hybrid embryos die
early, or new individuals die before they
can reproduce.
Hybrid sterility: Hybrid individuals or
their offspring do not make functional
gametes.
No offspring, sterile offspring, or weak offspring that die before reproducing
Fig. 18-17, p. 290
Animation: Reproductive isolating
mechanisms
18.10 Allopatric Speciation
 Allopatric speciation
• A physical barrier arises and ends gene flow
between populations
• Genetic divergence results in speciation
• Example: llamas, vicunas, and camels
Allopatric Speciation
The Inviting 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
Allopatric Speciation
on an Isolated Archipelago
A A few individuals of a
mainland species reach
isolated island 1. In the new
habitat, populations of their
descendants diverge, and
speciation occurs.
B Later, a few
individuals of a new
species colonize
nearby island 2.
Speciation follows
genetic divergence
in the new habitat.
C Genetically different
descendants of the
ancestral species may
colonize islands 3 and 4
or even invade island 1.
Genetic divergence and
speciation may follow.
Fig. 18-21a, p. 293
Akepa
Akekee (L.
Nihoa finch Palila(Loxioides
Maui parrotbill
(Loxops
caeruleirostris)
(Telespiza
(Pseudonestor
bailleui)
Mamane
coccineus)
Insects,
ultima)
xanthophrys)
Insects, spiders spiders, some Insects, buds,
seeds ripped Rips dry branches
from buds
nectar; high
seeds, flowers, from pods;
for insect larvae,
twisted apart
buds,
mountain rain seabird eggs;
pupae, caterpillars;
by bill, some
flowers,
forest
rocky or
mountain forest
nectar; high
some
berries,
shrubby slopes
with open canopy,
mountain rain
insects; high dense underbrush
forest
mountain dry
forests
Apapane
(Himatione
sanguinea)
Nectar, especially
of ohialehua flowers;
caterpillars and other
insects; spiders;
high mountain
forests
Fig. 18-21b, p. 293
Poouli
Maui Alauahio Kauai Amakihi
(Melamprosops (Paroreomyza (Hemignathus
phaeosoma)
montana)
kauaiensis)
Bark-picker;
Tree snails,
Bark or leaf
insects in
insects, some insects, spiders,
nectar; high
understory;
nectar; high
last known
mountain rain mountain rain
forest
male died in
forest
2004
Akiapolaau
(Hemignathus
munroi)
Probes, digs
insects from
big trees; high
mountain rain
forest
Akohekohe
(Palmeria
dolei)
Mostly nectar
from flowering
trees, some
insects, pollen;
high mountain
rain forest
Iiwi (Vestiaria
coccinea)
Mostly nectar
(ohia flowers,
lobelias, mints),
some insects;
high mountain
rain forest
Fig. 18-21c, p. 293
Animation: Allopatric speciation on an
archipelago
18.11 Other Speciation Models
 Populations sometimes speciate even without a
physical barrier that blocks gene flow
• Sympatric speciation
• Parapatric speciation
Sympatric Speciation
 In sympatric speciation, new species form
within a home range of an existing species, in the
absence of a physical barrier
 A change in chromosome number (polyploidy)
can cause instant speciation
 On Lord Howe Island, species of palms are
reproductively isolated
Sympatric Speciation in Wheat
T. turgidum
(wild emmer)
Triticum
monococcum
(einkorn)
14AA
Unknown spontaneous
species of chromosome
Triticum
doubling
X
14BB
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.
14AB
T. tauschii
(a wild
relative)
28AABB
X
B About 8,000 years ago,
the allopolyploid 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.
14DD
T. aestivum
(one of the
common
bread
wheats)
42AABBDD
C AABB emmer probably
hybridized with T.
tauschii, a wild relative
of wheat. Its diploid
chromosome number
s 14 (two sets of 7 DD).
Common bread wheats
have a chromosome
number of 42 (six sets
of 7 AABBDD).
Fig. 18-22, p. 294
Animation: Sympatric speciation in
wheat
Sympatric Speciation in Palms
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
Parapatric Speciation
T. barretti
hybrid zone
T. anophthalmus
Fig. 18-24c, p. 295
Different Speciation Models
18.9-18.11 Key Concepts
How Species Arise
 Speciation varies in its details, but it typically
starts after gene flow ends
 Microevolutionary events that occur
independently lead to genetic divergences,
which are reinforced as reproductive isolation
mechanisms evolve
18.12 Macroevolution
 Macroevolution
• Large-scale patterns of evolutioary change
• Includes patterns of change such as one species
giving rise to multiple species, the origin of major
groups, and major extinction events
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
Coevolution
proboscis
nectar tube
10 cm
Fig. 18-25, p. 296
Stasis and Exaptation
 Stasis
• A lineage exists for millions of years with little or no
change (e.g. coelacanth)
 Exaptation (preadaptation)
• Some complex traits in modern species held
different adaptive value in ancestral lineages (e.g.
feathers in birds and dinosaurs)
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
Extinction
 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 of Mammals
Following the K-T Extinction
Eomaia scansoria
Cenozoic
Mesozoic
Fig. 18-26a, p. 297
rabbits
rodents
primates
armadillos
shrews, other
insectivores;
bats
carnivores
whales,
dolphins
deer, other
artiodactyls
horses, other
perissodactyls
anteaters
platypus, other
monotremes
kangaroos, other
marsupials
elephants, other
proboscideans
manatees
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
18.12 Key Concepts
Macroevolutionary Patterns
 Patterns of genetic change that involve more
than one species are called macroevolution
 Recurring patterns of macroevolution include
exaptation, adaptive radiation, and extinction
Animation: Models of speciation
Animation: Morphological differences
within a species
Animation: Simulation of genetic drift
Animation: Temporal isolation among
cicadas
Video: Rise of the super rats
Video: Humpback whales
Video: Salamander gills