EVOLUTION AND
NATURAL SELECTION
CHAPTER 14
Darwin’s Voyage on HMS Beagle
• Charles Darwin (1809–
1882).
• English naturalist who first
suggested an explanation
for why evolution
occurred.
• Published in 1859, On the
Origin of Species by
Means of Natural
Selection.
Darwin’s Voyage on HMS Beagle
• Charles Darwin’s work on evolution
challenged established worldviews.
• Darwin proposed a mechanism for
evolutionary change called natural selection.
• Darwin’s hypothesis for evolutionary change,
after much testing, eventually became
accepted as theory.
Darwin’s Voyage on HMS Beagle
• Darwin voyaged from 1831–1836 on the HMS
Beagle, a ship mapping the world’s
coastlines.
• Darwin observed firsthand different plants
and animals in various locales.
• These observations played an important role
in the development of his thoughts about
the nature of life on earth.
THE FIVE-YEAR VOYAGE OF HMS BEAGLE
North America
Asia
North Pacific
Ocean
North Atlantic
Ocean
Canary
Islands
Cape Verde
Islands
Marquesas
Indian
Ocean
South
America
Galápagos
Islands
Society
Islands
Africa
Philippine
Islands
Keeling
Islands
Equator
Madagascar
Bahia
Ascension
Valparaiso
St. Helena
Rio de Janeiro
Montevideo
Buenos Aires
Straits of Magellan
Cape Horn
Mauritius
Bourbon Island
Port Desire
Falkland
IIslands
Tierra del Fuego
Australia
Friendly
Islands
Sydney
Cape of
Good Hope
King George’s
Sound
Hobart
South Atlantic
Ocean
New
Zealand
Darwin’s Evidence
•
Darwin made several observations that
helped lead him to believe that species
evolve rather than remain fixed.
1) Fossils of extinct organisms resembled those
of living organisms.
2) Geographical patterns suggested that
organismal lineages change gradually as
individuals move into new habitats.
3) Islands have diverse animals and plants that
are related to, yet different from, their
mainland sources.
FOSSIL EVIDENCE OF EVOLUTION
Armadillo
Glyptodont
Four Galápagos finches and what they eat
Large ground finch (seeds)
Vegetarian finch (buds)
Cactus finch
(cactus fruits and flowers)
Woodpecker finch (insects)
• Darwin observed that, although all the finches shared a
common ancestor, their beak sizes had evolved to suit their
food. Darwin termed this “descent with modification.”
THE THEORY OF NATURAL
SELECTION
• Thomas Malthus’ Essay on the Principle of
Population (1798) provided Darwin with a
key insight.
• While human populations tend to increase
geometrically, the capacity for humans to feed
this population only grows arithmetically.
GEOMETRIC AND ARITHMETIC
PROGRESSIONS
54
Geometric
progression
18
Arithmetic
progression
6
2
4
6
8
• What happens to a population when this gap between
population density and resource availability gets wider?
EXPANDING ON MALTHUS
• Darwin expanded Malthus’ view to include
every organism.
• All organisms have the capacity to overreproduce.
• Only a limited number of these offspring survive
and produce the next generation.
THE THEORY OF NATURAL SELECTION
• Darwin associated survivors with having
certain physical, behavioral, or other
attributes that help them to live in their
environment.
• By surviving, they can pass their favorable
characteristics on to their own offspring.
THE THEORY OF NATURAL SELECTION
• Darwin envisioned the frequency of
favorable characteristics increasing in a
population through a process called natural
selection.
• Favorable characteristics are specific to an
environment; they may be favored in one but not
in another.
• Organisms whose characteristics are best suited
to their particular environment survive more often
and leave more offspring.
THE THEORY OF NATURAL SELECTION
• Darwin’s selection concept is often referred
to as the “survival of the fittest.”
• Darwinian fitness does not always refer to the
biggest or the strongest.
• Fitness, in evolutionary theory, refers to organisms
who, due to their characteristics, survive more
often and leave more offspring.
THE THEORY OF NATURAL SELECTION
• Domesticated animals evolved through
selective breeding for certain traits that
breeders preferred.
• The resulting differences between breeds of
domesticated species are more extreme than
what exists in nature.
• Darwin termed this form of selection artificial
selection, because breeders determined
which traits were successful, rather than
nature.
THE THEORY OF NATURAL SELECTION
• Darwin drafted his ideas in 1842 but
hesitated to publish them for 16 years.
• Another researcher, Alfred Russel Wallace,
sent an essay to Darwin outlining a theory of
evolution by natural selection.
• Wallace had independently arrived at the same
mechanism for evolution that Darwin had.
• Lyell and Hooker arranged for a joint presentation
of their ideas in London.
• Darwin finally published On the Origin of
Species in 1859.
THE THEORY OF NATURAL SELECTION
• Darwin’s 1859 publication ignited
controversy.
• But the scientific community soon accepted
Darwin’s arguments.
The Beaks of Darwin’s Finches
• Darwin’s finches are a closely related group of
distinct species.
• All the birds are similar to each other except for the
shape of their beaks.
• Genetic differences account for the physical
differences in the beaks.
• Birds with larger beaks make more of a protein
called BMP4.
A DIVERSITY OF FINCHES ON A SINGLE
ISLAND
Cactus finch
(Geospiza scandens)
Warbler finch
(Certhidea olivacea)
Sharp-beaked
finch
(G. difficilis)
Woodpecker finch
(Cactospiza pallida)
Small
insectivorous
tree finch
(Camarhynchus parvulus)
Small ground
finch
(G. fuliginosa)
Cactus
eater
Large
insectivorous
tree finch
(Camarhynchus
psittacula)
Medium ground
finch
(G. fortis)
Insect eaters
Seed eaters
Vegetarian
tree finch
(Platyspiza
crassirostris)
Bud eater
Large
ground
finch
(G.
magnirostris)
The Beaks of Darwin’s Finches
• Darwin supposed that the birds evolved from
a single ancestor to become individual
species who specialized in particular foods.
The Beaks of Darwin’s Finches
• Peter and Rosemary Grant studied the
medium ground finch on the island of
Daphne Major in the Galápagos.
• They measured beak shape over many years
and recorded feeding preferences.
The Beaks of Darwin’s Finches
• The finches preferred to feed on small,
tender seeds.
• The finches switched to larger, harder-tocrack seeds when the small seeds
became hard to find.
• Beak depth increased when only large,
tough seeds were available.
Beak depth
EVIDENCE THAT NATURAL SELECTION
ALTERS BEAK SIZE IN GEOSPIZA FORTIS
Wet year
Dry year
Dry year
Dry year
1977
1980
1982
1984
23
The Beaks of Darwin’s Finches
• The Grants’ work with the medium
ground finch is an example of
evolution in action.
• Average beak depth increased after
a drought.
• Only large-beaked birds were able
to crush the bigger seeds and
survive to make the next
generation.
• When wet periods returned, smaller
beaks prevailed at handling the then
more plentiful small seeds.
HOW NATURAL SELECTION
PRODUCES DIVERSITY
Geospiza
fuliginosa
• Darwin’s finches on the
Galápagos are an example of
adaptive radiation.
• In adaptive radiation, a cluster of
species changes to occupy a series
of different habitats within a region.
• Each habitat offers different niches
to occupy.
• A niche represents how a species
interacts both biologically and
physically with its environment in
order to survive.
• Each species evolves to become
adapted to that niche.
Geospiza
fortis
Geospiza
magnirostris
Geospiza
scandens
Ground
and
cactus
finches
Geospiza
conirostris
Geospiza
difficilis
Camarhynchus
parvulus
Camarhynchus
psittacula
Camarhynchus
Tree
pauper
finches
Cactospiza
heliobates
Cactospiza
pallida
(woodpecker
finch)
Platyspiza Vegetarian
crassirostris tree finch
Certhidea
fusca
Certhidea
olivacea
Warbler
finches
THE EVIDENCE FOR EVOLUTION
• There are many lines of evidence supporting
Darwin’s theory of evolution.
• The fossil record comprises the most direct
evidence of macroevolution.
• Fossils are the preserved remains, tracks, or traces
of once-living organisms.
• They are created when organisms or their traces
become buried in sediment.
• By dating the rocks in which the fossils occur,
one can get an accurate idea of how old the
fossils are.
THE EVIDENCE FOR EVOLUTION
• Fossils in rock
represent a history of
evolutionary change.
• Fossils are treated as
samples of data and
are dated
independently of
what the samples are
like.
• Successive changes through time are a data
statement.
• Thus, the statement that macroevolution has
occurred is a factual observation.
TESTING THE THEORY OF EVOLUTION
WITH FOSSIL TITANOTHERES
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
50
45
40
Millions of years ago
35
THE EVIDENCE FOR EVOLUTION
• The anatomical record also reflects
evolutionary history.
• Example: all vertebrate embryos share a similar
set of developmental instructions and features.
Pharyngeal pouches
Pharyngeal pouches
Tail
Tail
Reptile
Bird
Courtesy of Michael Richardson and Ronan O’Rahilly
Human
THE EVIDENCE FOR EVOLUTION
• Homologous
structures are
derived from the
same body part
present in an
ancestor.
• For example, the
same bones might
be put to different
uses in related
species.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Bat
Human
Horse
Porpoise
THE EVIDENCE FOR EVOLUTION
• Analogous structures
are similar-looking
structures in
unrelated lineages.
Taking Flight
To take to the air, three very
different vertebrates lightened bones and transformed
hands into wings.
• These are the result of
parallel evolutionary
adaptations to similar
environments.
• This form of
evolutionary change
is referred to as
convergent
evolution.
Pterosaur
(extinct)
Eastern bluebird
Samoan
flying fox
(fruitbat)
Wolf
Mouse
Marsupial mouse
Two Worlds
Marsupials evolved the same
sort of adaptations in
isolation in Australia that
placental mammals did
elsewhere.
Tasmanian
wolf
Flying phalanger
Flying squirrel
THE EVIDENCE FOR EVOLUTION
• Traces of our evolutionary past are also
evident at the molecular level.
• Organisms that are more distantly related should
have accumulated a greater number of
evolutionary differences than two species that
are more closely related.
• The same pattern of divergence can be seen at
the protein level.
MOLECULES REFLECT EVOLUTIONARY
DIVERGENCE
Human Macaque Dog
8
0
32
Bird
45
Frog
67
Lamprey
125
10 20 30 40 50 60 70 80 90 100 110 120
Number of amino acid differences between hemoglobin
of vertebrate species and that of humans
THE EVIDENCE FOR EVOLUTION
• Evolutionary changes appear to
accumulate at a constant rate.
• This permits changes in an individual gene,
compared over a broad array of organisms, to be
dated from the time of divergence.
• This dating is referred to as a molecular clock.
• For example,
changes have
accumulated
in the
cytochrome c
gene at a
constant rate.
Nucleotide substitutions
THE MOLECULAR CLOCK OF
CYTOCHROME C
Human/kangaro
100
Human/
Dog/ cow
75
cow
Rabbit/
rodent
Human/rodent
50 Horse/ Llama/
donkey cow
Horse/cow
Sheep/goat Pig/
25
cow
Goat/cow
0
0
25
50
75
100
125
Millions of years ago
GENETIC CHANGE IN POPULATIONS:
THE HARDY-WEINBERG RULE
• Population genetics is the study of the
properties of genes in populations.
• Gene pool is the sum of all of the genes in a
population, including all alleles in all
individuals.
GENETIC CHANGE IN POPULATIONS:
THE HARDY-WEINBERG RULE
• Variation in
populations puzzled
many scientists.
• Why don’t dominant
alleles drive recessive
alleles out of
populations?
GENETIC CHANGE IN POPULATIONS: THE
HARDY-WEINBERG RULE
• G.H. Hardy and W. Weinberg, in 1908,
studied allele frequencies in a gene pool.
• In a large population in which there is random
mating, and in the absence of forces that
change allele frequencies, the original genotype
proportions remain constant from generation to
generation.
• Because the proportions do not change, the
genotypes are said to be in Hardy-Weinberg
equilibrium.
• If the allele frequencies are not changing, the
population is not evolving.
GENETIC CHANGE IN POPULATIONS:
THE HARDY-WEINBERG RULE
• Hardy and Weinberg arrived at their
conclusion by analyzing the frequencies of
alleles in successive generations.
• Frequency is the proportion of something
compared to the total.
• Knowing the frequency of the phenotype, one
can calculate the frequency of the genotypes
and alleles in the population.
• Population of 1000 cats
• 840 black 160 white
• 840/1000 = 0.84 or 84% Proportion of black cats
GENETIC CHANGE IN POPULATIONS:
THE HARDY-WEINBERG RULE
• By convention, the frequency of the more
common of two alleles is designated by the
letter p and that of the less common allele
by the letter q.
• Because there are only two alleles, the sum
of p and q must always equal 1.
• p+q=1
GENETIC CHANGE IN POPULATIONS:
THE HARDY-WEINBERG RULE
• In algebraic terms, the Hardy-Weinberg equilibrium
is written as an equation.
(p + q) x (p + q) = p2 + 2pq + q2 = 1
Individuals Individuals Individuals
homozygous heterozygous homozygous
for allele B for alleles B for allele b
&b
GENETIC CHANGE IN POPULATIONS:
THE HARDY-WEINBERG RULE
• Population of 1000 cats
• 840 black, 160 white
• But how many BB vs Bb?
• Frequency of p & q
Sperm
p = 0.6
Phenotypes
q = 0.4
B
B
p = 0.6
BB
Genotypes
Frequency of genotype in
the population (number in
a population of 1,000 cats)
BB
360 cats
360/1,000 = 0.36
Bb
bb
480 cats
480/1,000 = 0.48
160 cats
160/1,000 = 0.16
p2 = 0.36
b
Frequency of alleles in the
population (total of 2,000)
720 B
480 B + 480 b
720 B + 480 B = 1,200 B
1,200/2,000 = 0.6 B
320 b
480 b + 320 b = 800 b
800/2,000 = 0.4 b
b
Bb
Bb
pq = 0.24
pq = 0.24
bb
Number of alleles in the
population (2 per cat)
q2
Eggs
= 0.16
q = 0.4
GENETIC CHANGE IN POPULATIONS: THE
HARDY-WEINBERG RULE
• Used to calculate allele frequencies (p & q) in a
simple way:
• 1000 cats, 160 white (bb) so q2 = 0.16
• q = square root of 0.16 = 0.40.
• Since p + q = 1; p = 1 – q = 0.60.
• p2 = 0.36 x 1000; so 360 homozygous dominant (BB)
• 2pq = 0.48 x 1000; 480 heterozygous (Bb)
GENETIC CHANGE IN POPULATIONS:
THE HARDY-WEINBERG RULE
• The Hardy-Weinberg equilibrium only works if the
following five assumptions are met:
– 1. The size of the population is very large or
effectively infinite.
– 2. Individuals can mate with one another at random.
– 3. There is no mutation.
– 4. There is no immigration or emigration.
– 5. All alleles are replaced equally from generation to
generation (natural selection is not occurring).
AGENTS OF EVOLUTION
• Five factors can alter the proportions of
homozygotes and heterozygotes enough to
produce significant deviations from HardyWeinberg predictions.
1. Mutation
2. Nonrandom mating
3. Genetic drift
4. Migration
5. Selection
AGENTS OF EVOLUTION
• Mutation is a change in a nucleotide
sequence in DNA.
• Mutation rates are generally too low to
significantly alter Hardy-Weinberg proportions.
• Mutations must affect the DNA of the germ cells
or the mutation will not be passed on to offspring.
• However, no matter how rare, mutation is the
ultimate source of variation in a population.
AGENTS OF EVOLUTION
• Nonrandom mating occurs when individuals
with certain genotypes mate with one
another either more or less commonly than
would be expected by chance.
• Sexual selection is choosing a mate often based
on physical characteristics.
AGENTS OF EVOLUTION
• Migration is the movement of individuals
between populations.
• The movement of individuals can be a powerful
force upsetting the genetic stability of natural
populations.
• The magnitude of the effects of migration is
based on two factors:
• The proportion of migrants in the population.
• The difference in allele frequencies between
the migrants and the original population.
AGENTS OF EVOLUTION
• Genetic drift describes random changes in
allele frequencies.
• In small populations, the frequencies of particular
alleles may be changed drastically by chance
alone.
• In extreme cases, individual alleles of a given
gene may be:
• All represented in few individuals.
• Accidentally lost if individuals fail to reproduce
or die.
AGENTS OF EVOLUTION
• A series of small populations that are
isolated from one another may come to
differ strongly as the result of genetic drift.
AGENTS OF EVOLUTION
• Founder effect occurs when
one of a few individuals
migrates and becomes the
founder of a new, isolated
population at some
distance from their place of
origin.
• The alleles that they carry
will become a significant
fraction of the new
population’s genetic
endowment.
AGENTS OF EVOLUTION
• Bottleneck effect occurs when a population
is drastically reduced in size.
• the surviving individuals constitute a random
genetic sample of the original population.
AGENTS OF EVOLUTION
• Selection occurs if some individuals leave
behind more progeny than others, and the
likelihood that they will do so is affected by
their individual characteristics.
• In artificial selection, a breeder selects for the
desired characteristics.
• In natural selection, conditions in nature
determine which kinds of individuals in a
population are the most fit.
SELECTION FOR COAT COLOR IN MICE
Light coat color pocket mouse
is vulnerable on lava rock
Light coat color favored by
natural selection because
it matches sand color
Dark coat color favored by
natural selection because
it matches black lava rock
54
AGENTS OF EVOLUTION
• There are three types of natural selection:
(b) Disruptive selection
(c) Directional selection
Number of individuals
(a) Stabilizing selection
0
25
50
75
100
Body size (g)
125
Number of individuals
Selection for midsized individuals
0
25
50
75
100
Body size (g)
50
75
100
Body size (g)
125
0
Selection for small and large individuals
Peak gets narrower
0
25
0
25
50
75
100
Body size (g)
50
75
100
Body size (g)
125
Selection for larger individuals
Peak shifts
Two peaks form
125
25
125
0
25
50
75
100
Body size (g)
125
• The result is an increase
in the frequency of the
already common
intermediate
phenotype.
• For example, human
birthweight is under
stabilizing selection.
20
100
70
50
30
20
10
15
10
7
5
3
2
5
2 3 4 5 6 7 8 9 10
Birth weight in pounds
Percent infant mortality
• Stabilizing selection is
a form of selection in
which both extremes
are eliminated.
Percent of births in population
AGENTS OF EVOLUTION
AGENTS OF EVOLUTION
• Disruptive selection is a form of selection in which the two
extremes in an array of phenotypes become more
common in the population.
• Selection acts to eliminate the intermediate
phenotypes.
• For example, beak size in African blackbellied
seedcracker finches is under disruptive selection
because the available seeds are only large or small.
• Directional selection is
a form of selection that
occurs when selection
acts to eliminate one
extreme from an array
of phenotypes.
Average tendency to fly toward light
AGENTS OF EVOLUTION
11
10
9
8
7
6
5
4
3
2
1
0
2
4
6 8 10 12 14 16 18 20
Number of generations
• For example, in Drosophila, flies that fly toward light
can be selected against and those that avoid light
selectively bred, producing a population of flies with a
greater tendency to avoid light.
SICKLE-CELL ANEMIA
• Sickle-cell anemia is a
hereditary disease
affecting hemoglobin
molecules in the blood.
Val 6
(a)
• The disorder results from a
single nucleotide change in
the gene for b-hemoglobin.
(b) Sickled red blood cells
(c) Normal red blood cells
• This causes the sixth amino acid in the chain to
change from glutamic acid (very polar) to valine
(nonpolar).
• The hemoglobin molecules clump together and
deform the red blood cell into a “sickle-shape”.
SICKLE-CELL ANEMIA
• People homozygous for the sickle-cell
genetic mutation frequently have a
reduced lifespan.
• The sickled form of hemoglobin does not carry
oxygen atoms well.
• The red blood cells that are sickled do not flow
smoothly through capillaries.
• Heterozygous individuals make enough
functional hemoglobin to keep their red
blood cells healthy.
SICKLE-CELL ANEMIA
• The frequency of the sickle-cell allele is
about 0.12 in Central Africa.
• One in 100 people is homozygous for the
defective allele and develops the fatal disorder.
• In contrast, sickle-cell anemia strikes only roughly
two African Americans out of every thousand.
• If natural selection drives evolution, why has
natural selection not acted against the
defective allele in Africa and eliminated it
from the population?
SICKLE-CELL ANEMIA
• The defective allele
has not been
eliminated from
Central Africa
because people who
are heterozygous are
much less susceptible
to malaria.
Sickle-cell
allele in Africa
1–5%
5–10%
10–20%
Falciparum
malaria in Africa
Malaria
• The payoff in survival of heterozygotes makes up for
the price in death of homozygotes.
• This is called heterozygote advantage.
• Stabilizing selection occurs because malarial
resistance counterbalances lethal anemia.
PEPPERED MOTHS AND INDUSTRIAL
MELANISM
• The peppered moth, Biston betularia, is a
European moth that rests on tree trunks during
the day.
• Until the mid-19th century, almost every captured
individual had light-colored wings.
• Since then, individuals with dark-colored wings
increased in frequency in industrialized areas.
• Can Darwin’s theory explain this?
• Yes it can!
PEPPERED MOTHS AND INDUSTRIAL
MELANISM
• Industrial melanism
describes the evolutionary
process in which darker
individuals come to
predominate over lighter
individuals since the
industrial revolution as a
result of natural selection.
• Dark organisms are better
concealed from their
predators in habitats that
have been darkened by
soot and other industrial
pollution.
PEPPERED MOTHS AND INDUSTRIAL
MELANISM
Percentage of melanic moths
• In England, the air pollution promoting industrial
melanism began to reverse following enactment
of the Clean Air legislation in 1956.
– As a result, the frequency of the melanic
(dark) form of Biston appears to decrease as
100
well.
90
80
70
60
50
40
30
20
10
0
59
63
67
71
75
79
Year
83
87
91
95
SELECTION ON COLOR IN GUPPIES
• On the island of Trinidad, guppies are found
in two very different stream environments.
• In pools above waterfalls, the guppies are
found along with the killifish, a seldom
predator of guppies.
• In pools below waterfalls, the guppies are
found in pools along with the pike cichlid, a
voracious predator of guppies.
• Guppies can move between pools by
swimming upstream during floods
SELECTION ON COLOR IN GUPPIES
• Guppy populations above
and below waterfalls exhibit
differences.
Guppy
(Poecilia reticulata)
• Guppies in high-predation
pools are not as colorful as
guppies in low-predation
pools.
• Guppies in high-predation
pools tend to reproduce at an
earlier age and attain
relatively smaller adult body
sizes.
• These differences suggest the
action of natural selection.
Pikecichlid
(Crenicichla alta)
Guppy
(Poecilia reticulata)
Killifish
(Rivulus hartii)
SELECTION ON COLOR IN GUPPIES
• John Endler conducted experiments on
guppies to determine whether predation risk
was really the driving selective force in this
system.
• In a controlled laboratory setting, he created
artificial pool environments in which he placed
guppies in one of three conditions:
• with no predator present
• with killifish present (low predation risk)
• with cichlid present (high predation risk)
FIGURE 14.31 EVOLUTIONARY CHANGE IN
SPOT NUMBER
• After 10 guppy generations, he found that the
guppies from no or low predation risk pools were
both larger and more colorful than the guppies
from the high predation risk pool.
• He later found the same results in field
experiments .
No
predation
14
Spots per fish
13
Low
predation
12
11
10
High
predation
9
8
0
4
8
Months
12
THE BIOLOGICAL SPECIES
CONCEPT
• Speciation is the macroevolutionary process
of forming new species from pre-existing
species.
• It involves successive change.
• First, local populations become increasingly
specialized.
• Then, if they become different enough, natural
selection may act to keep them that way.
THE BIOLOGICAL SPECIES CONCEPT
• Ernst Mayr coined the biological species
concept, which defines species as “groups of
actually or potentially interbreeding natural
populations which are reproductively isolated
from other such groups”.
• Populations whose members do not mate
with each other and cannot produce fertile
offspring are said to be reproductively
isolated and, thus, members of different
species.
THE BIOLOGICAL SPECIES
CONCEPT
• Barriers called reproductive isolating
mechanisms cause reproductive isolation by
preventing genetic exchange between
species.
• Prezygotic isolating mechanisms prevent the
formation of zygotes.
• Postzygotic isolating mechanisms prevent the
proper functioning of zygotes once they have
formed.
ISOLATING MECHANISMS
• There are six different prezygotic
reproductive isolating mechanisms:
•
•
•
•
•
•
geographical isolation
ecological isolation
temporal isolation
behavioral isolation
mechanical isolation
prevention of gamete fusion
ISOLATING MECHANISMS
• Geographical isolation occurs in cases
when species exist in different areas and are
not able to interbreed.
ISOLATING MECHANISMS
• Ecological isolation results from two species
that occur in the same area but utilize
different portions of the environment and
are unlikely to hybridize.
ISOLATING MECHANISMS
• Temporal isolation results from two species
having different reproductive periods, or
breeding seasons, that preclude
hybridization.
• Behavioral isolation refers to the often
elaborate courtship and mating rituals of
some groups of animals, which tend to keep
these species distinct in nature even if they
inhabit the same places.
ISOLATING MECHANISMS
• Mechanical isolation results from structural
differences that prevent mating between
related species of animals and plants.
• Prevention of gamete fusion blocks the
union of gametes even following successful
mating.
ISOLATING MECHANISMS
• If hybrid matings do occur, and
zygotes are produced, postzygotic
factors may prevent those zygotes
from developing into normal
individuals.
• In hybrids, the genetic
complements of two species may
be so different that they cannot
function together normally in
embryonic development.
• Even if hybrids survive the embryo stage, they may
not develop normally.
• Finally, many hybrids are sterile.