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Teresa Audesirk • Gerald Audesirk • Bruce E. Byers
Biology: Life on Earth
Eighth Edition
Lecture for Chapter 16
The Origin of Species
Copyright © 2008 Pearson Prentice Hall, Inc.
Chapter 16 Outline
• 16.1 What Is a Species? p. 316
• 16.2 How Is Reproductive Isolation Between
Species Maintained? p. 317
• 16.3 How Do New Species Form? p. 320
• 16.4 What Causes Extinction? p. 326
Section 16.1 Outline
• 16.1 What Is a Species?
– Biologists Need a Clear Definition of Species
– Species Are Groups of Interbreeding
– Appearance Can Be Misleading
Clear Definition of Species Needed
• In Pre-Darwinian times, the term
“species” referred to different kinds of
• Species were classified initially based
upon appearance
• Today a species is defined as a group of
actually or potentially interbreeding natural
populations, which are reproductively
isolated from other such groups
(biological species concept)
• Limitations of species definition
– Cannot be used to determine species identity
among asexually reproducing organisms or
among fossils
– Often difficult to observe whether members
of two different groups interbreed
Appearance Can Be Misleading
• Organisms of similar appearance
sometimes belong to different species
• Organisms that differ in appearance may
belong to the same species
Appearance Can Be Misleading
• The cordilleran flycatcher and Pacific
slope flycatcher are so similar that
birdwatchers can’t tell them apart
• These birds were considered to be a
single species, but research revealed that
they do not interbreed and are in fact two
different species
Appearance Can Be Misleading
• The myrtle warbler and Audubon’s
warbler used to be classified as different
– Discovered that the warblers interbreed where
their ranges overlap
– Today, considered a single species
Section 16.2 Outline
• 16.2 How Is Reproductive Isolation
Between Species Maintained?
– Premating Isolating Mechanisms Prevent
Mating Between Species
– Postmating Isolating Mechanisms Limit Hybrid
Reproductive Isolation
• Reproductive isolation occurs when
members of one population are unable to
interbreed with members of another
Isolating Mechanisms
• Isolating mechanisms prevent
interbreeding and maintain reproductive
– Premating isolating mechanisms prevent
mating between species
– Postmating isolating mechanisms prevent
the formation of viable, fertile offspring after
mating has occurred
Isolating Mechanisms
• For most species, two or more isolating
mechanisms interact to prevent the
formation of fertile offspring
Premating Isolating Mechanisms
• Premating isolating mechanisms include:
– Geographical isolation
– Ecological isolation
– Temporal isolation
– Behavioral isolation
– Mechanical incompatibility
Premating Isolating Mechanisms
• Geographical isolation occurs when
populations cannot mate because of
physical barriers
– In nature, lions do not mate with tigers
• Lions live in Africa
• Tigers live in India
Premating Isolating Mechanisms
• Geographically separated populations
aren’t necessarily distinct species
• The Kaibab and Abert squirrels living in
geographically separate areas of the
Grand Canyon are physically separated,
but still very similar
• It is currently unknown if the two
populations have diverged into two
Premating Isolating Mechanisms
• Ecological isolation occurs when species
can’t mate because they occupy different
Premating Isolating Mechanisms
• White-crowned sparrows inhabit fields and
meadows, while white-throated sparrows
inhabit dense thickets
• Each species of fig wasp breeds in the
fruits of a particular species of fig
Premating Isolating Mechanisms
• Temporal isolation occurs when species
can’t mate because they breed at different
Premating Isolating Mechanisms
• In nature, Bishop pines and Monterey
pines do not interbreed
– Bishop pine pollination occurs in
– Monterey pine pollination occurs in early
Premating Isolating Mechanisms
• Behavioral isolation occurs when species
can’t mate because they have different
courtship and mating rituals
Premating Isolating Mechanisms
• Songs and plumage of male songbirds are
species specific
– Attract females of the same species
– Females of other species are
Premating Isolating Mechanisms
• Male frogs embrace any female regardless
of species
– Female frogs encountering males of a
different species utter the “release call”
Premating Isolating Mechanisms
• Mechanical incompatibility occurs when
species cannot mate because their
reproductive structures are incompatible
Premating Isolating Mechanisms
• In animals with internal fertilization, male
and female sexual organs may not fit
– e.g., snails of species whose shells have lefthanded spirals may be unable to successfully
copulate with snails whose shells have righthanded spirals
Premating Isolating Mechanisms
• In plants, differences in flower size or
structure may attract different pollinators
Postmating Isolating Mechanisms
• Postmating isolating mechanisms include:
– Gametic incompatibility
– Hybrid inviability
– Hybrid infertility
Postmating Isolating Mechanisms
• Gametic incompatibility occurs when
sperm from one species cannot fertilize
eggs of another
Postmating Isolating Mechanisms
• In animals, fluids of the female
reproductive tract my weaken or kill sperm
of another species
• In plants, pollen from one species may fail
to germinate when it lands on the stigma
of another species
Postmating Isolating Mechanisms
• Hybrid inviability occurs when hybrid
offspring fail to survive to maturity
Postmating Isolating Mechanisms
• Hybrid may abort early in development
• Hybrid may be unable to reproduce
because it display behaviors that are
mixtures of the two parental types
– Lovebird hybrids have great difficulty
learning to carry nest materials during
Postmating Isolating Mechanisms
• Hybrid infertility occurs when hybrid
offspring are sterile or have reduced fertility
– Mule hybrids (a cross between a horse and a
donkey) are sterile
– Liger hybrids (a zoo-based cross between a
male lion and a female tiger) are sterile
Postmating Isolating Mechanisms
• Infertility is caused by the failure of
chromosomes to pair properly during
meiosis, so eggs and sperm never develop
Section 16.3 Outline
• 16.3 How Do New Species Form?
– Geographic Separation of a Population Can
Lead to Allopatric Speciation
– Ecological Isolation of a Population Can Lead
to Sympatric Speciation
– Under Some Conditions, Many New Species
May Arise
• Speciation is the process by which new
species form
How Do New Species Form?
• Speciation depends on two factors
– Isolation
– Genetic divergence
How Do New Species Form?
• When populations are isolated, gene flow
between them is blocked
• Isolated populations subsequently evolve
genetic differences large enough to
prevent interbreeding
– Differences arise by chance (genetic drift) or
through natural selection
Mechanisms of Speciation
• Allopatric speciation
– Isolating mechanism is a physical barrier
• Sympatric speciation
– Isolation occurs without geographic
Allopatric Speciation
• Allopatric speciation occurs when two
populations of a species become
separated by a geographical barrier
– Colonization of remote islands by mainland
– Geological changes such as volcanism,
earthquakes, continental drift, and rivers
changing course
Allopatric Speciation
• Allopatric speciation occurs when
isolated populations diverge genetically
Allopatric Speciation
• Is believed to be the most common type of
speciation, especially among animals
• Two allopatric populations, the Kaibab
squirrel and Abert’s squirrel, may be
evolving into two separate species
Sympatric Speciation
• Sympatric speciation occurs when
– Two populations of a species living in the
same geographical area become restricted to
different habitats
– Isolated populations diverge genetically
Sympatric Speciation
• Two sympatric populations of fruit flies
(Rhagoletis pomonella) may be evolving
into two separate species
Sympatric Speciation
• One population lays its eggs in hawthorn
fruit, while the other prefers apples
• The two populations experience very little
Sympatric Speciation
• Males and females prefer the same type of
fruit in which they developed
• Apples mature two or three weeks later
than hawthorn fruit (flies mature and mate
at different times)
New Species
• The above mechanisms of speciation and
reproductive isolation lead to forking
branches in the evolutionary tree of life,
as one species splits into two
Adaptive Radiation
• Adaptive radiation is the rise of many
new species over a relatively short period
of time
Adaptive Radiation
• Occurs when populations of one species
invade a variety of new habitats
– Finch colonization of the Galápagos Islands
– Cichlid fish colonization of Lake Malawi
– Tarweed plant colonization of the Hawaiian
Section 16.4 Outline
• 16.4 What Causes Extinction?
– Localized Distribution and Overspecialization
Make Species Vulnerable in Changing
– Interactions with Other Organisms May Drive
a Species to Extinction
– Habitat Change and Destruction Are the
Leading Causes of Extinction
What Causes Extinction?
• Extinction is the death of all members of
a species
• At least 99.9% of all species that ever
existed are now extinct
What Causes Extinction?
• Factors that may make a species
vulnerable to extinction include:
– Localized distribution
– Overspecialization
– Competition among species
– Habitat destruction
Localized Distribution
• Wide-ranging species normally do not
succumb to local environmental
Localized Distribution
• Species inhabiting extremely limited
ranges may become extinct if the area is
– Devil’s Hole pupfish is found in only one
spring-fed water hole in the Nevada desert
• Species that develop adaptations that
favor survival in a specific environment are
at risk of becoming extinct
– The Karner blue butterfly feeds only on the
blue lupine plant
– The habitat of the lupine has been
significantly reduced by development
– Loss of the lupine will lead to extinction of the
Karner blue butterfly
Competition Among Species
• Species that are unable to exploit
resources more efficiently and effectively
than their competitors may become extinct
Competition Among Species
• 2.5 million years ago a land bridge (the
isthmus of Panama) formed between
North and South America
• North American species displaced the vast
majority of South American species, many
of which became extinct
Habitat Destruction
• Habitat destruction is the single greatest
cause of extinction
Habitat Destruction
• Extinctions due to prehistoric habitat
change will be discussed in Chapter 17
• Human activities are the primary cause of
present-day habitat destruction
– Clearing of tropical rainforests could lead to
loss of up to half of all current species over
the next 50 years