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BIOLOGY Life on Earth
WITH PHYSIOLOGY Tenth Edition
Audesirk Audesirk Byers
16
The Origin
of Species
Lecture Presentations by
Carol R. Anderson
Westwood College, River Oaks Campus
© 2014 Pearson Education, Inc.
16.1 What Is a Species?
 Each species evolves independently
– Today, a species is defined as a group of populations
that evolves independently
– Each species follows a separate evolutionary path
because alleles rarely move between the gene pools of
different species
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16.1 What Is a Species?
 Each species evolves independently (continued)
– The biological species concept is based on the
observation that reproductive isolation (inability to
successfully breed outside the group) ensures
evolutionary independence
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Traits that prevent interbreeding and maintain
reproductive isolation are called isolating
mechanisms
– The benefit of isolating mechanisms is the production
of no offspring that are unfit or sterile
– Isolating mechanisms prevent the wasting of
reproductive effort and failing to contribute to future
generations
 Natural selection favors traits that prevent
reproduction across species boundaries
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Table 16-1
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms prevent mating
between species
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Members of different species may be prevented from
meeting
– Geographical isolation prevents interbreeding
between populations that do not come into contact
because they live in different, physically separated
places
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Members of different species may be prevented from
meeting (continued)
– This type of isolation is usually considered to be a
mechanism that allows new species to form rather than
one that maintains reproductive isolation between
species
– (Albert moved into Grand Canyon area and made the
Kaibab)
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Figure 16-3 Geographic isolation
Kaibab squirrel
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Abert squirrel
16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Different species may occupy different habitats
– Ecological isolation occurs when species do not mate
because they occupy different habitats
– White-crowned sparrows inhabit fields and meadows,
while white-throated sparrows inhabit dense thickets
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Ecological isolation occurs when species do not mate
because they occupy different habitats (continued)
– In another example, each species of fig wasp breeds in
the fruit of a different species of fig, and the wasps thus
do not come into contact with one another
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Figure 16-4 Ecological isolation
© 2014 Pearson Education, Inc.
16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Different species may breed at different times
– Temporal isolation (time-based isolation) prevents
breeding between two species occupying the same
habitat because of different breeding seasons
– The spring field cricket and the fall field cricket both
occur in many areas of North America
– They are unable to interbreed because of their varying
breeding seasons
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Different species may breed at different times
(continued)
– In nature, Bishop pines and Monterey pines do not
interbreed
– Bishop pine pollination occurs in summer
– Monterey pine pollination occurs in early spring
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Figure 16-5 Temporal isolation
Bishop pine
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Monterey pine
16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Different species may have different courtship signals
– Among animals, elaborate courtship colors and
behaviors can prevent mating between species
– Behavioral isolation is created by signals and
behaviors that differ from species to species
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Different species may have different courtship signals
(continued)
– The plumes and arresting pose of a courting male
Raggiana bird of paradise are conspicuous indicators of
this species
– There is little chance females of another species will
mate with him by mistake
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Figure 16-6 Behavioral isolation
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Different species may have different courtship signals
(continued)
– Male frogs embrace any female regardless of species
– Female frogs encountering males of a different species
utter the “release call,” which causes the males to let
go
– As a result, few hybrids—offspring of parents of different
species—are produced
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Differing sexual organs may foil mating attempts
– In animals with internal fertilization, male and female
sexual organs may not fit together
– Incompatible body shapes may make copulation
between species impossible
– For example, snails of species whose shells have lefthanded spirals may be unable to copulate successfully
with snails whose shells have right-handed spirals
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Premating isolating mechanisms include the
following (continued)
– Differing sexual organs may foil mating attempts
(continued)
– In plants, differences in flower size or structure may
attract different pollinators
– Mechanical incompatibility occurs when species
cannot mate because their reproductive structures are
incompatible
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Postmating isolating mechanisms limit hybrid
offspring (continued)
– If the resulting offspring die during development, the
two species remain reproductively isolated from each
other
– Mechanisms that prevent the formation of vigorous,
fertile hybrids between species are called postmating
isolating mechanisms
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Postmating isolating mechanisms limit hybrid
offspring (continued)
– One species’ sperm may fail to fertilize another
species’ egg
– Gametic incompatibility occurs when sperm from one
species cannot fertilize eggs of another
– In animals with internal fertilization, fluids of the female
reproductive tract may weaken or kill sperm of another
species
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Postmating isolating mechanisms limit hybrid
offspring (continued)
– One species’ sperm may fail to fertilize another
species’ egg (continued)
– Gametic incompatibility may be an especially important
isolating mechanism in certain species
– Marine invertebrates and wind-pollinated plants
reproduce by scattering gametes in the water or in the air
© 2014 Pearson Education, Inc.
16.2 How Is Reproductive Isolation Between Species
Maintained?
 Postmating isolating mechanisms limit hybrid
offspring (continued)
– One species’ sperm may fail to fertilize another
species’ egg (continued)
– Among plants, chemical incompatibility may prevent the
germination of pollen from one species that lands on
the stigma (pollen-catching structure) of the flower of
another species
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Postmating isolating mechanisms limit hybrid
offspring (continued)
– Hybrid offspring may fail to survive or reproduce
– If cross-species fertilization occurs, the resulting hybrid
may be unable to survive, a situation called hybrid
inviability
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Postmating isolating mechanisms limit hybrid
offspring (continued)
– Hybrid offspring may fail to survive or reproduce
(continued)
– The genetic instructions directing development of the two
species may be so different that hybrids abort early in
development
– The hybrid may abort early in development or die shortly
after birth
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16.2 How Is Reproductive Isolation Between Species
Maintained?
 Postmating isolating mechanisms limit hybrid
offspring (continued)
– Hybrid offspring may be infertile
– 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
– Hybrid infertility prevents hybrids from passing on
their genetic material to offspring, thus blocking gene
flow between the two parent populations
© 2014 Pearson Education, Inc.