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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITIONJane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
The Origin of Species
Chapter 24
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Overview: That “Mystery of Mysteries”
• In the Galápagos Islands Darwin discovered
plants and animals found nowhere else on Earth
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• Speciation, the origin of new species, is at the focal point of evolutionary theory
• Evolutionary theory must explain how new species originate and how populations evolve
• Microevolution consists of changes in allele frequency in a population over time
• Macroevolution refers to broad patterns of evolutionary change above the species level
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© 2011 Pearson Education, Inc.
Animation: Macroevolution Right-click slide / select “Play”
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Concept 24.1: The biological species
concept emphasizes reproductive isolation
• Species is a Latin word meaning “kind” or
“appearance”
• Biologists compare morphology, physiology,
biochemistry, and DNA sequences when
grouping organisms
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The Biological Species Concept
• The biological species concept states that a
species is a group of populations whose members
have the potential to interbreed in nature and
produce viable, fertile offspring; they do not breed
successfully with other populations
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Figure 24.2
(a) Similarity between different species
(b) Diversity within a species
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Reproductive Isolation
• Reproductive isolation is the existence of
biological factors (barriers) that impede two
species from producing viable, fertile offspring
• Hybrids are the offspring of crosses between
different species
• Reproductive isolation can be classified by
whether factors act before or after fertilization
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Figure 24.3_a
Prezygotic barriers
HabitatIsolation
TemporalIsolation
BehavioralIsolation
MechanicalIsolation
GameticIsolation
Reduced HybridViability
Reduced HybridFertility
HybridBreakdown
Individuals of
differentspecies
MATINGATTEMPT FERTILIZATION
VIABLE,FERTILE
OFFSPRING
Postzygotic barriers
(a) (c) (e)
(d)
(b)
(g)
(k)
(h) (i)
(j)
(l)(f)
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• Prezygotic barriers block fertilization from
occurring by:
– Impeding different species from attempting to mate
– Preventing the successful completion of mating
– Hindering fertilization if mating is successful
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• Habitat isolation: Two species encounter each
other rarely, or not at all, because they occupy
different habitats, even though not isolated by
physical barriers
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• Temporal isolation: Species that breed at
different times of the day, different seasons,
or different years cannot mix their gametes
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Western Spotted Skunk
delayed implantation – mating in Sept – Oct
fertilized eggs free float until March – April
Eastern Spotted Skunk
mating March – April
7-14 day implantation
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• Behavioral isolation: Courtship rituals and other
behaviors unique to a species are effective
barriers
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• Mechanical isolation: Morphological differences
can prevent successful mating
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ex: bushbaby
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• Mechanical isolation: Morphological differences
can prevent successful mating – plants, too.
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black sage white sage
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• Gametic Isolation: Sperm of one species may not
be able to fertilize eggs of another species
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important in broadcast spawners
isolation at the molecular level
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• Postzygotic barriers prevent the hybrid zygote
from developing into a viable, fertile adult:
– Reduced hybrid viability
– Reduced hybrid fertility
– Hybrid breakdown
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• Reduced hybrid viability: Genes of the different
parent species may interact and impair the
hybrid’s development
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• Reduced hybrid fertility: Even if hybrids are
vigorous, they may be sterile
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• Hybrid breakdown: Some first-generation hybrids
are fertile, but when they mate with another
species or with either parent species, offspring of
the next generation are feeble or sterile
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Limitations of the Biological Species Concept
• The biological species concept cannot be
applied to fossils or asexual organisms
(including all prokaryotes)
• The biological species concept emphasizes
absence of gene flow
• However, gene flow can occur between
distinct species
– For example, grizzly bears and polar bears can mate to produce “grolar bears”
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Figure 24.4Grizzly bear (U. arctos)
Polar bear (U. maritimus)
Hybrid “grolar bear”
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Other Definitions of Species
• Other species concepts emphasize the unity within
a species rather than the separateness of different
species
• The morphological species concept defines a
species by structural features
– It applies to sexual and asexual species but relies on subjective criteria
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• The ecological species concept views a species
in terms of its ecological niche
– It applies to sexual and asexual species and emphasizes the role of disruptive selection
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Scrub Jay
insects, fruit in summer
nuts in winter
Pinyon Jay
social
primarily pine nuts
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Concept 24.2: Speciation can take place
with or without geographic separation
• Speciation can occur in two ways:
– Allopatric speciation
– Sympatric speciation
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Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is
interrupted or reduced when a population is
divided into geographically isolated
subpopulations
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The Process of Allopatric Speciation
• The definition of barrier depends on the ability of a
population to disperse
– For example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen
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Harris Antelope Squirrel White Tailed Antelope Squirrel
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Figure 24.8
A. formosus
Atlantic Ocean
A. nuttingi
Isthmus of Panama
Pacific Ocean
A. panamensis A. millsae
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• Regions with many geographic barriers typically have more species than do regions with fewer barriers
• Geographic isolation is followed by reproductive isolation.
• Reproductive isolation between populations generally increases as the distance between them increases
– For example, reproductive isolation increases between salamanders that live further apart
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Figure 24.10 EXPERIMENT
RESULTS
Initial populationof fruit flies(Drosophila
pseudoobscura)
Some flies raisedon starch medium
Mating experimentsafter 40 generations
Some flies raised onmaltose medium
Female
Starch Maltose
Male
Malt
ose
Sta
rch
Number of matingsin experimental group
22 9
8 20
Female
Starchpopulation 1
Male
Sta
rch
po
pu
lati
on
2
Number of matingsin control group
18 15
12 15
Starchpopulation 2
Sta
rch
po
pu
lati
on
1
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Figure 24.10a
EXPERIMENT
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Mating experiments
after 40 generations
Some flies raised on
maltose medium
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Figure 24.10b
RESULTS
Female
Starch Maltose
Male
Malt
ose
Sta
rch
Number of matings
in experimental group
22 9
8 20
Female
Starch
population 1
Ma
leS
tarc
h
po
pu
lati
on
2Number of matings
in control group
18 15
12 15
Starch
population 2
Sta
rch
po
pu
lati
on
1
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Increased separation (time and/or distance) increases reproductive isolation and results in increasingly distinct speciation.
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• In sympatric speciation, speciation takes place
in geographically overlapping populations
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Polyploidy
• Polyploidy is the presence of extra sets of
chromosomes due to accidents during cell division
• Polyploidy is much more common in plants than in
animals
• An autopolyploid is an individual with more than two
chromosome sets, derived from one species
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• An allopolyploid is a species with multiple sets of
chromosomes derived from different species
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Figure 24.11-4
Species A2n = 6
Species B2n = 4
Normal
gamete
n = 3
Meiotic error;chromosome number not
reduced from 2n to n
Unreduced gametewith 4 chromosomes
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Normalgamete
n = 3
New species:viable fertile hybrid(allopolyploid) 2n = 10
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Speciation can occur rapidly or slowly and can
result from changes in few or many genes
• Many questions remain concerning how long it takes for
new species to form, or how many genes need to differ
between species
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Patterns in the Fossil Record
• The fossil record includes examples of species
that appear suddenly, persist essentially
unchanged for some time, and then apparently
disappear
• Niles Eldredge and Stephen Jay Gould coined the
term punctuated equilibria to describe periods of
apparent stasis punctuated by sudden change
• The punctuated equilibrium model contrasts with a
model of gradual change in a species’ existence
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• The interval between speciation events can range
from 4,000 years (some cichlids) to 40 million
years (some beetles), with an average of 6.5
million years
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