Download Chapter 24 PowerPoint 2016

Chapter 24
The Origin of Species
Overview: That “Mystery of Mysteries” (Darwin)
• 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 adaptations that
evolve within a population; changes in allele
frequency in a single gene pool
• Macroevolution refers to evolutionary change
above the species level (origin of mammals)
– Macroevolution is the cumulative effect of many
speciation and extinction events
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
• Reproductively compatible
• Gene flow between populations holds the
phenotype of a population together
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Reproductive Isolation
• Reproductive isolation is the existence of
biological factors (barriers) that impede two
species from producing viable, fertile offspring
– Reproductive isolation can be classified by
whether factors act before or after
fertilization
• Hybrids are the offspring of crosses between
different species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
– 5 types
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-4a
Habitat Isolation
Temporal Isolation
Individuals
of
different
species
(a)
(c)
(d)
(b)
• Habitat isolation: Two
species encounter each
other rarely, or not at all,
because they occupy
different habitats, even
though (e)not isolated by
physical barriers
(f)
• Temporal isolation:
Species that breed at
different times of the
day, different seasons,
or different years cannot
mix their gametes
• Behavioral isolation: Courtship rituals and
other behaviors unique to a species are
effective barriers
Courtship ritual of bluefooted boobies
• Mechanical isolation: Morphological
differences can prevent successful mating
Bradybaena with shells
spiraling in opposite
directions
• Gametic isolation: Sperm of one species may
not be able to fertilize eggs of another species
Sea urchins
• Postzygotic barriers prevent the hybrid
zygote from developing into a viable, fertile
adult:
– Reduced hybrid viability
– Reduced hybrid fertility
– Hybrid breakdown
– 3 types
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-4i
Prezygotic barriers
Gametic Isolation
Postzygotic barriers
Reduced Hybrid Viability Reduced Hybrid Fertility
Hybrid Breakdown
Viable,
fertile
offspring
Fertilization
(g)
(h)
(i)
(j)
(k)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
(l)
• Reduced hybrid viability: Genes of the
different parent species may interact in ways
that impair the hybrid’s development
Ensatina hybrid
• Reduced hybrid fertility: Even if hybrids are
vigorous, they may be sterile
Mule (sterile hybrid)
• Hybrid breakdown: Some first-generation
hybrids are fertile, but when they mate with
another or with either parent species, offspring
of the next generation are feeble or sterile
Hybrid cultivated rice plants with
stunted offspring (center)
Limitations of the Biological Species Concept
• The biological species concept cannot be
applied to:
• fossils (no way for us to evaluate)
• asexual organisms (including all prokaryotes)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
• The phylogenetic species concept: defines a
species as the smallest group of individuals on a
phylogenetic tree
– It applies to sexual and asexual species, but it
can be difficult to determine the degree of
difference required for separate species
• More than 20 other definitions of species proposed
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 24.2: Speciation can take place with or
without geographic separation
• Speciation can occur in two ways:
– Allopatric speciation
– Sympatric speciation
(a) Allopatric speciation
(b) Sympatric speciation
Two main modes of speciation:
Allopatric Speciation
Sympatric Speciation
“other” “homeland”
“together” “homeland”
Geographically isolated
populations
Overlapping populations within
home range
• Caused by geologic events
or processes
• Evolves by natural selection
& genetic drift
Gene flow between
subpopulations blocked by:
• polyploidy
• sexual selection
• habitat differentiation
Eg. Squirrels on N/S rims of
Grand Canyon
Eg. polyploidy in crops (oats,
cotton, potatoes, wheat)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is interrupted
or reduced when a population is divided into
geographically isolated subpopulations
• The definition of barrier depends on the ability of a
population to disperse
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Evidence of Allopatric Speciation
• Regions with many geographic barriers typically
have more species than do regions with fewer
barriers
• Reproductive isolation between populations
generally increases as the distance between them
increases
• Barriers to reproduction are intrinsic; separation
itself is not a biological barrier
– Some other difference must occur (courtship
behavior) to prevent populations from mating
if/when they do come in contact again
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Polyploidy
• Polyploidy is the presence of extra sets of
chromosomes due to accidents during cell division
– autopolyploid is an individual with more than two
chromosome sets, derived from one species
– allopolyploid is a species with multiple sets of
chromosomes derived from different species
• Much more common in plants than in animals
– Many important crops (oats, cotton, potatoes,
tobacco, and wheat) are polyploids
• Autopolyploid: extra sets of chromosomes
▫ Failure of cell division (2n  4n)
▫ Eg. Strawberries are 4n, 6n, 8n, 10n (decaploid)
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
2n
Gametes
produced
are diploid..
4n
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
Allopolyploidy
• 2 species
produce a hybrid
▫ Species A
(2n=6) +
Species B
(2n=4) 
Hybrid
(2n=10)
Habitat Differentiation
• Sympatric speciation can also result from the
appearance of new ecological niches
• For example, the North American maggot fly
can live on native hawthorn trees as well as
more recently introduced apple trees
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Sexual Selection
• Sexual selection can drive sympatric speciation
• Sexual selection for mates of different colors
has likely contributed to the speciation in cichlid
fish in Lake Victoria
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Allopatric and Sympatric Speciation: A Review
• In allopatric speciation, geographic isolation
restricts gene flow between populations
– Reproductive isolation arises by natural
selection, genetic drift, or sexual selection
• Even if contact is restored between
populations, interbreeding is prevented
• In sympatric speciation, a reproductive barrier
isolates a subset of a population without
geographic separation from the parent species
– Sympatric speciation can result from polyploidy,
habitat differentiation, or sexual selection
Concept 24.3: Hybrid zones provide opportunities
to study factors that cause reproductive isolation
• A hybrid zone is a region in which members of
different species mate and produce hybrids
• A hybrid zone can occur in a single band where
adjacent species meet
• Hybrids often have reduced fitness compared
with parent species
• The distribution of hybrid zones can be more
complex if parent species are found in multiple
habitats within the same region
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-13
EUROPE
Fire-bellied
toad range
Hybrid zone
0.99
Allele frequency (log scale)
Yellow-bellied toad,
Bombina variegata
Yellow-bellied
toad range
Fire-bellied toad,
Bombina bombina
0.9
0.5
0.1
0.01
40
20
30
10
0
10
20
Distance from hybrid zone center (km)
Fig. 24-14-4
• Incomplete reproductive barriers
• Possible outcomes: reinforcement, fusion, stability
Isolated population
diverges
Possible
outcomes:
Hybrid
zone
Reinforcement
OR
Fusion
Gene flow
Hybrid
Population
(five individuals
are shown)
OR
Barrier to
gene flow
Stability
Reinforcement: Strengthening Reproductive
Barriers
• The reinforcement of barriers occurs when
hybrids are less fit than the parent species
• Over time, the rate of hybridization decreases
• Where reinforcement occurs, reproductive
barriers should be stronger for sympatric than
allopatric species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-15
Sympatric male
pied flycatcher
28
Allopatric male
pied flycatcher
Pied flycatchers
24
Number of females
Collared flycatchers
20
16
12
8
4
(none)
0
Females mating Own
Other
with males from: species species
Sympatric males
Own
Other
species species
Allopatric males
Fusion: Weakening Reproductive Barriers
• If hybrids are as fit as parents, there can be
substantial gene flow between species
• If gene flow is great enough, the parent species
can fuse into a single species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-16
Pundamilia nyererei
Pundamilia pundamilia
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water
Stability: Continued Formation of Hybrid
Individuals
• Extensive gene flow from outside the hybrid
zone can overwhelm selection for increased
reproductive isolation inside the hybrid zone
• In cases where hybrids have increased fitness,
local extinctions of parent species within the
hybrid zone can prevent the breakdown of
reproductive barriers
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Tempo of Evolution
Gradualism
Punctuated Equilibium
• Common ancestor
• Eldridge & Gould
• Slow, constant change
• Long period of stasis
punctuated by short bursts of
significant change
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-17
(a) Punctuated pattern
Time
(b) Gradual pattern
Speciation Rates
• The punctuated pattern in the fossil record and
evidence from lab studies suggests that
speciation can be rapid
• The interval between speciation events can
range from 4,000 years (some cichlids) to
40,000,000 years (some beetles), with an
average of 6,500,000 years
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Define and discuss the limitations of the four
main species concepts/definitions
2. Describe and provide examples of prezygotic
and postzygotic reproductive barriers
3. Distinguish between and provide examples of
allopatric and sympatric speciation
4. Explain how polyploidy can cause
reproductive isolation
5. Define the term hybrid zone and describe
three outcomes for hybrid zones over time
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings