Download species

Chapter 24
The Origin of Species
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: That “Mystery of Mysteries”
• In the Galápagos
Islands Darwin
discovered plants
and animals found
nowhere else on
Earth
Video: Galápagos Tortoise
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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, confined to one gene
pool
• Macroevolution refers to evolutionary change
above the species level. Ex: how groups of
organisms such as mammals evolved.
Animation: Macroevolution
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
• Gene flow between populations holds the
phenotype of a population together
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-2a
(a) Similarity between different species
Although the eastern and western meadowlarks look similar, they are distinct
species because their songs and behaviors prevent interbreeding should they
meet in the wild.
Fig. 24-2b
(b) Diversity within a species
Although they look different, all human beings belong to the same
species.
Reproductive Isolation
• The formation of new species hinges on reproductive
isolation.
– Reproductive isolation is the existence of biological
factors (barriers) that impede two species from
producing viable, fertile offspring
• Such barriers block gene flow and limit the formation of
hybrids.
– Hybrids are the offspring of crosses between different
species
• Reproductive isolation can be classified by whether factors
act before or after fertilization
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Prezygotic barriers include:
– Habitat Isolation
– Temporal Isolation
– Behavioral Isolation
– Mechanical Isolation
– Gametic Isolation
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Habitat isolation: Two species encounter
each other rarely, or not at all, because they
occupy different habitats, even though not
isolated by physical barriers
Two species of garter snake
occur in the same geographic
areas, but one lives in water
and the other on land.
Water-dwelling Thamnophis
Terrestrial Thamnophis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Temporal isolation: Species that breed at
different times of the day, different seasons, or
different years cannot mix their gametes
The ranges of these
two skunks overlap,
but the eastern skunk
mates in winter and the
western skunk mates
in the summer.
Eastern spotted skunk
(Spilogale putorius)
Western spotted skunk
(Spilogale gracilis)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Behavioral isolation: Courtship rituals and
other behaviors unique to a species are
effective barriers
Female blue-footed boobies will
only choose a mate who performs
the correct courtship display. This
provides for the proper
“mate recognition”
Video: Albatross Courtship Ritual
Video: Giraffe Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Mechanical isolation: Morphological
differences can prevent successful mating
As a result of their shells spiraling
in opposite directions, these two
species of snails cannot physically
mate. Their genital openings,
indicated by arrows, do not line up.
Bradybaena with shells spiraling
in opposite directions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Gametic isolation: Sperm of one species may
not be able to fertilize eggs of another species
Sperm and eggs from sea urchins
are released into the water where they
fuse and form zygotes. However,
sperm and eggs from the red and purple
sea urchins seen here are unable
to fuse because their surface proteins
cannot bind to each other.
Sea urchins
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Postzygotic barriers prevent the hybrid
zygote from developing into a viable, fertile
adult:
– Reduced hybrid viability
– Reduced hybrid fertility
– Hybrid breakdown
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Reduced hybrid viability: Genes of the
different parent species may interact and
impair the hybrid’s development
Some salamander subspecies will
occasionally hybridize but the hybrids
do not usually complete their
development, and those that do are
usually very frail
Ensatina hybrid
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Reduced hybrid fertility: Even if hybrids are
vigorous, they may be sterile
The hybrid offspring of
a donkey and a horse is
a mule…it is sterile.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
Both of the healthy hybrid parents
carry recessive alleles that show
up in the offspring.
Hybrid cultivated rice plants with
stunted offspring (center)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-4
Recap Summary of Pre/Postzygotic barriers
Prezygotic barriers
Habitat Isolation
Temporal Isolation
Individuals
of
different
species
(a)
Postzygotic barriers
Behavioral Isolation
Mechanical Isolation
Gametic Isolation
Mating
attempt
(c)
(d)
(e)
(f)
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Viable,
fertile
offspring
Fertilization
(g)
(h)
(i)
(j)
(b)
(k)
(l)
Limitations of the Biological Species Concept
• The biological species concept cannot be
applied to fossils or asexual organisms
(including all prokaryotes)
• Thus, we have alternative species concepts
that are useful in some situations.
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
(organisms adapting to different environmental conditions)
• 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
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 (with geographic
separation)
– Sympatric speciation (without geographic
separation)
– (In both cases: gene flow is interrupted)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-5
(a) Allopatric speciation
(b) Sympatric speciation
Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is
interrupted or reduced when a population is
divided into geographically isolated
subpopulations
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Process of Allopatric Speciation
• The definition of barrier depends on the ability of a
population to disperse. Ex: birds can travers the Grand
Canyon but squirrels cannot..thus, two species of antelope
squirrels have evolved on the rims of the canyon.
• Separate populations may evolve independently through
mutation, natural selection, and genetic drift
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Reproductive isolation between populations
generally increases as the distance between
them increases
• Barriers to reproduction are intrinsic;
separation itself is not a biological barrier
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-9
EXPERIMENT
Initial population
Some flies
raised on
starch medium
Mating experiments
after 40 generations
Some flies
raised on
maltose medium
RESULTS
Female
9
8
20
Mating frequencies
in experimental group
Starch
Starch
population 2 population 1
22
Starch
Starch
population 1 population 2
Male
Male
Maltose Starch
Female
Starch Maltose
18
15
12
15
Mating frequencies
in control group
Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes
place in geographically overlapping populations
– It can occur if gene flow is reduced by such
factors as polyploidy, habitat differentiation
and sexual selection.
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. A new species may originate if an
accident during cell division produces a
polyploid individual.
• An autopolyploid is an individual with more
than two chromosome sets, derived from one
species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-10-1
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Fig. 24-10-2
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
2n
Gametes
produced
are diploid..
Fig. 24-10-3
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—creating
a new biological
species
• An allopolyploid is a species with multiple
sets of chromosomes derived from different
species:
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-11-1
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Fig. 24-11-2
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Hybrid
with 7
chromosomes
Fig. 24-11-3
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Hybrid
with 7
chromosomes
Unreduced
gamete
with 7
chromosomes
Normal
gamete
n=3
Fig. 24-11-4
Allopolyploids are fertile when mating with each other,
but cannot interbreed with either parent species; thus
they represent a new biological species.
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Species A
2n = 6
Normal
gamete
n=3
Hybrid
with 7
chromosomes
Unreduced
gamete
with 7
chromosomes
Normal
gamete
n=3
Viable fertile
hybrid
(allopolyploid)
2n = 10
• Polyploidy is much more common in plants
than in animals
• Many important crops (oats, cotton, potatoes,
tobacco, and wheat) are polyploids
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
–
Natural selection has favored the apple-feeding populations and they are
now beginning to show temporal isolation from each other. They are still
considered subspecies—but gene flow is being reduced and they appear
to be on their way to forming two new species.
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
Fig. 24-12
EXPERIMENT
Normal light
P.
pundamilia
P. nyererei
Monochromatic
orange light
Allopatric and Sympatric Speciation: A Review
• In allopatric speciation, geographic isolation
restricts gene flow between populations
• Reproductive isolation may then arise by
natural selection, genetic drift, or sexual
selection in the isolated populations
• Even if contact is restored between
populations, interbreeding is prevented
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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, natural selection, or sexual
selection
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Patterns Within Hybrid Zones
• 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)
Hybrid Zones over Time
• When closely related species meet in a hybrid
zone, there are three possible outcomes:
– Strengthening of reproductive barriers
– Weakening of reproductive barriers
– Continued formation of hybrid individuals
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-14-1
Gene flow
Population
(five individuals
are shown)
Barrier to
gene flow
Fig. 24-14-2
Isolated population
diverges
Gene flow
Population
(five individuals
are shown)
Barrier to
gene flow
Fig. 24-14-3
Isolated population
diverges
Hybrid
zone
Gene flow
Hybrid
Population
(five individuals
are shown)
Barrier to
gene flow
Fig. 24-14-4
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
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
Concept 24.4: 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Time Course of Speciation
• Broad patterns in speciation can be studied
using the fossil record, morphological data,
or molecular data
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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 equilibrium 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
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
Studying the Genetics of Speciation
• The explosion of genomics is enabling
researchers to identify specific genes involved
in some cases of speciation
• Depending on the species in question,
speciation might require the change of only a
single allele or many alleles
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
From Speciation to Macroevolution
• Macroevolution is the cumulative effect of
many speciation and extinction events
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 24-UN1
Original population
Allopatric speciation
Sympatric speciation
Fig. 24-UN2
Ancestral species:
AA
Triticum
monococcum
(2n = 14)
BB
Wild
Triticum
(2n = 14)
Product:
AA BB DD
T. aestivum
(bread wheat)
(2n = 42)
DD
Wild
T. tauschii
(2n = 14)
Fig. 24-UN3
You should now be able to:
1. Define and discuss the limitations of the four
species concepts
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