Download PoL2e Ch17 Lecture-Speciation

17
Speciation
Chapter 17 Speciation
Key Concepts
17.1 Species Are Reproductively Isolated
Lineages on the Tree of Life
17.2 Speciation Is a Natural Consequence of
Population Subdivision
17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Chapter 17 Opening Question
Can biologists study the process of
speciation in the laboratory?
Concept 17.1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Species: groups of organisms that mate with
one another
Speciation: divergence of biological lineages
and emergence of reproductive isolation
between lineages
Most of the species concepts proposed by
biologists are different ways of approaching the
question “What are species?”
Concept 17.1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Linnaeus described species based on their
appearance—the morphological species
concept.
Members of species look alike because they
share many alleles.
He originated the binomial system of
nomenclature.
Figure 17.1 Members of the Same Species Look Alike—or Not
Concept 17.1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Limitations:
•  Members of the same species do not
always look alike (e.g., male and female
hooded mergansers)
•  Cryptic species—two or more species that
are morphologically indistinguishable but
do not interbreed
Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed
Concept 17.1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Ernst Mayr’s Biological species concept
•  Species are groups of actually or
potentially interbreeding natural
populations that are reproductively isolated
from other such groups.
Reproductive isolation: two groups of
organisms can no longer exchange genes
Concept 17.1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Lineage species concept: species as
branches on the tree of life
A lineage is an ancestor–descendant series of
populations followed over time.
Each species has a history that starts with a
speciation event—one lineage is split into two
and ends either at extinction or at another
speciation event.
Concept 17.1 Species Are Reproductively Isolated
Lineages on the Tree of Life
The various species concepts emphasize
different aspects of species or speciation.
Reproductive isolation allows sexual species to
evolve independently and is necessary for
lineages to remain distinct over evolutionary
time.
The lineage concept accommodates asexually
reproducing species and extends over
evolutionary time.
Concept 17.2 Speciation Is a Natural Consequence
of Population Subdivision
Not all evolutionary changes result in new
species.
Speciation requires interruption of gene flow.
How can one lineage ever split into two
reproductively isolated species?
Concept 17.2 Speciation Is a Natural Consequence
of Population Subdivision
The Dobzhansky–Muller model:
•  A population is subdivided and then the
two groups evolve independently.
•  In each lineage, new alleles become fixed
at different loci. The new alleles at the two
loci are incompatible with one another.
•  Genetic incompatibility between the two
isolated populations will develop over time.
Figure 17.3 The Dobzhansky–Muller Model
Concept 17.2 Speciation Is a Natural Consequence
of Population Subdivision
In bats of the genus Rhogeessa, chromosomal
rearrangements in different lineages have led
to speciation.
Hybrids between some types will not be able to
produce normal gametes because the
chromosomes cannot pair normally in
meiosis.
Figure 17.4 Speciation by Centric Fusion
Concept 17.2 Speciation Is a Natural Consequence
of Population Subdivision
As species diverge genetically, reproductive
isolation increases.
Development of reproductive isolation may take
millions of years or may develop in a few
generations (e.g., the Rhogeessa bats).
Figure 17.5 Reproductive Isolation Increases with Genetic Divergence
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
Allopatric speciation—when populations are
separated by a physical or geographic barrier
Barriers can form when continents drift, sea
levels rise and fall, glaciers advance and
retreat, or climates change.
The populations evolve through genetic drift
and adaptation to different environments in
the two areas.
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
Pairs of sister species (species that are each
other’s closest relatives) may exist on
opposite sides of a geographic barrier.
•  Example: The Pleistocene glaciation
isolated fish populations in the Ozark and
Ouachita mountains from fish populations
in the eastern highlands, resulting in sister
species pairs.
Figure 17.6 Allopatric Speciation (Part 1)
Figure 17.6 Allopatric Speciation (Part 2)
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
Some members of a population may cross an
existing barrier and establish an isolated
population.
Finch species in the Galápagos Islands evolved
from a single South American species that
colonized the islands.
The islands are far apart and have different
environmental conditions.
Figure 17.7 Allopatric Speciation among Darwin’s Finches (Part 1)
Figure 17.7 Allopatric Speciation among Darwin’s Finches (Part 2)
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
Sympatric speciation—speciation without
physical isolation
May occur with disruptive selection—individuals
with certain genotypes prefer distinct
microhabitats where mating takes place.
This appears to be taking place with apple
maggot flies. One group prefers to lay eggs
on hawthorne fruits, the other group lays eggs
on apples. They are partially reproductively
isolated.
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
Sympatric speciation most commonly occurs by
polyploidy—duplication of sets of
chromosomes within individuals.
Autopolyploidy—chromosome duplication in a
single species
Allopolyploidy—combining chromosomes of
two different species
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
A tetraploid can result if two accidentally
unreduced diploid gametes combine.
Tetraploid and diploid individuals are
reproductively isolated because their hybrid
offspring are triploid.
But tetraploids can self-fertilize or mate with
another tetraploid.
Thus polyploidy can result in complete
reproductive isolation in two generations.
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
Hybridization between closely related species
can disrupt normal meiosis and result in
chromosomal doubling.
Allopolyploids are often fertile: each
chromosome has a partner to pair with in
meiosis.
Concept 17.3 Speciation May Occur through Geographic
Isolation or in Sympatry
Speciation by polyploidy is common in plants—
about 70% of flowering plant species and
95% of fern species.
It is more common in plants because many can
self-fertilize.
It has also occurred in animals (e.g., the gray
tree frogs).
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Over many generations, differences
accumulate in isolated lineages, reducing the
probability that individuals could mate
successfully when they come back into
contact.
If reproductive isolation is incomplete,
hybridization can occur.
Hybrids may be less fit, and selection favors
non-hybridizing parents.
Selection results in reinforcement of isolating
mechanisms.
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Prezygotic isolating mechanisms prevent
hybridization from occurring.
Postzygotic isolating mechanisms reduce
fitness of hybrid offspring.
Postzygotic mechanisms result in selection
against hybridization, which leads to
reinforcement of prezygotic mechanisms.
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Prezygotic isolating mechanisms:
•  Mechanical isolation—differences in
sizes and shapes of reproductive organs
§ 
In plants, it may involve pollinators.
Figure 17.8 Mechanical Isolation through Mimicry
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
•  Temporal isolation—species may breed
at different times of year or different times
of day
§  Example: closely related leopard frog
species that breed at different times of
the year
Figure 17.9 Temporal Isolation of Breeding Seasons
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
•  Behavioral isolation—individuals reject or
fail to recognize mating behaviors of other
species
§  Examples: mating calls of male frogs
and coloration of male cichlid fish
species
Figure 17.10 Behavioral Isolation in Mating Calls
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Whether plant species hybridize may depend
on their pollinators.
Flower color and shape influence which
pollinators are attracted or alters where pollen
is deposited.
Two sympatric species of columbines
(Aquilegia) have diverged in flower color,
structure, and orientation. One is pollinated by
hummingbirds, the other by hawkmoths.
Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
•  Habitat isolation—when two closely
related species evolve preferences for
living or mating in different habitats
§ 
Example: apple maggot flies and Lake
Malawi cichlids)
•  Gametic isolation—sperm and eggs of
different species will not fuse—important
for aquatic animals that release gametes
into the water
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Postzygotic isolating mechanisms
Genetic differences in diverging lineages may
reduce fitness of hybrid offspring:
•  Low hybrid zygote viability
•  Low hybrid adult viability
•  Hybrid infertility
Concept 17.4 Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Reinforcement of prezygotic isolating
mechanisms is often detected by comparing
sympatric and allopatric populations of
potentially hybridizing species.
Sympatric populations are expected to evolve
more effective prezygotic reproductive
barriers than do allopatric populations.
Experiments with Phlox flower species are an
example.
Figure 17.12 Flower Color and Reproductive Isolation (Part 1)
Figure 17.12 Flower Color and Reproductive Isolation (Part 2)
Figure 17.12 Flower Color and Reproductive Isolation (Part 3)
Concept 17.4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
If reproductive isolation is incomplete, hybrid
zones may form where population ranges
overlap.
Hybrid zones may persist for long periods, such
as between ranges of two European Bombina
toad species.
Hybrids suffer from a range of defects and are
not as fit as purebred individuals. The hybrid
zone is very narrow because there is strong
selection pressure against hybrids.
Figure 17.13 A Hybrid Zone
Answer to Opening Question
Some aspects of speciation can be studied in
the laboratory, using organisms with short
generation times.
Experiments using fruit flies showed that
divergence in habitat preference resulted in
reproductive isolation in 35 generations.
Similar habitat selection is thought to have had
a role in early speciation of cichlids in Lake
Malawi.
Figure 17.14 Evolution in the Laboratory