Speciation

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Speciation
Speciation
Key Concepts
• Species Are Reproductively Isolated Lineages
on the Tree of Life
• Speciation Is a Natural Consequence of
Population Subdivision
• Speciation May Occur through Geographic
Isolation or in Sympatry
• Reproductive Isolation Is Reinforced When
Diverging Species Come into Contact
Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
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?”
Species—groups of organisms that mate with
one another.
Concept 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.
Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Limitations: members of the same species don’t
always look alike (e.g., male and female wood
ducks).
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 (Part 1)
Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
Biological species concept: “Species are
groups of actually or potentially interbreeding
natural populations which are reproductively
isolated from other such groups.” Ernst Mayr
Reproductive isolation: two groups of
organisms can no longer exchange genes.
Concept 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.
Concept 1 Species Are Reproductively Isolated
Lineages on the Tree of Life
The various species concepts emphasize
different aspects of species or speciation.
Reproductive isolation is necessary for lineages
to remain distinct over evolutionary time.
It is also responsible for morphological
differences—mutations that result in
morphological changes cannot spread between
reproductively isolated species.
Concept 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 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
in meiosis.
Concept 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).
Concept 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.
Figure 17.6 Allopatric Speciation (Part 1)
Figure 17.6 Allopatric Speciation (Part 2)
Concept 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
Concept 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 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 of chromosomes of
two different species
Concept 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 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 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., tree frogs).
Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Reproductive isolation may be incomplete when
the incipient species come back into contact
and hybridization occurs.
But hybrids may be less fit, and selection favors
non-hybridizing parents.
Selection results in reinforcement of isolating
mechanisms.
Concept 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 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 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms
Temporal isolation—species may breed at
different times of year or different times of day.
Example: closely related leopard frog species.
Figure 17.9 Temporal Isolation of Breeding Seasons (Part 1)
Figure 17.9 Temporal Isolation of Breeding Seasons (Part 2)
Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms
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 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Whether plant species hybridize may depend on
their pollinators.
Flower color and shape influences 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 (Part 1)
Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 2)
Concept 4 Reproductive Isolation Is Reinforced
When Diverging Species Come into Contact
Prezygotic isolating mechanisms
Habitat isolation—when two closely related
species evolve preferences for living or mating
in different habitats.
Gametic isolation—sperm and eggs of different
species will not fuse. Important for aquatic
animals that release gametes into the water.
Concept 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 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)
Concept 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 European Bombina toad
species.
Figure 17.13 A Hybrid Zone
Chapter 17 Opening Question
Can biologists study the process of
speciation in the laboratory?
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.
Habitat selection is thought to have had a role in
speciation of cichlids in Lake Malawi.
Figure 17.14 Evolution in the Laboratory
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