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23
Species and Their Formation
23 Species and Their Formation
• 23.1 What Are Species?
• 23.2 How Do New Species Arise?
• 23.3 What Happens when Newly Formed
Species Come Together?
• 23.4 Why Do Rates of Speciation Vary?
• 23.5 Why Do Adaptive Radiations Occur?
23.1 What Are Species?
Species literally means “kinds.”
We recognize most species by their
appearance.
Many species change little over large
geographic ranges.
Figure 23.1 Members of the Same Species Look Alike—or Not (A)
23.1 What Are Species?
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.
23.1 What Are Species?
But males and females may not look
alike.
Immature individuals may not look like
their parents.
Other types of information must be used
to determine species.
Figure 23.1 Members of the Same Species Look Alike—or Not
23.1 What Are Species?
Species can be thought of as branches
on the tree of life.
Speciation: The process by which one
species splits into two or more daughter
species, often gradually.
Figure 23.2 Speciation May Be a Gradual Process
23.1 What Are Species?
Speciation involves reproductive
isolation—when individuals of a
population mate with each other, but not
with individuals in another population,
they are a distinct evolutionary unit.
23.1 What Are Species?
The biological species concept:
proposed by Ernst Mayr:
“Species are groups of actually or
potentially interbreeding natural
populations which are reproductively
isolated from other such groups.”
This does not apply to asexually
reproducing organisms.
23.2 How Do New Species Arise?
Darwin called speciation the “mystery of
mysteries.”
Not all evolutionary change results in
new species.
But if two populations are isolated, over
time their genetic structure may change
enough so that interbreeding is no
longer possible: gene flow must be
interrupted.
23.2 How Do New Species Arise?
Allopatric speciation occurs when
populations are separated by a physical
barrier. Also called geographic
speciation.
Thought to be the dominant mode of
speciation.
Figure 23.3 Allopatric Speciation
23.2 How Do New Species Arise?
Barriers can form as continents drift, sea
level changes, glaciers advance and
retreat, climate changes.
The environments in which the isolated
populations live are different, and so the
populations evolve differently.
23.2 How Do New Species Arise?
Allopatric speciation can also occur if
some individuals cross a barrier to form
a new, isolated population.
The 14 species of finches on the
Galapagos (Darwin’s finches) arose
from a single species that colonized the
islands from South America.
The islands have different environments,
and are sufficiently far apart for
speciation to occur.
Figure 23.4 Allopatric Speciation among Darwin’s Finches (Part 1)
Figure 23.4 Allopatric Speciation among Darwin’s Finches (Part 2)
23.2 How Do New Species Arise?
The Hawaiian Islands have 800 species
of Drosophila, many restricted to one
island.
Many of these species have resulted
from founder events—species are
descendents of individuals that
dispersed among the islands.
Figure 23.5 Founder Events Lead to Allopatric Speciation
23.2 How Do New Species Arise?
The effectiveness of physical barriers
depends on the size and mobility of the
organisms.
Example: An 8-lane highway would be
impossible for a snail to cross, but
easily crossed by a bird.
23.2 How Do New Species Arise?
Sympatric speciation does not require
physical isolation.
Disruptive selection is required, such as
in black-bellied seed crackers.
Figure 22.15 Disruptive Selection Results in a Bimodal Distribution
23.2 How Do New Species Arise?
Sympatric speciation may be occurring in
a fruit fly in New York state.
The flies previously deposited eggs only
on hawthorn fruits.
When large apple orchards were started
in the region, some began laying eggs
on apples.
Figure 23.6 Sympatric Speciation May Be Underway in Rhagoletis pomonella
23.2 How Do New Species Arise?
The flies are somewhat reproductively
isolated because the populations are
ecologically isolated—they feed on
different resources.
They also emerge from their pupae at
different times—apple-feeding flies
develop faster.
23.2 How Do New Species Arise?
Sympatric speciation most commonly
occurs by polyploidy—duplication of
the whole set of chromosomes.
Chromosome duplication in a single
species is autopolyploidy; combining
of chromosomes from two species is
allopolyploidy.
23.2 How Do New Species Arise?
Autopolyploidy can occur if a cell
accidentally duplicates the
chromosomes, resulting in a tetraploid
individual.
Tetraploid and diploid populations are
soon separated because their hybrid
offspring are triploid and sterile—the
chromosome do not properly synapse
during meiosis.
Figure 23.7 Tetraploids Are Soon Reproductively Isolated from Diploids
23.2 How Do New Species Arise?
Allopolyploids can arise if two closely
related species mate, or hybridize.
Allopolyploids are often fertile.
Many species of flowering plants and
ferns are polyploids. Most have arisen
through hybridization followed by selffertilization.
23.2 How Do New Species Arise?
Speciation by allopolyploidy has occurred
in the salsifies (Tragopogon). These
weedy plants have been carried around
the world by humans.
Three species were introduced into North
America. Two tetraploid hybrids have
been produced.
The hybrids have formed several times
and are now more widespread than the
parental strains.
Figure 23.8 Polyploids May Outperform Their Parent Species
23.3 What Happens when Newly Formed Species Come
Together?
When populations have been isolated,
differences can accumulate that reduce
the probability that members of the two
populations could interbreed.
Partial reproductive isolation has been
shown for Phlox drummondii.
Plant breeders created many strains by
selecting for many characteristics, and
inadvertently reduced reproductive
compatibility.
23.3 What Happens when Newly Formed Species Come
Together?
Geographic isolation does not always
lead to reproductive isolation.
American sycamores and European
plane trees have been geographically
isolated for 20 million years;
but they can form fertile hybrid offspring.
Figure 23.9 Geographically Separated, Morphologically Similar
23.3 What Happens when Newly Formed Species Come
Together?
Mechanisms of reproductive
incompatibility fall into two categories:
• Prezygotic reproductive barriers
• Postzygotic reproductive barriers
23.3 What Happens when Newly Formed Species Come
Together?
Prezygotic reproductive barriers
operate before fertilization occurs.
• Habitat isolation—e.g., Rhagoletis flies
in the Hudson River valley.
• Temporal isolation—mating periods do
not overlap.
• Mechanical isolation—differences in
size and shape of reproductive organs;
common in insects.
23.3 What Happens when Newly Formed Species Come
Together?
Gametic isolation—eggs of one species
don’t have appropriate chemical signals
for sperm of another species; or sperm
is not able to attach to and penetrate
the egg.
Behavioral isolation—individuals reject or
fail to recognize potential mating
partners.
23.3 What Happens when Newly Formed Species Come
Together?
Floral traits of plants can influence the
behavior of pollinators, and thus
whether plants can hybridize.
Two species of columbines (Aquilegia) in
California can produce fertile hybrids,
but flower structure determines that one
species is pollinated by hummingbirds,
the other by hawkmoths.
Figure 23.10 Hawkmoths Favor Flowers of One Columbine Species (Part 1)
Figure 23.10 Hawkmoths Favor Flowers of One Columbine Species (Part 2)
23.3 What Happens when Newly Formed Species Come
Together?
Postzygotic reproductive barriers:
survival and reproduction of hybrid
offspring are reduced.
Low hybrid zygote viability—fail to mature
or have severe abnormalities.
Low hybrid adult viability—lower survival
rate.
Hybrid infertility—e.g., mules.
23.3 What Happens when Newly Formed Species Come
Together?
If hybrid offspring survive poorly, natural
selection may favor prezygotic barriers.
Strengthening of prezygotic barriers is
known as reinforcement.
Where two species of Phlox are
sympatric, P. drummondii (normally pink
flowers) has red flowers.
An experiment indicated reinforcement.
Figure 23.11 Prezygotic Reproductive Barriers (Part 1)
Figure 23.11 Prezygotic Reproductive Barriers (Part 2)
23.3 What Happens when Newly Formed Species Come
Together?
If reinforcement is occurring, related
species should evolve prezygotic
barriers faster than allopatric pairs of
species.
This has been shown in Agrodiaetus
butterflies. Wing color in males has
diverged much faster in sympatric than
allopatric populations.
23.3 What Happens when Newly Formed Species Come
Together?
If populations are reunited before
complete reproductive isolation has
developed, interbreeding can occur.
If hybrid offspring are fit and interbreed
with both populations, gene pools are
combined and no speciation occurs.
If hybrid offspring are less fit,
reinforcement may result in more
prezygotic barriers.
23.3 What Happens when Newly Formed Species Come
Together?
A hybrid zone can develop, and may
persist for a long time while
reinforcement develops.
Hybrid zones make good natural
laboratories for the study of speciation.
Example: two species of European toads;
hybrids have many defects. Hybrids are
only half as fit as pure-bred individuals.
Figure 23.12 Hybrid Zones May Be Long and Narrow
23.3 What Happens when Newly Formed Species Come
Together?
The hybrid zone is narrow, because there
is strong selection against hybrids.
Pure-bred individuals move only a short
distance into the hybrid zone, so there
are few encounters with the other
species, and reinforcement has not
evolved.
23.3 What Happens when Newly Formed Species Come
Together?
Human activities can change hybrid
zones.
Habitat disruption has created a hybrid
zone between two Banksia species in
Australia. The flowering seasons have
expanded and overlapped.
23.4 Why Do Rates of Speciation Vary?
Rates of speciation among groups vary
greatly.
Many factors influence the likelihood of
speciation.
Species-rich groups are more likely to
speciate faster than species-poor
groups.
23.4 Why Do Rates of Speciation Vary?
Speciation rates are likely to be faster in
species with poor dispersal abilities,
which can be separated by even narrow
barriers.
Example: There are several thousand
species of land snails in the Hawaiian
Islands, many restricted to a single
valley.
23.4 Why Do Rates of Speciation Vary?
Populations with specialized diets are
more likely to speciate.
Groups of closely related true bugs
(hemipterans) have a common ancestor
that was a predator on other insects.
Herbivory has evolved twice in this
group, and these lineages have many
more species. Herbivores tend to
specialize on one or a few plant types.
Figure 23.13 Dietary Shifts Can Promote Speciation
23.4 Why Do Rates of Speciation Vary?
In plants, speciation rates are higher in
insect-pollinated than wind-pollinated
plants.
Speciation rate in columbines (Aquilegia),
has been three times faster than
lineages that lack nectar spurs.
Long spurs limit the number of possible
pollinator species, leading to
reproductive isolation.
Figure 23.10 Hawkmoths Favor Flowers of One Columbine Species (A)
23.4 Why Do Rates of Speciation Vary?
Sexual selection also appears to
increase rates of speciation.
Examples: birds with promiscuous mating
systems
Birds of paradise have strong sexual
dimorphism; males assemble at display
grounds, females come and choose
mates. Females then build nests and
care for young alone. Males remain
and mate with more females.
23.4 Why Do Rates of Speciation Vary?
In closely related manucodes, males and
females resemble each other, form
monogamous mating bonds, and both
parents participate in raising young.
Manucodes—Five species
Birds of paradise—Thirty-three species
Figure 23.14 Sexual Selection in Birds Can Lead to Higher Speciation Rates
23.4 Why Do Rates of Speciation Vary?
Animals with complex sexually selected
behaviors are likely to form new species
because of the high degree of
discrimination in mate selection.
Discrimination on subtle differences in
color, size, appearances, and behaviors
can lead to rapid evolution of species.
23.5 Why Do Adaptive Radiations Occur?
An evolutionary radiation is the
proliferation of a large number of
species from a single ancestor.
If the resulting species live in a wide
array of environments, it is called an
adaptive radiation.
23.5 Why Do Adaptive Radiations Occur?
Adaptive radiation is likely to occur in
environments with abundant resources.
A population may encounter underutilized
resources when colonizing a new area
that contains few species, such as
islands.
Adaptive radiations have followed mass
extinctions.
23.5 Why Do Adaptive Radiations Occur?
Many adaptive radiations have occurred
on the Hawaiian Islands.
Native biota included many bird, insect,
and flowering plant species, but no
terrestrial reptiles or amphibians, and
only one mammal (a bat) species.
23.5 Why Do Adaptive Radiations Occur?
The 100 species of birds are thought to
have arisen from only seven colonizing
species.
More than 90 percent of plant species
are endemic—they are found nowhere
else.
Twenty-eight species of silverswords
have evolved diverse morphological
differences.
Figure 23.15 Rapid Evolution among Hawaiian Silverswords (Part 1)
Figure 23.15 Rapid Evolution among Hawaiian Silverswords (Part 2)
23.5 Why Do Adaptive Radiations Occur?
The original silversword colonizers
arrived on islands that had very few
plant species.
There were few trees and shrubs,
because such large-seeded plants
rarely disperse to islands.
Many silverswords developed tree and
shrub forms.
23.5 Why Do Adaptive Radiations Occur?
Adaptive radiations can occur in other
places as well:
1,563 species of ice plants in southern
Africa radiated within the last 9 million
years.
Ice plants are succulents with CAM
metabolism.
The region is known as the Succulent
Karoo.